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Drukuj Troubleshooting Analog Circuits.tif (230 Strony)

1. 22 23 24 25 26 156 13 Letters to Bob Probes work into a certain specified scope input capacitance You can t always take a probe that came with one scope and use it at high frequencies on another scope A simple test technique is waving your hand over a circuit to feel for the hot spot If something has gone into a latch up but is not smoking you can frequently find it this way g Edmund Scientific Barrington NJ sells thermally sensitive liquid crystal sheets which you can lay over a circuit to find moderate hot spots This material works well when you have a known good PC board to compare with the circuit under test Drafting departments sometimes erroneously think that they own the schematic and that its only purpose is to serve as a wiring diagram for the PC board layout Long after PC board layout the production test sustaining engineering and service de partments will still need the schematic Drafting tends to lose notes that I place on the schematic such as filter poles and zeros temperature coefficients normal AC and DC voltages waveforms and thermal information I save myself a lot of calls by putting this information in front of the techs from the beginning You can make an extremely low distortion and slightly microphonic sine wave oscillator from a light bulb and an op amp I got the circuit from Linear Technology s Milpitas CA AN 5 application note I built a 3 frequency 400
2. After you operate a high voltage power supply at full voltage if you turn off the power and decide that for safety s sake you should short out the filters with a few hundred ohms be careful A few minutes later the voltage on the Capacitors may come back up to 60 or 80 V and give you a shocking experience The partial recovery of voltage on a discharged capacitor is caused by soakage or dielectric absorption which causes the dielectric of the capacitor to remember the voltage it was recently charged up to In high voltage equipment it is wise to install a 2 W resistor of a few hundred kilohms across each large high voltage filter capacitor to bleed off the charge continuously and decrease the chance of shocks Ref 1 40 Nonpolar Capacitors Can Be a Bear 4l The last problem with old vacuum tube equipment is that the heat tends to dry up the capacitors electrolyte thus causing their capacitance to decrease This decrease is evidenced by excessive ripple or hum on various signals and of course on the power supply output of unregulated supplies Although I have presented these mal adies as problems afflicting old equipment you should consider them even in new designs In modem power supply designs it is critical that you choose a filter capacitor whose effective series impedance is low at all rated temperatures and frequencies Otherwise the rms filter current multiplied by the resistive component of the series
3. One technique for measuring and defining aperture delay is to maintain Vin at a constant level and issue the HOLD command If after a short delay Vin jumps by a few volts the smallest spacing between the HOLD command and the Vin jump that causes no false movement of Vou is one possible definition of the tAPERTURE DELAY Another way of defining and measuring aperture delay is to let Vin Move smoothly at a well defined rate Shortly after you issue the command to switch the circuit to the HOLD mode V stops changing The value at which V stops corresponds to the value of V at a particular point in time You can define the aperture delay as the difference between this point and the time at which the mode control signal crossed the logic threshold The uncertainty in the value of the aperture delay is then the aperture uncertainty Depending on how the circuit was optimized that delay can be positive or negative or practically zero perhaps only 1 ns or less Now will the real definition of aperture time please stand up I think that both of the characteristics I have described are of interest to people at different times But how can you avoid the problem of a person expecting one of these characteristics and actually getting the other I invite your comments on who wants to buy which characteristic and where to find a definition I ve looked in mili tary specs and at many data sheets and the issue still seems pretty unclear Another in
4. old art will probably have the best results Nasty Latches 119 SYSTEM 1 EDGE PC BOARD POWER 1 CONNECTOR SUPPLIES Figure 9 6 Installing antireversal diodes in the system power supplies and also on each PC board greatly reduces the chances of damaging the supplies or the circuitry with inadvertent short circuits or polarity reversals I certainly do not want to say that technicians can t troubleshoot oscillations simply because they don t know the theory of why circuits oscillate that s not my point at all I will only argue that a green or insensitive person whether a technician or an engineer can fail to appreciate when a circuit is getting much too close to the edge of its safety margins for comfort Conversely everyone knows the tale of the old time unschooled technician who saves the project by spotting a clue that leads to a solution when all the brightest engineers can t guess what the problem is References I Linear Brief LB 32 Microvolt Comparator in NSC Linear Applications Book 1980 1990 10 The Analog Digital Boundary A Never Never Land Previous chapters have dealt with circuit elements and circuits usually thought of as purely analog Now we turn to an area that confounds and frightens all too many engineers the boundary between the analog and digital worlds Armed with a solid theoretical foundation and the insights presented here you can keep your journey into the analog digital inter
5. 1000 and 2800 Hz oscillator in a small metal Bud box It had a THD lower than 80 dB Ifa circuit s DC values change when you breathe on it you may have dirty circuit boards When testing high gain low signal level circuits repeat the measurements with the lights off You may be surprised to learn that many components are photosensitive and have infrared transparent bodies One of my colleagues had a photosensitive metal can op amp that leaked light in around the leads Protection diodes can rectify high frequency noise and oscillations Micro Technical Industries Laguna Hills CA makes a handy thermal probe with which you can individually heat components The probe has tips to fit various compo nents such as small and large resistors metal can op amps and DIPs of various sizes Some sample and hold circuits are sensitive to slew rates on the digital inputs Even Schmitt triggers can exhibit metastability As paraphrased from an Analog Devices Norwood MA application note You may be able to trust your mother but you should never trust your ground Wrapped wire circuits work pretty well if you can distribute power and ground prop erly I use large diameter bus wire in a rectangular grid for high frequency logic if I don t have a wrapped wire board with internal power and ground distribution Sometimes powering your test circuit with batteries breaks ground loops and elimi nates power line noise A han
6. AE of 2 to4 worse se than a single bimn p t because th the mectani al layout of a a single turn pot is more stable and balanced Does anyone know of an example in which the multi turn pot is better A full year after this statement was originally published nobody has tried to contradict me although people who sell multi turn pots still brag in the vaguest possible terms about infinite resolution bleah Don t Exceed Your Pot s and V Ratings How do variable resistors fail If you put a constant voltage between the wiper and one end and turn the resistance way down you will exceed the maximum wiper cur rent rating and soon damage or destroy the wiper contact Note that the power rating of most variable resistors is based on the assumption that the power dissipation is uniformly distributed over the entire element If half of the element is required to dissipate the device s rated power the pot may last for a short while However if a 32 3 Getting Down to the Component Level quarter of the element is required to dissipate this same amount of power the pot will fail quickly For example many years ago the only ohmmeters available might put as much as 50 mA into a 1 Q resistor When a 50 kQ 10 tum precision potentiometer think of an item costing 20 was tested at incoming inspection using such an ohm meter the test technician would turn the pot down to the end where the 50 mA was sufficient to bu
7. EPPM C Parasitic Effects Cost Composition 1 22M High Low Low Metal Film 10 1M Low Medium Medium Carbon Film 10 10M Medium Medium Medium Wirewound Precision I 1M Low High High Wirewound Power 0 01 100k Medium High Medium Thin Film 25 100k Low Low Medium Thick Film 10 1M Low Low Medium Diffused 20 50k High High Low cementite nt tk etre aa e e a a aaa ne Range may vary by manufacturer a Figure 3 1 28 3 Getting Down to the Component Level RESISTIVE MATERIAL METAL ALLOY WIRE METAL ALLOY FILM CERAMIC CERAMIC CORE CORE b te Film resistors a are made by cutting a spiral into a layer of metal or carbon deposited ona nonconductive core Carbon composition types b have a solid core of resistive material wirewound resistors are formed by winding resistive wire on a nonconductive core c 100 or 50 ppm C these resistors have TCs as good as 20 10 5 or 2 ppm C and accuracies as good as 0 01 These resistors are comparable to small precision wire wound resistors but are generally smaller and slightly less expensive They also have much less inductance than the wirewound types and thus are suitable for higher speed operation A few spirals on a film substrate add negligible inductance compared to the hundreds or thousands of turns on a wirewound resistor s bobbin Precision film resistors are also available in matched sets of discrete resistors whose relative accuracy and TC
8. ROBERT A PEASE Troubleshooting Analog Circuits Troubleshooting Analog Circuits National Semiconductor Troubleshooting Analog Circuits son aie G2 392 eee Ww 2 4486S Robert A Pease Butterworth Heinemann Boston kondon Oxford Singapore Sydney Toronto Wellington Copyright 1991 by Butterworth Heinemann a division of Reed Publishing USA Inc All rights reserved Paperback reprint 1993 No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording or otherwise without the prior written permission of the publisher Recognizing the importance of preserving what has been written it is the policy of Butterworth Heinemann to have the books it publishes printed on acid free paper and we exert our best efforts to that end Library of Congress Cataloging in Publication Data Pease Robert A Troubleshooting analog circuits Robert A Pease p cm The EDN series for design engineers Includes bibliographical references p and index ISBN 0 7506 9184 0 alk paper ISBN 0 7506 9499 8 softcover l Linear integrated circuits Maintenance and repair 2 Electronic circuit design I Title II Series TK7874 P42 1991 91 1192 621 381 5 dc20 CIP British Library Cataloguing in Publication Data Pease Robert A Troubleshooting analog circuits 1 Analogue circuits I T
9. along Bayshore witha newly purchased 1970 VW Bus At the time we couldn t figure out what was wrong with it In the letter he explained that the gas gauge always read 3 4 full because it was broken which was why he ran it out of gas and didn t know it Unfortunately the guy he bought it from wasn t helpful enough to warn him So if I keep on driving this basic car forever I will know everything I need to Back to Electronic Circuits 173 know about it Go ahead Say OK Pease you can t keep on driving a 1968 VW forever You can say that but you are wrong I can buy enough 1968 VWs to last me for another 50 years There are a lot of 1968 VWs in very good shape here in California Back to Electronic Circuits Just as many cars are designed to be repaired by swapping out a large modular seg ment and swapping in a replacement so many electrical circuits and systems are designed with swap in cards which are non field repairable Even circuits which are pretty easy to repair are said by habit to be nonrepairable In fact the advent of throw away modules has been debated Personally I don t approve of it not a darned bit A couple weeks ago this little Compaq portable computer quit the one I m using for word processing When I tried to read its technical literature and find out its advice on how to repair the computer it told me to use various software in quiries to
10. etc etc R2 broken or missing R3 shorted R1 shorted QI s base shorted Q s collector open R3 broken or missing or open R1 or R2 has wrong value Collector base short Short from collector to ground Solder connection missing Base emitter junction probably blown PNP transistor Wrong resistor values Input signal much too big Capacitor missing or too small Bad DC bias Q installed back wards General Power supply oscil lating Load is oscillating Load causes QI to oscillate Transistor broken PNP transistor Check ohms and volts Touch in 47 k across R2 Look for shorted foil Look for shorted foil Measure base voltage Check c b diode Check resistor Touch in 120 k across R3 Check resistors Look for shorted foil or transistor Look for shorted foil or shorted transistor Make sure base and emitter are actually connected Replace Q1 Double check Check resistor values Check with scope Add a good capacitor across CI or C2 Check for DC levels as above Check Study frequency of ose n Check power supply add more bypasses Short QI base to ground look at collector Remove load study load Check as above Check eee Figure 14 3 Table 14 3 Back to Electronic Circuits 177 Comments on Troubleshooting Table for Single transistor Amplifiers This circuit has many things in common with many transistor circu
11. feature There are many other fine DVMs that have 10 MQ inputs above 2 or 3 V and if a 10 MQ input impedance is not a problem they are acceptable The most important reason to use a high input impedance DVM is because sometimes it s necessary to put 33 kQ or 100 kQ resistors in series with the probe right near the circuit under test to prevent the DVM s input capacitance from causing the circuit to oscillate If you re using a DVM with a 10 MQ input impedance and you have a 100 KQ resistor in series with the probe the DVM s measurements would lose 1 of their accuracy Fortunately many good DVMs have less than 500 pA of input cur Figure 2 4 6 Choosing the Right Equipment 17 10K or 100K CIRCUIT UNDER TEST 1K or 10K or 100K 0 01 or 0 1 or 1 0 Even if it s battery powered a DVM is capable of spitting out noise pulses into your delicate circuit The RC filters shown here can help minimize this Pick the values that work for your circuit rent which would cause less than 50 pV of error in the case of 100 kO source resis tance A high resolution DVM lets you detect 100 to 200 V deviations in an 11 V signal You can best spot many semiconductor problems by finding these minor changes A 4 digit DVM is a relatively poor tool however if you are desperate you can detect small voltage changes if you refer the DVM s low or ground side to a stable reference such as a 10 V bus
12. mmmm I asked if he had made up a breadboard and did it oscillate He said he had made it and it did not oscillate H mmmmm I asked him If you built up a breadboard and a computer model and the real breadboard oscillated but the computer did not you wouldn t be calling up to complain about the computer would you He stopped and thought about it He cogitated for a while He said I ll call you back And he hung up And he never did call back I mean what would you do Appendix H Pease s Troubleshooting Articles as Originally Published i in EDN o Part Troubleshooting i is more effective with the right philosophy EDN January 5 1989 14 arene Part 10 The analog digital boundary n needn t be a never never land EDN September 28 1989 p 145 Part 11 Presid Vv 12 1989 pi Part 12 Troubibs p 171 Part 13 Pease pointers rouse readers Hatters tp p 119 ery a lose n EDN October eria Bob p EDN Mey 10 1990 208 Index A References to figures are printed in boldface type acquisition time See sample and hold circuit active components 65 71 74 77 onward adapters 20 23 25 ADCs See Analog to Digital Converters add on technique 47 ADJUST pin 10 11 177 179 191 193 adjustable voltage regulator See under volt segulator adjustable adjustment 17 19 21 71 100 153 adjustment without trim pots 152 153
13. see the upper trace in Figure 8 6 If you think that is the CM error you might say the CMRR is as low as 5 000 at kHz and falling rapidly as the frequency increases But the actual CMRR error is about 0 2 mV p p see the lower trace of Figure 8 6 and thus the CMRR is about Vom Vout Gan 1 TO SCOPE OR TO ATE Cos SERVO AMPLIFIER Ros FUNCTION GENERATOR SINE SQUARE TRIANGLE This circuit is considered acceptable when used for DC CMRR tests in ATE systems However nobody ever tells you what are good values for the Rs and Cs nor whether it is valid up to any particular frequency Trace A shows the CMRR error taken using the circuit of Figure 8 3 But it s not the CMRR error it s really the gain error you are looking at 4 mV p p at kHz Trace B shows the actual common mode error about 1 20 the size of the gain error measured using the circuit of Figure 8 7 a b 94 8 Operational Amplifiers The Supreme Activators 100 000 at 1 kHz or any lower frequency Note also that on this unit the CM error is not really linear as you get near 9 V the error gets more nonlinear This is a 9 V 12 V CM range on a 12 V supply I chose a 12 V supply so my function generator could over drive the inputs So the business of CMRR is not trivial at least not to do it right gt How to Do It Right As we discussed in the previous section there are circuits that people
14. t show trends well And if you want to cover a wide range of currents you have to switch in different resistors or wait for the DVM to autorange which is not my idea of fun On the other hand you can find a DVM almost anywhere so this approach is easy to implement In either case if you put 15 V across 1 000 000 MQ and measure 15 pA that is at least 50 000 x higher resolution than most meters that can only measure up to 20 MQ Whichever detector you use apply a reasonable voltage across the unknown impedance and see where the leakage gets interesting This method can also be used for diodes and transistor junctions The op amp Circuit is not especially recommended for measuring the leakage of large value capacitors neither is the DVM approach because of the slow charging of the large capacitance and because of soakage or dielectric absorption effects But if you re desperate and start out with a low value of Royse You can eventually get some approximate measurements Recently a customer had a problem with a simple basic design using an LM317 regulator in which the circuit s impedances were fairly low just a few hundred ohms The same basic circuit as in chapter 14 Figure 14 3 After just a Sow minutes of operation the output of the LM317 would start drifting badty Phe cause turned 4 5 Preventing Material and Assembly Problems out to be not the LM317 or the resistors or the capacitors but the flux build up where the board
15. 166 13 Letters to Bob Dear Mr Sturgeon Thanks for your tip Most of the guys in our lab don t use Tempilaq but it s a good tip We use thermocouples on the can or diodes in the chip RAP Dear Bob So what do you think about spreadsheets Anonymous Dear Anonymous On beds they are fine but for linear circuit design they can be Bad News Nor mally I hate despise and detest them because they give you an answer but they do not give you a feel for what is important Also sometimes spreadsheets lie When they lie most people still trust them and never check on them We have found several cases of a spreadsheet with an error in it and the error went uncorrected unsuspected and even undoubted for a long time Finally we ran a sanity check and the answer was so silly that we realized nobody had ever checked to see if it even made sense Like any other form of computer output you should not trust spreadsheets and their results blindly RAP Dear Bob You re right Most people even technical people who should know better tend to treat any numeric display or computer readout as if it were engraved in stone ig noring whatever imperfect mechanism generated digits They re even in awe of num bers scratched in Jello Back in the sixties there was a Story circulating about the Apprentice Engineer who had been experimenting with the plant s new analog computer He ran up to the Chief Engineer waving a she
16. 79 80 See also transistors MOSFETs 2N4117A 2N4118A 2N4 119A 66 2N5485 2N5486 16 problems with JFETs excess gate current 80 gate connections 80 low transconductance 79 Vgs match 79 Vos Stability and tempco 79 80 transistors MOSFETs 81 87 88 3N160 81 IRF511 164 analog switches 81 CMOS digital ICs See under digital ICs problems with MOSFETs 81 87 88 DMOS FETS 88 parasitic bipolar transistor 88 ESD tolerance 81 88 ESD precautions 81 88 unreliability after ESD 81 excessive bandwidth 87 88 freedom from secondary break down 87 Index secondary breakdown possible at high voltage 87 gate voltage max ratings amp 8 oscillations 87 88 unprotected gates used in op amps 81 gt used in picoammeters femtoam meters 81 88 transistors structures 84 85 85 86 epi base 84 85 85 planar 84 85 single diffused obsolete 84 85 86 trim pots See under resistors adjustable troubleshooting techniques specific techniques add on 47 48 ask your buddy 12 beer check 4 4 gimmick capacitors 20 help your buddy 12 Paralysis by Analysis 6 planning 5 12 spring loading components 48 substitution 18 43 47 touch in technique 47 48 176 Twiddle box 18 TV receivers See television receivers two layer metal 7 Typicals or non guaranteed character istics 81 96 98 104 105 199 UA741 A741 77 See also LM741 Vos VS tempera
17. If an amplifier circuit isn t running at all sometimes the right thing to do is to reach into the circuit and grab one amplifier s input and force it to go above and below the other input If the output doesn t respond at all you have a dead amplifier an amplifier with no connections or a stuck output It is not obvious to try this open loop test no book tells you that this is a good idea but after you try it you will agree that its results usually tell you an obvious story Refer to Figure 14 1 in Chapter 14 for more detailed techniques and notes on troubleshooting basic op amp circuits Many of these op amp troubleshooting tips are applicable to other components as well The next chapter will continue with buffers comparators and related devices Data Converter Handbook Analog Devices P O Box 9106 Norwood MA 02062 1974 Pease Robert A Improved unity gain follower delivers fast stable response EDN February 20 1979 p 93 Also available as LB 42 in NSC s Linear Applications Book 1980 1986 1989 etc Get Fast Stable Response from Improved Unity Gain Followers DeMichele Glenn Compensate op amps without capacitors EDN July 21 1988 p 331 Frederiksen Thomas M ntuitive Operational Amplifiers McGraw Hill New York NY 1985 Available from Heath Company P O Box 8589 Benton Harbor MI 49022 800 253 0570 Part No EBM 1 19 95 Nelson Carl T Super Matched
18. Other power supply may be missing Output is shorted Bad amplifier Input oscillating Power supply osc illating P S bypass caps miss ing or inadequate Cap load too heavy No feedback cap Oscillation in air Comp cap too small LM301A or similar Use lower Rg or better op amp with lower I Use scope check for oscillation Check for dirty leaky PC board or connectors Make R Rj Il Ry Vout should be lt Vos x R R IJ Use a Vos trimpot or a better amplifier for low Vog If all other causes are negative remove op amp and test it Check voltage on each pin of the part not just the PC board See if amplifier is hot Check continuity Pull out unit test it Check input Check each supply Try more caps closer to unit or bigger or better ones Look for cables measure the C load See text try different values of Cy Turn off power and watch Try adding more capac itance Continued Back to Electronic Circuits Output distorts Bad gain In general No output Output is Zero volts Output oscillation is intermittent Load too heavy Input is distorted Slew rate distortion Resistors have bad tol erance or wrong value Oscillations at various levels Amplifier is suspected to be bad Swapped amplifier is bad too Output shorted to ground Amplifier has low Vog a very good op amp 175 Check
19. PC layout strays Enough said gt And besides how good are those models if you ask their makers Are the models guaranteed to give such a good representation of reality that if SPICE gives good results the op amps are guaranteed to work Well no not exactly In fact from what I have read none of the op amp models are guaranteed for anything The only thing they can do guaranteeably is to give a customer something when he begs for SPICE models It s guaranteed to make the customer go away happy and to keep him busy for a while But it s not guaranteed to make him happy in the long run Because the performance of high speed op amps and precision circuits depends so critically on the layout and on the resistors and capacitors that the model itself is almost irrelevant CP iy stl oy Pero Now some people might say How oes Pease dare to say that It s easy I haven t got any SPICE models of my op amps to give away Not at this _ time And if I did or when I do Iwonit be ableto guarantee them either At best I 4i may be able to say If you are a good engineer and use these models as a tool to _ pioneer some experiments that are inconvenient to test on the breadboard you may _ find these models are helpful so long as you then check it out on your breadboard to confirm the circuit For example you can use SPICE to measure some voltages or currents that are so small and delicate that you
20. The 2N5039 also has a switching speed faster than the 2N3771 when used as a saturated switch but the 2N3771 has a considerably larger safe area if used for switching inductive loads You can select the characteristics you prefer and order the type you need But be careful If you breadboard with one type and then start building in produc tion with the other you might suddenly find that the bandwidth of the transistor has changed by a factor of 10 or a factor of 0 1 or that the safe area doesn t match that of the prototypes Also be aware that the planar power devices like the familiar 2N2222 and 2N3904 are quite capable of oscillating at high frequencies in the dozens of megahertz when operated in the linear region so you should plan to use beads in the base and or the emitter to quash the oscillation The slower epi base devices don t need that help very often When I first wrote these articles on troubleshooting back in 1988 you could still buy the older single diffused transistors such as 2N3055H and the old 2N3771 Fabrication Structures Make a Difference 85 BASE EMITTER BASE BASE EMITTER BASE N SUBSTRATE COLLECTOR CONNECTIONS b COLLECTOR CONNECTIONS N SUBSTRATE a Figure 7 5 The characteristics of power transistors depend on their fabrication structure The epitaxial base structure a takes advantage of the properties of several different epitaxial layers to achieve good beta good s
21. Usually a good scrub with soap and water works better than any other solvent to clean off the residual contaminants that cause leakage under humid conditions Fingerprints for example Refer to Chapter 5 for notes on how a dishwasher can clean up a leaky PC board or a leaky dirty IC package When Computers Replace Troubleshooters Look Out Now let s think what needs troubleshooting Circuits Television receivers Cars People Surely doctors have a lot of troubleshooting to do they listen to 2 If you don t think troubleshooting of cars can be entertaining tune in Car Talk with Tom and Ray Magliozzi Ask your local National Public Radio station for the broadcast time GOOD STUFF Vs OUTPUT O TO LOAD SPEED CONTROL AT FINGERTIPS Figure 1 3 When you walk across a dry carpet and reach for the speed control you draw an arc and most of the current from the wiper of the pot goes right into the LM317 s ADJ pin The Computer Is Your Helper and Friend V1 Vs OUTPUT TO LOAD SPEED CONTROL AT FINGERTIPS Figure 1 4 By merely swapping two wires the ESD pulse is now sent to ground and does no harm symptoms and try to figure out the solution What is the natural temptation To let a computer do all the work After all a computer is quite good at listening to com computer sysler i is sometimes called an Expert System part of the general field of Artificial Intelligence B
22. current limiting Beware of removing the input power and then re applying it before the thermistors have had a chance to cool A hot thermistor has low resistance and will fail to limit the current thus you are again likely to blow a fuse or a rectifier The second general problem with a line transformer occurs when you have a smal output filter capacitor In our old LM317 and LM350 data sheets we used to show typical applications for battery chargers with just a 10 pF filter Our premise was that when the transformer s secondary voltage dropped every 8 ms there was no harm in Jas 3 Getting Down to the Component Level letting the regulator saturate That premise was correct but we began to see occa sional failed regulators that blew up when we turned the power on After extensive investigations we found the problem in the transformer If the line power switch was turned off at exactly the wrong time of the cycle the flux in the transformer s steel core could be stored at a high level Then if the line power switch was reconnected at exactly the wrong time in the cycle the flux in the transformer would continue to build up until the transformer saturated and produced a voltage spike of 70 90 V on its secondary This spike was enough to damage and destroy the regulator The solution was to install a filter capacitor of at least 1000 HF instead of just 10 uF This change cut the failure rate from about 0 25 to near zero Another pr
23. fuses circuit breakers reputations and power com panies A soft start circuit forces the switcher to bring the output up to its working levels gradually and draws only a finite amount of current from the mains as it does I could show you a good soft start design but F11 do better than that I ll show you a bad one The LM3524 data sheet shows well that s what it used to show the circuit in Figure 11 2 fora 15 V 0 5 A step up Boost switching regulator I should mention that this circuit and the LM3524 data sheet have been in National Semiconductor s 1978 1980 and 1982 data books they were mistakenly left out of the 1986 data book but have been restored to the 1989 and future editions A Boost or step up switcher needs a soft start circuit to prevent it from satu rating its transformer and from just sitting there at start up For this reason C and D were included to provide soft start in Figure 11 2 s circuit But Figure 11 2 is still a bad circuit if R and D are not added Let s say that the regulator is running at a low duty cycle and the voltage on the COMP pin is relatively low Now as the input voltage changes the duty cycle may have to increase suddenly But the control am plifier that drives the COMP pin not only has to pull up the series RC network at the COMP pin but it must also pull C up to the new voltage level This load is unfairly heavy too much for the COMP pin s control amp and the output
24. impedance can cause excessive self heating And if the heat can t flow out of the capacitor the temperature will rise and i cause early failure Excessive heating is one hich is the frequency of ji ull wave rectifier operated fn K N REAC source capacitors at 2 A rms for each th 1000 ue Because the shotgun blast if ouap Ny very carefull wi youn gives you lower inductance but also allows you to insert the device into a board two ways and both are correct neither way is wrong Tantalum capacitors have many churact ristios similar to those of aluminum elec trolytic capacitors and for the extra price you pay you can get less leakage and somewhat lower series resistance Designers often try out a timing circuit using a tantalum capacitor and a high value resistor But when they try to buy a tantalum capacitor with leakage guaranteed low enough t make the circuit work every time they get quite angry when nobody is interested in selling such a device Of course if you were a manufacturer of tantalum capacitors and someone asked you to measure the leakage you would refuse the business too because testing is so difficult Even 42 4 Getting Down to the Component Level Capacitor Problems though this leakage is usually quite low nobody wants to have to measure it in pro duction nor to guarantee it for the lifetime of the component Wound film and stacked film capacitors cover wide
25. in G More on Spice 205 finitely versatile expanded scale oscilloscope and if it has a dV dt that suddenly changes well that is quite a surprise So I immediately wrote an open message to all my friends at NSC warning them about this potential problem and now I am writing about this to warn all my friends everywhere So these are just some of the reasons I am not enthusiastic about SPICE It s goofed me up me and my friends too many times _ My boss points out that it s not necessarily true that all kinds of SPICE have such bad problems with convergence or bad computations or spurious signals And that may be so If somebody who knows all about W SPICE or J SPICE wants to write in and assure me that his SPICE will never do that well that is fine by me But mean while don t get me wrong I don t hate these digital computers They hate me I despise them The other day I was standing out in the rain talking with a design engineer from the East Coast He said all the other engineers at his company ridicule him because they rely on SPICE while he depends on the breadboards he builds There s just one hitch his circuits work the first time and theirs don t To add insult to injury his boss forces him to help his colleagues get their circuits working since he has so much time left over I said that sounds pretty good to me so long as his boss remembers who is able to get out the circuits when it comes to doing
26. other engineers are re porting These spec limits and conditions have to be re vised and Those application circuits don t work like we thought they would we ll have one running in a couple of be finalized and frozen right away so that they can start printing copies to go out with evaluation samples These trying conditions may explain why data sheets always ct W requirements Getting a data sheet as Pogsible and as accurately as possi nig it ne wants to get the data sheet as The reader should always question the manufacturer What tse arise getting angry with the manufac Ns advantage been staff scientist at National Semicon Appendix G More on SPICE Recently I was down in New Orleans at one of the IEEE conferences the International Symposium on Circuits and Systems The keynote speaker Professor Ron Rohrer from Carnegie Mellon University commented thoughtfully about many aspects ofied or ers But what he said that really stunned me was his observation 3 C inobody d signs on the back of envelopes any more Ouch ing tore and mor true that young or lazy engineers cannot do muc ing yithout some computers or high powered calculators SPICE just hap Me of my pet peeves and will start gnawing on its ankles today E a a gt nd of analogies analogues analogs similes models and metaphors When I rked at George A Phil
27. t need to worry about is the differential input impe dance Every year I still get asked How do we measure the input impedance of an op amp And every year trot out the same answer We don t Instead we mea sure the bias current There s a close correlation between the bias current and the input impedance of most op amps so if the bias current is low enough the input impedance differential and common mode must be high enough So let s not even think about how to measure the low frequency differential input impedance or input resistance because I haven t measured it in the last seven years Generally an ordinary differential bipolar stage has a differential input impedance of 1 20 X I where I is the bias current But this number varies if the op amp in cludes emitter degeneration resistors or internal bias compensation circuitry You can easily test the common mode input resistance by measuring l as a function of Voy Measure Bias Current Rather Than Impedance 97 Ci 1M 1M 100 mV p p t MHz a 10x PROB 4 i t I 1 1 i CROSSPLOT 1 SCOPE i i 1 i 100 mV p p i 1 MHz i oc COUPLED 1 b Figure 8 8 Circuit a lets you test an op amp s common mode input capacitance When C Ca 5 pF measure V Va when C Cg 1000 pF measure V Vg Then Cin Ca X Va VAIVA Ye X Ca Ca For best results connect the signal to the plus input of the DUT with a small g
28. 0 anes manufacturers devices Such design diy 30 he circuit esigniniay have been di din t really ork all that well Ifthey re still isee they hide margin to spare If the ithe failures ot thal were unrealistic After all every engineer tine seenieitenits that had no right to work but they did work for a while Aind then when they began to fail it was obviously just a hopeless case So which will burn you quickest a marginal design or marginal components That s impossible to say If you buildin some safety margin you may survive some of each But you can t design with big margins to cover every possibility or your a k ud 4 a i a MOSFETS Avoid Secondary Breakdown 87 design will become a monster That s where experience and judgment must be invoked An old friend wrote to me from Japan Why do you talk about having to trou bleshoot 40 of the units in a batch of switching regulators In Japan that would be considered a bad design I replied that I agreed that it sounds like a problem but until you see what is the cause of the problems it is unfair to throw any blame around What if it was a bad workmanship problem Then that does not sound like a bad design aunless the design was so difficult to execute that the assembly instruc tions could not be followed Or maybe a bad part was put in the circuit Or maybe it was a ma
29. 100 pF in series with 1500 Q With discrete transistors whose junctions are considerably larger than the small geometries found in ICs ESD damage may not be as severe But in some cases ESD damage can still happen Delicate RF transistors such as 2N918s 2N4275s and 2N2369s sometimes blow up when you just look at em because their Junctions are so small Other transistor related problems arise when engineers make design assumptions Every beginner learns that the Vee Of a transistor decreases by about 2 mV per de gree Celsius and increases by about 60 mV per decade of current Don t forget about the side effects of these rules or misapply them at extreme temperatures Don t make sloppy assumptions about Vprs For instance it s not fair to ask a pair of transistors to have well matched Vpgs if they re located more than 0 1 in apart and there are heat sources power sources cold drafts or hot breezes in the neighborhood Matched pairs of transistors should be glued together for better results Of course for best results monolithic dual transistors like the LM394 give the best matching I ve seen people get patents on circuits that don t even work based on miscon ceptions of the relationships between Vee and current It s fair to assume that two matched transistors with the same Vape at the same small current will have about the same temperature coefficient of Vgg But you wouldn t want to make any rash as sumptions i
30. 109 126 129 non standard pins for digital ICs 187 operational amplifier 12 30 89 108 128 131 148 149 156 offset voltage See under operational amplifier op amps See operational amplifier ohmme See under digital voltmeter es r analog voltmeter 7 Onion Syndrome 8 Aneta 120 124 128 155 3 174 it cao Ads 99 100 101 102 15 dampit for unity gain follower uM AOJEM310 101 102 drivihgcables 100 102 101 102 155 how to accommodate cap loads decoupling 100 101 Noise gain damping 94 95 100 101 107 CMRR curve is same as Bode plot 91 93 feedback capacitor needed 101 102 103 generally advisable 102 103 value changes from breadboard to PC board 103 gain error 89 91 93 g noise 81 103 104 107 offset voltage trim 90 153 188 189 vs bias current errors 90 91 oscillations 89 99 105 155 output impedance 98 100 passive components 89 power supply bypass capacitors 100 See also unde foripower supply bypasses step response 89 98 104 typicals 96 98 104 105 199 200 st perature gradients 98 scopes 2 3 6 8 25 91 94 99 106 109 14 123 130 136 150 168 176 178 180 164 205 ating S digital istorage DSO 15 143 150 dual trace 14 p p automatic triggering 14 storage See under analog storage or digital storage oxide 6 p p automatic triggering Se
31. 19th Century Prussian Chancellor Bob Pease is one of the legends of analog design Over the years he s developed techniques and methods to expedite the often dificult tasks of debugging and troubleshooting analog circuits Now Bob has Compiled his battle tested methods in the pages of this hook Based on his immensely popular series in AON Waga tre the book combats a wealth of new material byer chapter has been expanded and Dvo new chapters and several usetul appendices bave been added Mnnerous tables samima zine Doublesbhoonng approaches tor vanots con ponents are another welcome additon Whether vou are prima an analog or digitabensimeer tes Dbimoan experienced or ncophs te this book has something for yor Your dT tind Bob s approach to problem adent bhcauon and asodaton to be apphoable lo wide specttum oberg neon disciplines Bab wii bevont perona Prenton pa these Pris SUV ETE Nor aD HIRES oot yas eh owdhieke proponis ae tiheds to shoes ap aand bosco posto eh doniga Tene Jhe bead Hie ude e doco asthe opio Cat priehb le thou bh trot thd hefies aopa hapat ANA prado l plo o wp te ee a aa pregbhosfroopire raoga e Caage de dpa Lid n idii poah Lil jil SERRET ESETERE kad S Poik rag mond ood irae Veh 7 aad a H pidid o ihig jhe ISBN O 506 9499 4 i
32. 203 207 Index computers aiding in troubleshooting 10 12 147 152 connectors 22 25 52 58 61 153 162 174 185 problems with connector fails to connect 60 glue in socket 60 153 7 intermittencies 60 nothing in socket 60 plugging into live socket 60 socket leakage 52 unexpected capacitance 60 61 153 copper clad material See under Printed Circuit Boards materials cube taps 25 current limits 17 21 34 38 58 88 105 106 118 137 139 164 180 184 192 current map 20 current meter leakage detector 17 52 53 53 88 111 logarithmic detector 52 53 wide range linear detector 53 53 current probes See under probes current current pumps 25 58 current surges 10 27 32 37 42 44 57 70 71 75 77 78 81 82 87 88 139 140 currents 32 34 37 41 57 61 62 65 67 73 99 100 107 110 111 115 116 130 133 136 138 141 145 160 164 192 196 198 204 206 curve tracers 20 Czar 7 135 136 of Band gaps 7 135 136 205 of Czener Czaps 7 of Floobydust 164 of Start up Circuits 7 142 Czarina of Data Sheets 7 DAC See digital to analog converter Darlington See under transistors bipolar data sheets 20 30 37 40 42 48 56 57 61 62 65 73 75 82 84 91 96 102 105 106 113 116 120 128 129 131 133 136 137 140 157 159 167 177 179 185 188 199 202 206 detector current See current detector detector settling 67 detector short circuit 21 21 diagn
33. AL See Artificial Intelligence All Tests mode 5 alligator clips 23 25 95 96 alligators 8 12 100 AM radio 21 22 155 158 AMP Inc 185 amplifier battery powered 18 156 157 analog computers 29 145 166 203 analog ground 63 129 130 analog switches 79 81 121 133 155 Analog Devices Data Converter Handbook 2 13 90 107 156 analog meters 17 32 36 38 147 150 186 accuracy 17 147 148 hysteresis friction 148 inductance 148 position sensitivity 148 150 analog digital circuits See analog to digital converters and digital to analog converters analog digital interface 120 134 124 125 analog to digital converters 126 128 131 207 See also under digital voltmeters 12 bit CMOS 126 fast successive approximation types 130 148 ground loops 129 130 ground systems 129 130 integrating types 130 148 noise 128 129 134 power supply rejection 130 pre load on outputs 130 VFCs 130 131 analogs See analogues analogues resistive 29 Analogy 206 anti bounce circuits 6 antereversal circuit with MOSFET 164 anti reversal circuits See also at Powel Supply anG reversal apti static 23 aperture delay See sample and hold circuit Ardizzoni John 164 165 Armitage 54 Artificial Intelligence 11 Artificial Stupidity 11 ATE See Automatic Test Equipment Automatic Test Equipment 66 92 93 94 ballast resistors 83 ball hooks 23 band gap references See voltage refer
34. Could Cost an Engineer Time and Money By Robert A Pease When a new product arrives in the marketplace it hopefully will have a good clear data sheet with it The data sheet can show the Prospective user how to apply the device what performance specifications are guaranteed and various typical applications and characteristics If the data sheet writer has done a good job the user can decide if the product will be valuable to him exactly how well it will be of use to him and what precautions to take to avoid problems SPECIFICATIONS The most important area of a data sheet specifies the char acteristics that are guaranteed and the test conditions that apply when the tests are done Ideally ali specifications that the users will need will be spelled out clearly If the product is similar to existing products one can expect the data shest to have a format similar to other devices But if there are significant changes and improvements that nobody has seen before then the writer must clarify what is meant by each specification Definitions of new phrases or characteristics may even have to be added as an appendix For example when fast settling operational amplifiers were first introduced some manutacturers defined settling time as the time after slewing before the output finally enters and Stays within the error band but other manufacturers includ ed the slewing time in their definition Because both groups made their definiti
35. First repair 20 40 or 60 of these complex units Switching regulated power supplies can also be quite complex If you manufacture them in batches of 100 you shouldn t be surprised to find some batches with 12 pieces that require troubleshooting and other batches that have 46 such pieces The troubleshooting may as you well know be tough with a new product whose bugs haven t been worked out But it can be even tougher when the design is old and the parts it now uses aren t quite like the ones you used to be able to buy Troubleshooting can be tougher still when there isn t much documentation describing how the product is supposed to work and the designer isn t around any more If there s ever a time when troubleshooting isn t needed it s just a temporary miracle You might try to duck your troubleshooting for a while You might pretend that you can avoid the issue And what if you decide that troubleshooting isn t necessary You may find that your first batch of products has only three or four failures so you decide that you don t need to worry The second batch has a 12 failure rate and most of the rejects have the same symptoms as those of the first batch The next three batches have failure rates of 23 49 and 76 respectively When you finally find the time to study the problems you will find that they would have been relatively easy to fix if only you had started a couple of months earlier That s what Murphy s
36. For example the overall current gain at 8 mA can vary from 15 to 104 even though the spec is simply 10 min Further the transfer efficiency from the LED to the photodiode varies over a range wider than 10 1 and the B of the transistor varies from 300 to 3000 Consequently the transistor s speed of response which is of course related to B and f3 ag would vary over a 10 1 range If your circuit doesn t allow for gains and frequency responses that vary so wildly and widely expect trouble For example two circuits one an optoisolated switching regulator Ref 3 and the other a detector for 4 to 20 mA currents Ref 4 have enough degeneration so that any 4N28 you can buy will work I used to have a group of several worst case 4N28s from various manufacturers that I would try out in prototypes and problem circuits Unfortunately I don t have those marginal devices anymore but they were pretty useful Also the data sheets for optoelectronic components often don t F a clear Vp a b Adding R and R to the inexpensive 4N28 optoisolator lets it handle faster signals with less delay 5 ms vs 60 us The scope photo s bottom trace is an input waveform the top trace ts the circuit s output without R and R3 and the center trace is the output with R 2 MQ and Ry tk Identifying and Avoiding Transistor Problems Although transistors both bipolars and MOSFETs are immune to many problems you can still h
37. Fuses are generally guaranteed to carry 100 of their rated current indefinitely and most will carry 120 for several hours Even the fast blow ones cannot open up much faster than 10 ms if overloaded by 10X their rated load or 100 ms if overloaded by 2X You may be able to get faster response than that if you shop for the new semiconductor rated fuses with very fast blow time If somebody in your organization a components engineer or an old timer can help you find the right information in a catalog of fuses he can save you a lot of time Without that kind of help you will probably not be able to find a catalog from a fuse maker or to figure it out when you get it The curves of various ratings are a little obscure until you get used to them You may not use fuses much modern solid state circuits have such good current limiters and thermal limiters that you may not see fuses every day So when you do see fuses you may be surprised The low current ones act pretty soft resistive Some fuses just happen to fail unprovoked The one in my clothes dryer fails every 3 or 4 years leaving my wife perplexed Finally I wrote down the list of symptoms so any time the fuse goes out and there is no heat we at least save time by recognizing the symptoms When my microwave oven quit working recently I was a little con cerned because the label on its back said No user serviceable components inside When I opened it up there was a fuse clip wit
38. HIGH CURRENT RECTIFIER 75V USE HEAT W SINK gv COMPOUND one ZENER EQUIVALENT a HIGH CURRENT RECTIFIERS OR BRIDGE wv COMPOUND SYMMETRICAL on ZENER EQUIVALENT b The power rating of this compound zener a is that of the Power transistor The second compound zener b is almost the same as a but acts as a symmetrical matched double ended compound zener ne ee weer Optoisolators 73 liamps of current LEDs are awfully reliable these days I have a thermometer display on my wall which has 650 inexpensive plastic packaged LEDS These LEDs have amassed 40 000 000 device hours with just one failure The only problem I ever have with LEDs is trying to remember which lead is plus I just measure the diode and re derive it every time Optoisolators Figure 6 5 An optoisolator also called a photo coupler or opto coupler usually consists of an infrared LED and a sensitive phototransistor to detect the LED s radiation In the course of working with the cheaper 4N28s I ve found it necessary to add circuitry to achieve moderate speeds For example if you tailor the biasses per Figure 6 5 you can get a 4N28 s response up toward 50 kHz otherwise the devices can t make even 4 kHz reliably The trick is decreasing the phototransistor s turn off time by using a resistor from pin 4 to pin 6 I ve evaluated many different makes and lots of 4N28s and have found widely divergent responses
39. They are invaluable for running various tests that can lead to the right answer You may need component values beyond what the twiddle boxes offer in our labs we built a couple of home brew versions Figure 2 7 The circuit shown in Figure 2 7a provides variable low values of capacitance and is useful for fooling around with the damping of op amps and other delicate circuits You can make your own calibra tion labels to mark the setting of the capacitance and resistance values The circuit shown in Figure 2 7b provides high capacitances of various types for testing power supplies and damping various regulators An isolation transformer If you are working on a line operated switching regulator this transformer helps you avoid lethal and illegal voltages on your test setup and on Figure 2 6 10 150 pF 10k a Choosing the Right Equipment 19 SELECTOR SWITCH Be ECTOR FOR MODE R Rp SELECTOR Q Oo e gt R2 SELECTOR NOTES 15 lt R lt 10k 15k lt Rz lt 10M 100 pF lt C lt 0 22 uF This general schematic is for a commercially available RC substitution box the VIZ Model WC 412A The unit costs around 139 in 1991 dollars and has resistor and capacitor values in the range of 15 Q to 10 MQ and 100 pF to 0 2 uF respectively It can be configured to be an open circuit a series RC resistors capacitors a parallel RC or a short circuit See text for availability C SELECTOR R 18 POSIT
40. Various Diodes 196 How to Get the Right Information from a Datasheet 199 More on SPICE 203 Pease s Troubleshooting Articles as Originally Published in EDN 208 index 209 Foreword Your idea is so good that if you give me 20 minutes I ll be sure that I was the first one to think of it Although I pass out that accolade sparingly if I were to do what the compliment implies I d surely claim credit for the idea of publishing Bob Pease s series on Troubleshooting Analog Circuits in EDN Magazine Edition The fact is though that the idea came from Jon Titus VP Editorial Director and Chief Editor of EDN magazine and from Tarlton Fleming then an EDN Associate Editor and now Manager of Applications Engineering at Maxim Integrated Products Corporation In early 1988 Jon and those EDN technical editors who work at the publication s and Cahners Publishing Company s Newton Massachusetts headquarters were brainstorming ideas for articles we could solicit from contributors in industry Jon ventured that because EDN readers always look to the magazine to provide practical ideas on how to do their jobs better and because trouble is ubiquitous articles on how to troubleshoot more effectively should be a natural for us Tarlton who edited EDN s popular Design Ideas section worked with Bob on a regular basis as Bob reviews the analog design ideas submitted by EDN readers Tarlton recalled Bob s mentioning a bo
41. York NY 1981 p 265 Understanding Diodes and Their Problems After the first five chapters about troubleshooting passive parts this chapter shifts over to troubleshooting active components We begin with the simple stuff diodes and rectifiers optically coupled devices solar cells and batteries Even the simplest active devices harbor the potential for causing baffling trouble shooting problems Consider the lowly diode The task of a diode sounds simple To conduct current when forward biassed and to block current when reverse biassed while allowing negligible leakage That task sounds easy but no diode is perfect and diodes imperfections are fascinating Even these two terminal devices are quite complex All diodes start conducting current exponentially at low levels nanoamperes and up An ideal diode may have an exponential characteristic with a slope AV AI of g 38 6 mS mA X Ip where mS milliSiemens millimhos and Ip forward current And indeed transis tors do have this slope of 38 6 mS mA at their emitters at room temperature This corresponds to 60 mV per decade of current But the slopes of the exponential curves of different real two terminal diodes vary considerably Some like a IN645 havea slope as good as 70 or 75 mV per decade Others like 1N914s have a slope as poor as 113 mV per decade Others have intermediate values such as 90 mV decade When you go shopping for a diode the data sheets never
42. a Vos pot There s a chapter in Analog Devices Data Converter Handbook it s in there Ref 1 Now I ve known about this trim problem for 25 years but this isn t a problem that a customer asks us about even every year these days and I guess that s it true for my colleagues too As it was not fresh in anybody s mind well we forgot to include it we didn t notice it when it was missing It just shows why you have to ih write things down 4 An Uncommon Mode A good example of misconstrued specs is the common mode error We often speak of an op amp as having a CMRR Common Mode Rejection Ratio of 100 dB Does this number mean that the common mode error is exactly one part in 100 000 and has a nice linear error of 10 uV per volt Well this performance is possible but not likely It s more likely that the offset voltage error as a function of common mode voltage is nonlinear Figure 8 2 In some regions the slope of AVog will be much better than 1 part in 100 000 In other regions it may be worse It really bugs me when people say The op amp has a common mode gain Ayc and a differential gain Ayp and the CMRR is the ratio of the two This statement is silly business It s not reasonable to say that the op amp has a differential gain or common mode gain that can be represented by a single number Neither of these gain numbers could ever be observed or measured with any precision or repeatability on any
43. all new circuits should be submitted to the Czar to allow him to spot any old errors So far we think we ve found and fixed over 50 of the possible errors before the wafers were fabricated and we re gaining In addition we have added Czars for start up circuits and for trim circuits and a Czarina for data sheet changes and we are con sidering other czardoms It s a bit of a game but it s also a serious business to use a game to try to prevent expensive errors haven t always been a good troubleshooter but my baptism of fire occurred quite a few years ago had designed a group of modular data converters We had 1o X bo irse Fhings Pirse ship 525 of them and some foolish person had bought only 535 PC printed circuit boards When less than half of the units worked I found myself in the trouble shooting business because nobody else could imagine how to repair them I discov ered that I needed my best triggering scope and my best DVM I burned a lot of mid night oil I got half a dozen copies each of the schematic and of the board layout scribbled notes on them of what the DC voltages ought to be what the correct AC waveforms looked like and where I could best probe the key waveforms made little lists of If this frequency is twice as fast as normal look for Q17 to be dam aged but if the frequency is low look for a short on bus B I learned where to look for solder shorts hairline opens cold soldered joints
44. and intermittents diagnosed the problems and sent each unit back for repair with a neat label of what to change When they came back did they work Some did and some still had another level or two of problems That s the Onion Syndrome You peel off one layer and you cry you peel off another layer and you cry some more By the time I was done I had fixed all but four of the units and I had gotten myself one hell of a good education in troubleshooting After I found a spot of trouble what did I do about it First of all I made some notes to make sure that the problem really was fixed when the offending part was changed Then I sent the units to a good neat technician who did precise repair work much better than a slob like me would do Lastly I sent memos to the manu facturing and QC departments to make sure that the types of parts that had proven troublesome were not used again and I confirmed the changes with ECOs Engineering Change Orders It is important to get the paperwork scrupulously cor rect or the alligators will surely circle back to vex you again Sloppy Documentation Can End in Chapter XI I once heard of a similar situation where an insidious problem was causing nasty reliability problems with a batch of modules The technician had struggled to find the solution for several days Finally when the technician went out for lunch the design engineer went to work on the problem When the technician came back from
45. and realized that the climate in a dentist s office in North Dakota in February is about as dry as it can be and is conducive torvery high electrostatic potentials The LM317 is normally safe against electrostatic dis charges as high as 3 or 4 kV but walking across a carpeted floor in North Dakota in February can generate much higher voltages than that To make matters worse the speed control rheostat for this dental instrument was right out in the handle The wiper and one end of the rheostat were wired directly to the LM317 s ADJUST pin the other end of the rheostat was connected to ground by way of a 1 KQ resistor located back in the main assembly see Figure 1 3 The speed control rheostat was just wired up to act as a lightning rod that conducted the ESD energy right into the ADJUST pin The problem was easily solved by rewiring the resistor in series with the IC s ADJUST pin By swapping the wires and connecting the rheostat wiper to ground see Figure 1 4 much less current would take the path to the ADJUST pin and the diffused resistors on the chip would not be damaged or zapped by the current surges Of course adding a small capacitor from the ADJ pin to ground would have done just as well but some customers find it easier to justify moving a component than adding one A similar situation occurs when you get a complaint from Boston in June Your op amps don t meet spec for bias current The solution is surprisingly simple
46. and similar adjustable positive regulators their substrate is tied to Vou The LM196 voltage regulator s substrate is tied to the positive supply voltage Vg as are the Power Transistors May Hog Current 8l MORRE LUNG Je gp ite STOFA Ju ave ON you might get some ys be warty of any devices liyi As you build a bipot Ar WANSISTOI bigger and bigger you may be tempted togo to extremes and make a huge power transistor But there are practical limitations Soon the circuit capacitances cause oppressive drive requirements and removing the heat is difficult Still no matter how big you build power transistors people will find a use for them Their most serious limitation on just building transistors bigger and bigger is secondary breakdown whichis what happens when you drive a transistor outside its safe operating area When you operate a power transistor at very high currents and low voltages the distributed emitter resistance of the device which includes the resistance of the emitter metal and the inherent emitter resistivity can 8 82 7 Identifying and Avoiding Transistor Problems Figure 7 2 When you hit a component or circuit with a pulse of real ESD you can never be sure what kind of trouble you ll get unless you ve already tested it with an ESD simulator Photo copyright Peggi Willis cause enough I X R drop to force the entire emitter and its periphery to share the current Now let s
47. aren t I even recall a case where I had to preload the TTL outputs of a modular DAC s internal storage register with a 2 kQ resistor from each line to ground Otherwise they would over shoot when going HIGH and then recover with a long slow tail an attenuated version of which would then appear on the DAC output 7 On chip buffers at a DAC s input can help cut down feedthrough from the bit lines to the analog output but will not completely eliminate it The bus can move around incessantly and capacitive coupling or even PC board leakage will sometimes cause significant crosstalk into the analog world Even IC sockets can contribute to this noise If you could prove that such noise wouldn t bother your circuit you could for get about it The problem is that you can only make meaningful measurements of such effects on an operating prototype computer modeling isn t going to simulate this Multiplying DACs are popular and quite versatile However a multiplying DAC s linearity can be degraded if the output amplifier s offset voltage isn t very close to zero I ve heard this d gradation of linearity estimated at 0 01 per millivolt of offset Fortunately low offset Op amps are pretty cheap these days At least a low offset op amp is cheaper than a trim pot f Another imperfection of any multiplying DAC is its AC response for different i codes If you put in a 30 kH sine wave as the reference you shouldn t really be i surprised
48. blinders off Don t get caught in the analog vs digital war Don t swear by your favorite meter or scope forsaking all others Instead realize that no instrument is the answer to every problem no generic circuit is best for all occasions In the same vein and plagiarizing an old joke Never trust anybody not even your fa vorite meter This also goes for just about any written specification And finally you can t fix it if you don t understand it This is the simplest and the hardest Even if there is a deadline take time to study the circuit diagrams service manuals operating guides and anything else you can find BEFORE you even take the cover off the box Thanks again for a jewel of a job Sincerely yours Frank R Borger Michael Reese University of Chicago Center for Radiation Therapy Chicago Illinois Dear Mr Borger You went to the circus and thought about my articles That s funny am usually reminded of the Zoo Boy when you get into the troubleshooting business it gets really strange Thank you for your letters I like your Three Commandments On your third Commandment I wouldn t quite say You can t fix it if you don t understand it but it does make it a lot harder to fix something if you don t understand it RAP PQ LELLES CO oo Dear Bob You have been writing an excellent series of articles as you always do This series reminds me of the old days when I used to be abl
49. can answer the question Still the telephone number is the key to getting the factory to heip ORIGINS OF DATA SHEETS Of course historically most data sheets for a class of prod ucts have been closely modeled on the data sheet of the forerunner of that class The first data sheet was copied to make new versions That s the way it happened with the UA708 the first mono lithic op amp and all its Copies as well as many other simi lar families of circuits 202 F How to Get the Right Information From a Data Sheet Even today an attempt is made to build on the good things learned from the past and add a few improvements when necessary But it s important to have real improvements not just change for the sake of change So while it s not easy to get the format and everything in it exactly right to please everybody new data sheets are con tinually surfacing with new features applications ideas specifications and aids for the user And if the users com plain loudly enough about misleading or inadequate data sheets they can help lead the way to change data sheets That s how many of today s improvements came about through customer demand Who writes data sheets in some cases a marketing per son does the actual writing and engineers do the checking In other companies the engineer writes while marketing people and other engineers check Sometimes a commit tee seems to be doing the writing None of these w
50. capacitors for power supply bypass runt pulses See under digital ICs safe to keep good parts in 104 105 137 safe operating area See under transis tors bipolar safety margins 6 82 86 87 99 112 118 119 safety goggles 23 41 67 139 safety 23 40 139 sample and hold circuit 131 133 de glitchers 132 acquisition time definition of 132 133 aperture delay definition of 132 LF6197 132 low soakage capacitors for 131 problems driving a Multiplexer MUX 132 schematic diagrams 8 9 20 57 106 156 157 171 screwdrivers 23 screws 165 170 Seddon Bruce x second sourcing 70 71 97 shielding See under RF shielding short circuit recovery 05 short circuit to or supply 105 118 short circuit detector 21 21 161 sin 12 single supply operation 96 115 Smith Marvin 163 Smith John 2 12 smip trim 152 153 SOA Sate Operating Area 81 84 85 Re soakape Sec under capacitors soap and water 10 54 81 sockets See under connectors socket capacitance 600 soft diodes dow conductance See under diodes soft start See under switch mode regulators software 1 133 140 146 155 166 173 solar cells 74 75 heating problems 74 packaging problems 74 solar powered night light 74 167 solder 8 21 24 48 51 58 59 62 81 105 153 162 167 170 186 types of solder acid core solder 59 aluminum solder 59 low temperature solder 46 59 rosin core solder 59 silver sold
51. circuit that had some JFETs whose gate connection was supposed to be through the back of the die I found that some of the dice didn t have proper metallurgical processing which caused some strange behavior Initially the gate acted as if it really were connected to the metal under the die and would act that way for a long period of time Then after a while the gate would act like an open circuit with as much as 1 V of error between the actual gate and the bottom of the die The amplifier s Vos would grow as large as 1 V The gate would remain discon nected until a voltage transient restored the connection for another week The inter mittency was awful because nothing would speed up the week cycle to failure time So we had to go back and add definite lead bonds to the gate s bond pad on the top of the chip which we had been told was unnecessary Ouch When designing hybrids you need to make sure to connect the substrate of a chip to the correct DC level The bottom of a FET chip is usually tied to the gate but the connection may be through a large and unspecified impedance You have to be a pretty good chemist or metallurgist to be sure that you don t have to add that bond to the gate s metal bonding pad on the top of the die just to get a good gate connection The substrate of a discrete bipolar transistor s die is the collector Most linear and digital IC substrates are tied to the negative supply Exceptions include the LM117
52. correct state If however either or both inputs are driven below V7 and either input current ex ceeds 10 A the output state is not guaranteed to be correct And this definition applies nominally to the LM339 LM393 and also to the LM324 and LM358 ampli fiers if you are applying them as a comparator So you cannot say we are not trying to make our data sheets more clear and precise even if it does sometimes take 20 years to get it just night Still if you stay within their rated common mode range comparators are not that hard to put to work Of course some people disdain reading data sheets so they get unhappy when we tell them that differential signals larger than 5V will damage the inputs of some fast comparators But this possibility has existed since the existence of the A710 so you ll have to clamp clip or attenuate the input signal differential or otherwise so the fast comparators can survive An Unspoken Problem Something else that does not usually get mentioned in a data sheet is common mode slew problems The good old LM311 is one part that is otherwise very well behaved but causes some confusion when common mode slew problems arise But to some extent all comparators can have these troubles If one input suddenly slews up to exceed the other s level you may see an unexpected extra delay before the com parator s output changes state This delay arises because the comparator s internal nodes do not slew fast
53. deterioration over time I usually use some form of reverse voltage limiting to ensure that the base emitter Junction doesn t undergo zener break down Referring to your comments on p 132 of the same article I have to disagree on the advisability of plugging MOS ICs into sockets with the power applied I consider it inadvisable because power may easily be sourced to the chip via its input and output pins in this situation Some manufacturers forbid this by implication in the Electrical Characteristics section of their data sheets and I have witnessed device destruction being caused by this practice am also unable to agree with your comments on not 160 13 Letters to Bob FLOATING GATE Figure 13 2 Letting TTL inputs float HIGH is a risky business They won t do any harm on your bread board only when you go into production It s wise to tie them up towards V with kQ or so wearing ground straps when handling MOS ICs My experience and that of others has been that MTBFs plummet if you don t handle MOS ICs with kid gloves The problem is that the device rarely fails on the spot but having possibly been over stressed will do so at a time and place of its own choosing Turning to the article in the September 28 1989 edition of EDN Chapter 4 I feel bound to present a caution regarding tantalum capacitors First they are even less tolerant of reverse polarity than electrolytic capacitors are Reverse polarity can ar
54. enough for its outputs to respond For example a 10 V step can accrue an extra 100 ns delay compared with the delay for a 100 mV step And if both inputs slew together the output can spew out indeterminate glitches or false pulses even if the differential inputs don t cross over Be careful if your circuit has comparator inputs of this sort yet cannot tolerate such glitches Come to think of it I get occasional complaints from engineers along the lines of T ve been using this comparator for years without any trouble but suddenly it doesn t work right How come When we inquire we find that the comparators have been operating very close to the edge of the typical common mode range well beyond 116 9 Quashing Spurious Oscillations any guaranteed values Although these engineers have been getting away with crowd ing the limits for years the latest batch of comparators gives them trouble Some of our best friends depend on us to have our parts meet those typical specs and it s always painful for us to tell them that they really ought to depend only on guaranteed specs If you need three op amps and one comparator can you use a single LM324 Well op amps are not necessarily bad as comparators but they sure are slow and the LM324 is among the slowest Not only is its slew rate slow but if you put in an over drive of just 5 mV more than Vog the output will respond at only 0 01 V us not even as fast as its specified slew r
55. error s TC If the errors are well behaved and fit inside a small box well that s a pretty good part There is one classical caveat and l I include it here because it never got men tioned in the EDN series I showed my typed draft to 45 people and then thousands of people looked at my articles and nobody told me that I had forgotten this Namely If you run an op amp in a high impedance circuit so that the bias current causes i significant errors when it flows through the input and feedback resistors do not use i the Vos pot to get the circuit s output to zero Example if you have an LM741 as a unity gain follower with a source impedance of 500 kQ and a feedback resistor of if 470 kQ the 741 s offset current of 200 nA worst case could cause an output offset of 100 mV If you try to use the Vog trim pot to trim out that error it won t be able to F do it If you have only 20 or 40 mV of this I X R error you may be able to trim it out but the TC and stability will be lousy So you should be aware that in any case where the Ios X R is more than a few millivolts you have a potential for bad DC error and there s hardly any way to trim out those errors without Causing other errors When you get a case like this unless you are willing to accept a crude error then these errors are trying to tell you that you ought to be using a better op amp with lower bias currents And where did I find this reminder not to trim out an I X R with
56. errors EDN September 15 1988 p 207 3 Getting Down to the Component Level Resistors and Inductors In earlier r ohapters wel ve covered the philosophy of troubleshooting analog circuits failure of the resistor is justa symptom of a larger robiem You may eventually have to track down a wide variety of problems involving resistors to achieve aworking a Some will seem co For example your circuitineeds a m ko instead aetna ip KO Te viere iar is supposed kawas Olteni there are aging overheating or temp nine cycling Recently we found a batch of 1 metal film resistors whos valties had increased by 20 to 900 after just a few dozen cycles of 55 to 125 C As it turned out our QC department had okayed only cer tain resistors to be usedi in burn in boards and these particular resistors had not been approved The QC people too had spotted this failure mode Resistor Characteristics Can Vary Widely You should be familiar with the different resistor types in order to select the most appropriate type for your application the most common types and some of their characteristics are summarized in Table 3 1 A component type that s good for one application can be disastrous in another For example I often see an engineer specify 26 Table 3 1 Resistor Characteristics Can Vary Widely 27 a carbon composition resistor in a case where stability and low TC Temperature Coe
57. halve the current and double the voltage The amount of dissipa tion is the same but the I X R drop is cut in half Now continue to halve the current and double the voltage Soon you ll reach a point where the ballasting Figure 7 3 won t be sufficient and a hot spot will develop at a high power point along the emitter The inherent decrease of Vgg will cause an increase of current in one small area Unless this current is turned OFF promptly it will continue to increase unchecked This current hogging will cause local overheating and may cause the area to melt or crater this is what happens in secondary breakdown By definition you have exceeded the secondary breakdown of the device The designers of linear ICs use ballasting cellular layouts and thermal limiting techniques all of which can prevent harm in these cases Ref 3 Some discrete transistors are beginning to in clude these features Fortunately many manufacturers data sheets include permitted safe area curves at various voltages and for various effective pulse widths So it s possible to design reliable power circuits with ordinary power transistors The probability of an unreli able design or trouble increases as the power level increases as the voltage increases as the adequacy of the heat sink decreases and as the safety margins shrink For example if the bolts on a heat sink aren t tightened enough the thermal path degrades and the part can run ex
58. handy to use your wits but then what One of my favorite quotes from Jiri Dostal s book says that troubleshooting should resemble fencing more closely than it resembles wrestling When your troubleshooting efforts seem like wrestling in the mud with an implacable opponent or component then you are probably not using the right approach Do you have the right tools and are you using them correctly I ll discuss that in the next chapter Do you know how a failed com ponent will affect your circuit and do you know what the most likely failure modes are I deal with components in subsequent chapters Ah but do you know how to think about Trouble That is this chapter s main lesson Even things that can t go wrong do One of the first things you might do is make a list of all the things that could be causing the problem This idea can be good up to a point I am an aficionado of stories about steam engines and here is a story from the book Master Builders of Steam Ref 4 A class of new 3 cylinder 4 6 0 four small pilot wheels in front of the drive wheels six drive wheels no little trailing wheels steam engines had just been designed by British designer W A Stanier and they were perfect stinkers They simply would not steam So the engines designers made a list of all the things that could go wrong and a list of all the things that could not be at fault they set the second list aside The designers specified chang
59. happened to anybody you know right Experts Have No Monopoly on Good Advice Another thing you can do is ask advice only of experts After all only an expert knows how to solve a difficult problem tright Wrong Sometimes a major reason vou can t find your problem is because you are too close to it you are blinded by vour familiarity You may get excellent results by simply consulting one or two of your colleagues who are not as familiar with your design they may make a good guess at a solution to your problem Often a technician can make a wise or lucky guess as Casily as Can a savvy engineer When that happens be sure to remember who saved vour neck Some people are not just lucky they may have a real knack 4 First Things First for solving tricky problems for finding clues and for deducing what is causing the trouble Friends like these can be more valuable than gold At National Semiconductor we usually submit a newly designed circuit layout to a review by our peers I invite everybody to try to win a Beverage of Their Choice bs catching a real mistake in my circuit What we really call this is a Beercheck It s fun because if I give away a few pitchers of brew I get some of my dumb mistakes corrected amistakes that I myself might not have found until a much later more painful and more expensive stage Furthermore we all get some education And you can never predict who will find the little picky e
60. have a poor TC so that as the circuit warms up it really is likely that there will be a temperature where the ringing frequency moves up to be a mul tiple of the clock frequency The standard solution is to add 2 pF of tantalum electrolytic bypass capacitors or 20 uF of aluminum electrolytic capacitors for every three to five ICs unless you can prove that they are unnecessary That s a good rule of thumb The ESR of the elec trolytic capacitors typically 1 Q is essential to damp out the ringing Some people say that this ESR is too high to do any good in a bypass capacitor but they do not understand the problem I have read a few ads in which some capacitor manufac turers claim that their ceramic bypass capacitors are so good have such low series resistance that ringing is no longer a problem I find the claims hard to believe I invite your comments ESR Friend or Foe Specifically some capacitor manufacturers claim that the series resistance Rg is so low that you won t have a problem with ringing But low Rg would seem to exacer bate the ringing problem Conversely I ve heard that one capacitor manufacturer is proposing to market ceramic capacitors whose series Rg has a lower limit a few ohms to help damp out any ringing I m going to have to look into that But if you have bypass capacitors with a very low Rg you can lower the Q of the resonator you have inadvertently constructed around them by adding a resistor
61. heat sink out of regulation Study the dissipation oad x Vin Vou Output shifts badly R1 connected too Connect R1 to LM317 s VS load close to load Bad Output pin directly Kelvin connections use different wire than for the load Load causes oscill n Monitor with scope at various loads try Pease s Principle Items above do not fix Could be a bad part Replace the regulator the problem etc etc oa 1 ny E 7 K i Comments on Troubleshooting Table for LM317 Regulators jandi friendly to apply but when people get absent minded listed lin the table they can have problems This covers people callus about The M350 LM338 and LM396 are byt same basic adjustable scheme But beware as the LM396 is The LM317 i8 usually eas and forgeti isome ofth f iked regulators such as the popular LM340s and 5 V 12V and 15 V versions ot ought to have alittle breadboard with a if the problem follows the IC or stays with the the load regulation willl probably be Kelvin socket andthe part will probably get hot Now let move on to the Negative Regulator which is aabstantall the same as the Positive Regulator so we will put into the table only the items that are different O O Vout Vour 1 25V B41 Figure 14 4 Negative Adjustable Regulator Back to Electronic Circuits 179 Table 14 4 Troubleshooting Negative Adjustable Regulat
62. how you are going to do it Remember this text started out as a single chapter for Al Kelsch s book on switching regulators I wouldn t have gone about this in such a dumb inefficient way if I could have imagined what a big project it would be But then I might never have even started As for technical and troubleshooting ideas well after all the tips I ve given you it s only fair that you share your comments with me Bob Pease Staff Scientist National Semiconductor Corp M S C2500A P O Box 58090 Santa Clara CA 95052 8090 x About the Author For the record Bob Pease is a senior scientist in industrial linear IC design at National Semiconductor Corporation in Santa Clara California he has worked at National since 1976 He is also one of the best known analog circuit designers in the world he s been creating practical producible analog products for fun his and profit both his and his employers and writing about analog topics for over a quarter of a century As you might expect though there s a lot more to Bob Pease than his impressive credentials Following untrodden paths to discover where they lead is one of Bob s avocations He s done it on foot on skis and on a bicycle sometimes by himself and sometimes with his wife and two sons often along abandoned railroad roadbeds throughout the United States and England Aside from the peace and quiet and the thrill of the journey itself the reward
63. in either case this is the best way I have seen i For use with ATE you do not have to look with a scope you can use a step or trapezoidal wave and look just at the DC levels at the ends or the middle or wherever you need Note that you do not have to trim that resistor network all the time nor do you have to trim it perfectly All you have to know is that when the noise gain changes from a low value to a high value and the output error changes it is the How to Do It Right 95 e Use a minigator to select R100 Let R3 40x Rio R4 Ry2 20 Ry Ry A Re Rio Q7 11 TION All Rs should be 1 GENERATOR Vom Vin X Ro R Ro SINE OR TRIANGLE Verror Vom CMRR Vout iS related to Verror Figure 8 7 Here s how to evaluate CMRR with confidence and precision both AC and DC change of the output error that is of interest not really the p p value before or after but the delta You do not have to trim the resistor to get the slope perfect but that is the easy way for the guy working at his bench to see the changes This is a great circuit to fool around with When you get it running you will want to test every op amp in your area because it gives you such a neat high resolution view It gives you a good feel for what is happening rather than just hard cold dumb numbers For example if you see a 22 mV p p output signal that is caused by a 22 HV error signal you know that the CMRR really is way up near a m
64. indicating paint from Tempil South Plainfield NJ to monitor the temperature of power devices in metal cans and plastic packages such as TO 220 devices I use the paint both in development and in units shipped to the field The substance comes in various temperature grades I anoint the top of each device with a range of paints When and if a device heats up each temperature grade paint melts and fuses at a specific temperature permanently altering the substance s ap pearance I also use the paint to ascertain the relative effectiveness of various thermally conducting insulating washers These washers isolate a power device from its heat sink I measure the time for the paste to change state after applying power to a device I discovered that silicon based elastomeric washers didn t work at all with TO 220 cases The mounting screw of these cases is off center One end of the case is tight and the other is loose which causes poor heat flow J don t understand why these washers are even manufactured Didn t the makers try one out before deciding to offer them Washers aren t the only problem One sample TO 220 device got hot so quickly that I performed a post mortem on it I opened the package and pried the chip loose and observed that only about 15 of the die had been bonded to the substrate Since that experience I have steered clear of that particular manufacturer Bill Sturgeon Sturgeon Engineering Co Petrolia CA
65. known good part a Vo 5V O Voar lo f 20kHz 1A Final Floobydust 185 Comments on Table for LM3524 Switching Regulator Obviously if a circuit of this complexity gives problems you will need to be almost as wise as the person who designed it Or maybe even wiser And you will need to memorize what all the correct waveforms ought to look like so you can detect errers and deviations You will need a compact breadboard so you can evaluate the critical components in sockets There may be in any given batch of badly running parts several different modes of unhappiness If you have to troubleshoot a lot of these you will be a good troubleshooter when you are done NOTE This circuit is based on Figure 15 from the 1989 National Semiconductor LM3524 data sheet which did not work because the diodes to pins 12 and 13 were drawn backwards sorry about that I noticed this problem even before I started to build this Then I did build this It ran Anyhow now you know why the LM2575 is called a Simple Switcher because of comparison to old circuits like this Final Floobydust e I almost forgot this but when you make up these little breadboards for evaluating circuits and their components you may need some tiny connectors for diodes or transistors or small capacitors You cannot use those nylon breadboarding panels as mentioned in Chapter 13 because the capacitances and inductances will be hope lessly b
66. l on oye Users get accustomed tose rated DC output They get indignant when the noise doubles or triples due to 1 f or popcorn noise The chances of finding a noisy regulator are quite small s when some noisy ones do show up it s a shock Unfortunately no high volume manufac turer of regulators is ina position to test for those low noise levels or to guarantee that you ll never see a noisy part Picase don t expect the manufacturer to admit the parts are bad or Minreliable or worthy of being replaced If you do depend on super quiet ICs or ICs with other specially selected characteristics it s wise to keep a spare stock of selected and tested parts in a safe Then you can use them when some of the ones you just bought happen toibe alittle too moigyei e ian EOS eg Py ee we lobe What Is Worst Case Once I designed a circuit to drive a 200 Q load a rather light load at the far end of a 2000 ft RG174U cabl The specifications call d for me to test the Circuit by driving the near end ofithe coaxial cable with a low impedance square wave I called the engineer who wrote the spec and recommended that we perform the test with about a 39 Q source impedance to avoid bad ringing and reflections along the unterminated cable He told me that this impedance wasn t necessary he had already checked out the worst case conditions with 116 cable and with 2000 ft of cable I asked him if he had checked it with
67. leakage inductance is nor mally much much smaller than the mutual inductance you are measuring the leak age inductance of the primary plus the reflected secondary leakage in the second case When you work with inductors or transformers you have to think in terms of cur rent In any transformer or inductor flux is directly proportional to the current and resistive losses are directly proportional to the current squared Therefore be sure to have several current probes so you can observe what the current waveforms are doing After all some of the weirdest ugliest and most nonideal waveforms you ll see are the waveforms associated with inductors Especially in a switch mode regulator In the absence of an instrument designed to measure inductance parallel the in ductor with a known capacitance to create a parallel resonant circuit If you use a high impedance source to apply a current pulse to this circuit you can determine the inductor s value from the resonant frequency and the capacitance f 1 2 7 VLC If you look at the inductor s waveform on a scope you can compare it to the wave form you get with a known good inductor This technique is also good for spotting a shorted turn which reduces inductance nearly to zero The L meter and the similar Q meter can help you ensure that good inductors haven t been damaged by saturation Incredible as it may sound you can permanently damage an inductor by saturating it Some fer
68. like that of an op amp includes no output capacitor You can buy such a supply or you can make it with an op amp and a few transistors The advantage of the supply shown in Figure 2 5 is that you can design it to slew fast when you want it to For speed use a quick LF356 rather than a slow LM741 Also if a circuit latches and pulls its power supply down the circuit won t destroy itself by discharging a big capacitor While we are on the topic of power another useful troubleshooting tool is a set of batteries You can use a stack of one two or four 9 V batteries ni cads gel cells or whatever is suitable and convenient Batteries are useful as an alternate power supply for low noise preamplifiers If the preamp s output doesn t get quieter when the batteries are substituted for the ordinary power supply don t blame your circuit trou bles on the power supply You can also use these batteries to power low noise cir cuits such as those sealed in a metal box without contaminating their signals with any external noise sources A few RC substitution boxes You can purchase the VIZ Model WC 412A which I refer to affectionately as a Twiddle box Figure 2 6 from R amp D Electronics 1432 South Main Street Milpitas CA 95035 408 262 7144 Or inquire at VIZ 175 Commerce Drive Fort Washington PA 19034 800 523 3696 You can set the unit in the following modes R C series RC parallel RC open circuit or short circuit
69. models of Op amps that have been raising questions for over a dozen years A guy calls up and asks me What is the maximum DC voltage gain on an LM108 I reply Weil it s 40 000 min but a lot of them run 300 000 or 500 000 and some of them are as high as 3 or 4 million The customer sighs Oh that s terrible When I ask why it s terrible he explains that when the gain gets high the gain bandwidth will get so high that it will be impossible to make a stable loop if the gain bandwidth gets up to dozens or hundreds of megahertz Sigh I sit down and explain that there is no correlation between the DC gain and its spread compared to the GBW Product and its spread The guy says Oh I read in a book somewhere that there s good correlation because the first pole is constant tell him to throw out the book or at least X out those pages because the first pole is not at a constant frequency These days read that several Op amp companies are giving away free SPICE models What do I think of these models Well on a ypical hasis F have read that 206 G More on Spice some of these models are pretty good as in several typical situations they slew and settle and ring just a little as real op amps do and have as good accuracy as a real typical op amp and its feedback resistors Maybe in a few years the slow ones will be trustworthy But I don t think you can get very good results from the fast ones Why
70. on PC 56 5 Preventing Material and Assembly Problems boards which scares the heck out of me If you have to use an eyelet don t count on it to connect the top foil to the bottom These days plated through holes are quite reliable but I still like to use two plated througt holes in parallel whenever I have room or to put a component s lead through the hole It just makes me feel more com fortable See Chapter 13 for more comments on eyelets More important than these layout conveniences is that your layout should i not interfere with and if possible should enhance the expected performance of the cir cuit I try to make my layouts so the PC runs and the metal runs on an IC chip are as short and compact as p ssible especially the sensitive ones that would receive a lot of noise or leakage or have a lot of capacitance if they were longer Otherwise you A ire runs that turn into a whole hank of spaghetti som net imes we haye to locate some of the components in odd ont te ch aby thie Kor human interface which leads to ooh tha ave to sp it it s a matter of engi as sey rane nib design a must st provide a list AI Tea narrow as possible Others like Neither one is wrong but you ng large areas of foil when you are laying The co tor of a TO 92 transistor can put a lot of heat down its copper collector lead and an extra square inch of PC foil on either
71. or on both sides of the PC board can spread that heat out and help keep the transistor cool The same is true for high power ICs if you look at the data sheet of a medium power IC such as the LM384 the cutves show that 2 square inches of copper foil can help keep the IC much cooler than minimum foil and 6 square inches is even better 5 ai w5 touse wide foils ami ano Kelvin Connections Improve Measurement Accuracy 57 when the device s 6 ground pins carry the heat out of the package into the foil But some people point out that leaving too much foil on a PC board can cause warping after wave soldering Engineers often assume that a printed circuit trace has virtually no resistance and no IR drop But when you run large currents through a foil run you will be uripleas antly surprised by the IR drop you ll see The classic example is a layout where the signal ground for a preamplifier is mixed and shared with the ground return path for the power supply s bridge and filter capacitor This return path will see ampere size surges 120 times per second Needless to say that preamplifier won t have low noise until the path for the current surges is essentially divorced from the preamp s ground For precision work your PC board layout must include well thought out ground paths for your sensitive circuitry When you think about it separating power supply grounds that carry lots of cur rent from voltage sen
72. part That s very helpful unless it s a high power op amp Then just repeat after me Heat is the enemy of precision because it is After all when an IC has to handle and dissipate a lot of power it s not going to be nearly as accurate as when it is not overheating and when all the components around it get heated too You should remember too that not all op amps of any one type have the exact same output voltage swing or current drive or frequency response I get phone calls every four or five months from customers who complain We have a new batch of your op amps and they don t have as good an output swing or output current or frequency response as the previous batches When I check it out 98 of the time find that a part with extremely good performance is just a random variation The customer had got into the habit of expecting ail the parts to be better than average When they got some parts that were still much better than the guaranteed spec but worse than average or typical they found themselves in trouble It s always painful to have to tell your friends that you love them when they like you and trust you but For many years I ve been cautioning people If you have a regulator or an amplifier circuit and it oscillates do not just add resistors or capacitors until the oscillation stops If you do the oscillation may go away for a while but after it lulls you into complacency it will come back
73. plated wire strands for Teflon insulated wire With this type of wire solder tends to wick up into the strands thus making the wire quite brittle Once I under stood the wire s structure I was able to solve my problems by adding strain relief for any bends or pulling stresses As I mentioned in Chapter 2 the ordinary plastic insulated single conductor wire that is used in telephones has just the right stiffness to make good twisted pair wire for making capacitors with values of 1 2 1 or 4 95 pF The wire doesn t have a E Teflon dielectric but it s good enough for most applications Consider Your Wire Type Shielded or coaxial cables such as RG 58U RG 174 shielded twisted pairs and other special flat cables all have their place in the job of getting signals from here to there without undue attenuation or crosstalk When you have a large number of wires mindlessly bundled together and you don t have any bad crosstalk you re witnessing Consider Your Wire Type 63 a miracle Often you have to unbundle the wires and separate the offending ones or the sensitive delicate signals from the rest Also you may end up rewiring some or all of the wires into shielded cables Remember Teflon is a good insulator but air is even better If you have to add struts standoffs or spacers to make sure that the critical wires stay put go right ahead If you have problems the wire manufacturers can give you some advice Conversely just a
74. probes for debugging circuits I glue capacitors or resistors on the ends of Popsicle sticks with epoxy I keep the components leads short about 1 4 in and cut at an angle to form precise contact points Thus you isolate your finger and body capacitance from the actual value of the component The Popsicle stick has a couple of advantages over simply grasping the component with your fingers or mounting it in a length of heat shrink tubing First the Popsicle stick is more rigid than heatshrink tubing Second the added length of the Popsicle Letters to Bob 165 stick often proves handy I ve got a whole box of capacitors and resistors mounted this way which makes for a very efficient and portable trimming kit John Ardizzoni M A COM T Lowell MA Dear Mr Ardizzoni And if you look in the right magazines the ones catering to RF designers you may find advertisements for similar tuning sticks that are available for purchase in values from 0 1 pF to 1000 pF You can buy these ready made for about 50 for a set of 20 from American Technical Ceramics Corp 1 Norden Lane Huntington Station NY 11746 2102 516 271 9600 Obviously they think your idea is an excellent one they believe their customers will pay for ready made versions with a neat plastic shaft instead of a Popsicle stick But when you make your own you can make up any value you want in just a few minutes time RAP Dear Bob I use Tempilaq temperature
75. ranges from small signal coupling capacitors to large high power filters The different dielectrics are their most interesting ingredients Often a designer installs a polyester capacitor techni cally polyethylene terephthalate often called Mylar a trademark of E I DuPont de Nemours and Co and wonders why something in the circuit is drifting 2 or 3 as the circuit warms up What s drifting is probably the polyester capacitor its TC of 600 to 900 ppm C is 10 times as high as that of a metal film resistor If you give up on polyester and go to polystyrene polypropylene or Teflon also a trademark of DuPont the TC gets better about 120 ppm C Polystyrene and polypropylene have low leakage and good dielectric absorption almost as good as Teflon s which is the best Ref b But Teflon is quite expensive and rather larger in package size than the other types Be car ful with polystyrene its maximum temper ature is 85 C so you might damiage it diring ordinary wave soldering unless you take special precautions to keep the capacitors from over heating Polycarbonate polysulfone and polyphenylene haye good TCs of about 100 ppm C and their i i nares have enough syllables that they sound as if they should be pretty good but a actually they have inferior soakage Glassiand porcelain are dielectrics that sound like they ought to have some re lly fancy characteristics and excellent dielectric _ absorption But
76. really could not measure them with a re ntiprohe ng tiin th real World But if you try to rely __ Scope or buffered probe of currentipro al Wor wrt solely on these models witho t breadboard ng they will not tell you the whole story it land your crutches will collapse sopnen op l t Ty and ou can t say I didn t warn you T showed this to Bettina Briz in Amplifier Marketing and she said Bob you ean t say that I said Oh tell me where will fix it She admitted that what I had aid probably was quite true Then I said Well why try to soft pedal the truth and pretend that you can trust computers iall the time Wouldn t that be a disservice to our customers And Bettina replied When we have models we ll educate our use vell point out when you can trust ithe models and when you shouldn t Softer that are we in disagreement Well maybe we did agree after albi y a eS E At present we have a small library of p amp 1 10dels released with Analogy Beaverton Oregon 97075 They are only level I models low precision and while we have made some progress on good predision ones level II they are not released yet These are behavioral models rather than SPICE models and we think they I have said anything that is untrue and I rise have several advantages over SPICE models There are some min typ max specifica t
77. required When you bang a device s output and that output rings with a high Q you know your fix doesn t have much margin When the output just goes flump and doesn t even ring or overshoot appreciably you know your damping is effective and has a large safety margin Good Now take a hair dryer and get the circuit good and warm Make sure that the damping is still pretty well behaved and that the output doesn t begin to ring or sing when you heat the capacitor or power transistor or control IC or anything don t mean to imply that you shouldn t do a full analysis of AC loop stability But the approach have outlined here can give you pretty good confidence in about five minutes that your circuit will or won t pass a full set of exhaustive tests Mw 0 1 pF TU e OR LARGER Loan 100 8 Operational Amplifiers The Supreme Activators they really shouldn t trust your parts to always be better than average Maybe in Lake Wobegon all the kids are better than average but you can t go shopping for op amps and complain when they are not all better than average Oscillations Do Occasionally Accompany Op Amps a One of the most troublesome problems you can have with op amps ts oscillation Just as you can build an oscillator out of any gain block then you must admit that any gain block can also oscillate when you don t want it to Op amps are no exception Fortunately most op amps t
78. requires rather high temperatures such that you need a small torch and some messy borax flux and I suspect that the high temperatures will do a lot more damage to the insulation and to the copper wire by oxidizing it excessively than any advantage you might get from a superior soldered joint Make Good Connecti gt ns Printed circuit boards aren t the only assembled component you Il have to contend with while trying to make circuits work In Tracy Kidder s Pulitzer Prize winning hook Phe Soul of a New Machine Ref 2 one of the crucial moments occurs when ihe cngineers explain to a management team that their new computer has a flaw that 60 5 Preventing Material and Assembly Problems occurs only rarely but is driving them to distraction One of the managers stands thoughtfully for a while and then reaches over and warps the main board Scrunch scrunch To the horror and shock of the project engineers the scrunching correlates with the terrible intermittent problem When the main board s DIP sockets were replaced the problem disappeared gt Like a faithful dog a socket or connector is expected to do its job without ques tion and it usually does However on the rare ccasion when one does go bad the connector usually becomes intermittent before t fails utterly Fortunately many engineers and technicians learn early on that the way to check an intermittent problem is to make it reveal itself when the board s connect
79. simple tips aren tmeant to overshadow the truly difficult areas of regulator design Youmight wonder if it s possible for a smart experienced engineer to design a switch mod regulator that works welliafter only minor redesigns andigoes into production without a major yield loss My answer is Just barely The weasel word here is smart Ifithe engineer forgets some little detail and doesn t have a contin gency plan to test forit screen it or repair the regulators that don t work then maybe he or she isn t very smart Those iof us who don t design switchers all the time would have avery poor batting average at getting a design to work righvoff the drawing board even if we re really good at designing other circuits After all a switcher is a complicated system composed of power transistors transformers in ductors one or more control ICs and lots of other passive components And the circuit s ayo yout must guard small signals against electrostatic interferenc di ven more importantly must contfoliand eject the ic meani for a switcher to be efficient the volts per micro second and amperes per microsecond get really large so it doesn t take many pico farads or nanohenries to couple a big noise into the rest of the circuit The paths for high currents are important and the paths for cooling air are even more critical So when someone asks me how to design a switcher Lask How ma
80. so that part of the resistive network shorts out and the Vog is improved In its LM108 National Semiconductor first used zener zapping although Precision Monolithics Santa Clara CA wrote about zener zapping first and used it extensively later on Although zener zapping is a useful technique you have to be sure that no body discharges a large electrostatic charge into any of the pins that are connected to the zener zaps If you like to zap zeners for fun and profit you probably know that they really do make a cute lightning flash in the dark when you zap them Otherwise be careful not to hit zeners hard if you don t want them to zap and short out These zener zaps are also becoming popular in digital ICs under the name of ver tical fuses or more correctly anti fuses If an IC designer uses platinum silicide instead of aluminum metallization for internal connections the diode resists zapping Diodes That Glow in the Dark Efficiently Figure 6 4 Once I needed 100 LEDS so I bought 200 LEDs from the cheapest supplier I hoped to find some good ones and maybe just a few units that were weak or performed poorly which I could use for worst case testing I lost out every one of the 200 was of uniformly good intensity In a variation on Murphy s Law worst case parts will typically appear only when you are depending on having uniform ones So long as you don t fry LEDs with your soldering iron or grossly excessive mil
81. solder connec tors wire and cable Also covered is PC board layout a poor layout can cause more than occasional problems it can determine completely how well your circuit works Some of the topics discussed in this troubleshooting series so far may have seemed obvious But far too often it is this obvious information that engineers overlook and it is this information that can make troubleshooting so much easier So be careful not to overlook the obvious Don t assume that your PC board materials or layout don t matter or that wire characteristics don t differ you ll find that PC boards connectors wire and cable can cause problems when you least expect it First of all the use of the term printed circuit board is a misnomer these days almost every board is an etched circuit board But I continue to use the abbrevia tion PC board because it s a familiar term There are six basic troubles you can get into with PC boards The board is made of the wrong material The quality of the vendor s board is so bad that there are opens or shorts in the board or worse yet intermittent connections in the plated through holes The foil starts to peel off the board because of mistreatment You were so concerned with cost that you neglected to specify a layer of solder mask you ended up with a board full of solder shorts The surface of the board is leaky or contaminated Your circuit layout is suc
82. take them by the hand and show them where the trouble was It s like a dumb cartoon I once saw showing three men walking out of a movie an old man a young man and a middle aged man The posters said the movie was fun for young and old And sure enough the old man and the young man were smiling but the middle aged man was frowning Even a dumb cartoon can be instructive if it reminds you that bad news is not only where you first expect it It may be lurking in other places too This story reminds me of a boss who asked me if my new regulator design was really short circuit proof I told him yes I had tested it with short bursts and long pulses and everything in between for days and weeks With a wry smile he went over to a tool cabinet and removed a really big heavy file He applied this file with rough uneven scraping motions to ground and to the output of my regulator He got showers of sparks outiofithe regulator but he couldn t kill it What a bastard of a test Then he explained to me that the random repetitive action of a file sets up pat terns of current loads and thermal stresses that can kill a regulator if its short circuit protection is marginal There are many many tricks you can use to show that a de sign really canisurvive every worst case condition Every industry has its own tests and mostiof them have nothing todo with computers Switch Mode Regulators A A Whole New Ball Game These
83. tell you about this To tell the truth didn t even really recognize this When I wrote the first version of this as published in EDN I assumed that the slopes started out from 60 mV per decade and then got worse shifted over to 120 mV per decade at higher current levels and I said so But I was wrong And nobody ever contradicted me Such a strange world Please refer to Figure 6 1 which shows just a few of the different curves you may get when you buy a diode None of these slopes are characterized or guaranteed if you change vendors all bets are off So qualify each vendor of diodes carefully for each application Note For a detailed explanation of this graph and a table detailing exactly which diode is which see Appendix E As the current level continues to increase the conductance per milliampere gets even worse due to series resistance and high level injection and other nonlinear fac tors Therefore at a large forward current a diode s forward voltage Ve will be considerably larger than predicted by simple theory and larger than desired Of course some rectifiers depending on their ratings can handle large currents from amperes to kiloamperes but the Vps of all diodes no matter what their ratings err from the theoretical at high current levels These days Schottky diodes have smaller Vrs than ordinary pn diodes However even germanium diodes and rectifiers still have their following because their low Vps are si
84. the answer quickly rather than just charging off in random direc tions Intermittents are the toughest most frustrating kind of troubleshooting prob lem And bench instrumer ts augment an engineer s senses and open the window of perception to the circuits he or she is troubleshooting Floobydust is an old expres sion around our lab that means potpourri catch all or miscellaneous In this chapter I ll throw into the floobydust category a collection of philosophical items such as advice about planning your r troubleshooting and practical hints about computers and instruments Troubleshooting Intermittent Problems The car that refuses to malfunction when you take itto the shop the circuit that re fuses to fail when you re looking i does it really fail only at 2 am these are the problems that often most extreme efforts to solve The following t intermittent problems te inig l 1 Look for correlation of e problem with something Does it correlate with the time of day The line voltage The phase of the moon Don t laugh 2 Get extra observers to help see what else may correlate with the problem This extra help includes both more people t to help you observe and more equipment to monitor more channels of information 3 Try to make something happen Applying heat or cold may give you a clue Adding some vibration or mechanit shock could cause a rain o
85. the meter gave an indication I thought I had designed an altimeter After some study I localized the problem to an FD300 diode whose body is a clear glass DO 35 package covered with black paint This particular diode s paint had been scratched a little bit so when I picked up the test box the light shone under the fixture and onto the diode Most of these diodes didn t exhibit this behavior the paint wasn t scratched on most of them To minimize problems such as the ones I have listed I recommend the following strategies Have each manufacturer s components specifically qualified for critical applications This is usually a full time job for a components engineer with help and advice from the design engineer and consultation with manufacturing engineers Establish a good relationship with each manufacturer Require that manufacturers notify you when or preferably before they make changes in their products l Keep an alternate source qualified and running in production whenever possible My boss may gripe if I say this too loudly but it is well known that having two good sources is better than having one The argument that One source is better than 7 Zener Zener Zener two falls hollow on my ears Two may be better than seven or eight but one is not better than two oo 1 EST oOo Zener Zener Zener gia wn if you apply too much reverse voltage but ik down in a predictable and we
86. the supply s regulation Even with a 15 V supply where the wasted power might be acceptable the bounce and poor regulation of the supply might hurt the accuracy of the circuits that the supply powers The diode s impedance may cause poor regulation So in cases where a regulator drives the power busses the shunt antireversal diodes are a good idea The solution presumes of course that the regulators are short circuit proof The worst part of your circuit Mr Smith comes to light when one of the power supply wires falls off or becomes disconnected Then the 14 V bus could get pulled to 5 or 10 V depending on what loads are between the 14 and 14 V busses Many linear circuits can get very unhappy if fate pulls their negative supply pin 6V or 9V a Figure 13 5 164 13 Letters to Bob gt J B 6V or 9V b This simple circuit avoids the disadvantage of the series and the parallel diode schemes The value of R is not criticaH MQ may protect against transients If the supply voltage is higher than I5 V you may need to clamp or attenuate the Vaate to avoid over stressing the gate There are many kinds of enhancement type MOSFETs that are suitable such as IRF51 I or similar above ground Even if the ICs didn t complain the electrolytic power supply bypass capacitors would be reverse biassed a potentially destructive situation So Mr Smith would you agree that adding a couple a
87. the tester No He just kept his computer cranking along because he knew the computer anal ysis was the most important thing We did finally get the tester fixed and we got the product out a little late but obvi ously I was not a fan of that test engineer nor his statistics as long as he was at our company And that is just one of a number of examples I trot out when anybody tries to use statistics when they are inappropriate I do like to use scatter plots in two dimensions to help me look for trends and to look for sports that run against the trend I don t look at a lot of data on good parts or good runs but I study the hell out of bad parts and bad runs And when I work with other test engineers who have computer programs that facilitate these plots I support and encourage those guys to use those programs and to look at their data and to think about those data Anything that facilitates thinking that I support Keep It Cool Fool A couple years ago I was approached by an engineer who was trying to use one of our good voltage references that had a typical characteristic of about 20 ppm per 1000 hours long term stability at 125 C He was using it around room temperature and he was furious because he expected it to drift about 0 1 ppm per 1000 hours at room temp and it was a lot worse than that Why was our reference no good he asked I pointed out that amplifiers drifts and references drifts do not keep i
88. they don t not very good at all Many years ago wound film capaci tors were made with oil impregnated paper but you won t see them unless you are gt Working on ancient radios Theywere Pretty crummy just adequate for audio coupling epia F iled Again ah eag Now let s discuss the differen between a polyester foil capacitor and a metallized capacitoriis ade of alternating layers of film and foil _ where both the delicate film and the metal foib are just a couple of tenths of a milli good capacitor at a nominal price and in a inch thick This construction makes a go o nominal size The metallize fi Capacitor ig made with only a very thin film of lt s poly ster with the metal depo i e on the polyester in a very thin layer This con _ struction leads to an ev maller s wA given capacitance and voltage rating but fe ae gy Pe ey i Ee RUT yt oe the deposited metal is so thin tha hit current carrying capacity is much less than that of the metal in the foil capacitor THis offe r advantages and disadvantages If a pin hole short develops in this metallized polye ter capacitor s plastic film the metal layer in the vicinity of the pinhole will briefly carry such a high current density that it will vaporize like a fuse and clear the short For many years metallized polyester capacitors were popular in vacuum tube television sets because they were small and cheap Th
89. tracking are better than those of individual resistors You can also buy custom thin film resistor networks on a single substrate if your requirements are critical and your bankroll large A more economical route is to use four seven or eight matched precision thin film resistors in a DIP I have found the TC tracking of these devices from several manufacturers to be better than 1 ppm C These sets are ideal for precision amplifier stages and D A converters For a good example of where they are useful refer to the matched resistors in the thermocouple amplifier in Figure 2 10 Also when you buy film resistors leave them on the tape When you need matched sets you can pull off adjacent resistors and be reasonably confident that they will match and track well Thick film resistors are usually found in hybrid circuits but are also available as small networks They are made of Cermet or other proprietary materials and are baked and fired after being screened onto a ceramic substrate Their TC is not quite as good as thin film resistors but they are popular because they have good TC tracking and they are inexpensive and easy to trim to 1 or 1 2 Traditionally the best most stable resistors have been wirewound resistors Figure 3 1c These days precision film resistors can match wirewound resistors quite well for almost any set of specifications However for a resistor whose value is between 200 kQ and 1 MQ wirewound resistors are more ex
90. unfair Ordinary aluminum electrolytic capacitors are most often used for power supply filtering and bypassing In the old vacuum tube days electrolytic capacitors were often used at levels of 150 V 300 V 500 V or more There are several basic prob lems with these old circuits First if the voltage across a Capacitor is much higher than 350V the capacitor s reliability is not nearly as good as that of units operated below 350 V Also if a piece of old equipment has not been powered up for years it is advisable to apply the AC power gradually by cranking up the line voltage slowly with a variable transformer so that the electrolytic film has a chance to form up If you hit it with full voltage instantly an old capacitor may fail Of course if you are hit by high voltage you may fail too At this point I should remind you that when working on high voltage circuits probe with one hand only and keep the other hand in your pocket Avoid grounding your body at any other place and stand or sit on an insulating slab of dry material These precautions can prevent a shock from Causing you serious harm When I start work on a high voltage circuit I solder a neon lamp in series with a 100 KQ resistor across the high voltage power supply as a glowing reminder that this circuit is pow ered by a voltage much higher than 15 V I mean I stick my fingers into low voltage circuits all the time but when I see the glow of a neon lamp I stop FAST
91. up as you go along For example the list above of resistors 80 112 120 104 88 or 72 are nominally binary choices You don t have to go to ex actly those places an approximate binary search would be just fine i iry a Law work for me For cca according to this arisini of Muiphy s Law if Idrive around with a fire extinguisher if I am prepared to put out any fire will that make sure that I never have a fire in my car When you first hear it the idea sounds dumb But if I m the kind of meticulous person who carries a fire extinguisher I may also be neat and refuse to do the dumb things that permit fires to start Similarly when designing a circuit I leave extra safety margins in areas where I cannot surely predict how the circuit will perform When I design a breadboard I often tell the technician Leave 20 extra space for this section because I m not sure that it will work without modifications And please leave extra space around this resistor and this capacitor because I might have to change those values When I design an IC I leave little pads of metal at strategic points on the chip s surface so that I can probe the critical nodes as easily as possible To facilitate probing when Consider Appointing a Czar for a Problem Area 7 5V R128 1K MID POINT R1 1K NOTE ALL RESISTORS 1K Figure 1 2 If you discovered that the midpoint was not at 2 5 V but at 0 V how would you troubleshoot this ci
92. use to try to test for CMRR that do not give valid results Just how then can we test for CMRR and get the right results Figure 8 7 is a darned fine circuit even if I did invent it myself about 22 years ago It has limitations but it s the best circuit I ve seen Let s choose R Ry 1k R3 Rj2 10k and R 200 k and R a 500 Q pot single turn carbon or similar In this case the noise gain is defined as 1 R Rin or about 11 See pages 100 101 for discussion of noise gain Let s put a 11 V sine wave into the signal input so the CM voltage is about 10 V The output error signal will be about 11 times the error voltage plus some function of the mismatch of all those resistors Okay first connect the output to a scope in cross plot X Y mode and trim that pot until the output error i is very small auntil the slope is nominally flat We don t know if the CMRR error is balanced out by the resistor error or what but as it turns Out we don t care Just observe that the output error as viewed on a cross plot scope is quite small Now connect in R100a a nice low value such as 200 Q If you sit down and compute it the noise gain rises from 11 to 111 Namely the noise gain was 1 R R and it then increases to 1 R R plus R Rj2 Rjoo In this example that is an increase of 100 So you are now looking at a change of Vout equal to 100 times the input error voltage and that is Voy divided by CMRR
93. value corresponding to a new value of Not Much Agreement on Acquisition Time Definition 133 Vin in the SAMPLE mode However the outputs of many S H circuits can settle to a new DC value faster than the hold capacitor charges to the correct value If the S H doesn t have to go back to HOLD the data may give false results even if the output seemed to give the correct answer when it was still in SAMPLE mode To avoid con fusion we define acquisition time as the pulse width required for precise sample and hold action If the circuit SAMPLEs and settles and then goes into HOLD and gives you the wrong answer the SAMPLE pulse should have been wider right Right There may be some S H circuits whose output voltage won t change if you switch them to the HOLD mode as soon as their output reaches a value that corresponds to a new Vin But if I had an analog switch that couldn t hold at all I could still get it to acquire a signal according to the data sheet definition just cited I consider the test implied by that definition to be too easy I believe some users and manufacturers in this field agree with my definition but the situation isn t really clear I would appre ciate reader comments Youifolks are getting all sorts of good ideas from me and if you have some good comments it s only fair that you bounce them off me E Pluribus Unum The Multiplexer Another type of circuit that depends on analog switches is the analog m
94. w ont oathode and overshoot S e slow turn on poor series resistance 65 slow turn off 67 181 183 slow turn on 68 67 69 181 soft low conduictance 65 66 thermostatic problems 70 Zenering and breakdown 71 See also under Zener diodes dish washer machine 54 170 disk drives 1 divider resistive 5 6 7 30 DM74L86 124 187 DM74LS86 187 DM7486 187 DMOS FETs See under transistors MOSFET documentation 2 8 Dostal Jiri 2 3 12 70 196 See also controlled avalanche rectifiers 7 re on diodes made from transistors See a is iiugh conductance 6 65 66 196 197 be light emitting S g 211 drafting 156 158 DVMs See digital voltmeters Early voltage 79 Edmund Scientific 156 EDN magazine 25 57 64 65 85 90 126 129 134 155 1 167 187 208 Electronic Design magazine xi electrostatic discharge ESD 10 11 23 78 81 82 88 _ ESD and CMOS ICs 23 _ ESD simulator 82 _ Engineering Change Order ECO 8 _ESD See electrostatic discharge Fetched circuit boards See printed circuit oar boards Everest Mt See Mt Everest 7 expert systems 11 experts 3 122 139 144 205 failure modes See the parca item of Aay t ni l tees 7 4044 89 97 BA 1355137 139 150 156 158 162 207 fire extinguisher 6 23 32 Fischer Thomas 163 Floobydust 143 153 164 185 flump 99 ng flux magnet
95. when there is a large voltage drop across a resistor the effect is most common in resistors with large values and small values ones built with high densities Therefore when you drive the reference input to a D A converter you should be aware that the R will only shift 1 3 over the entire temperature range However there may still be a broad tolerance as it is not easy to keep tight tolerances on the sheet rho or resistivity during the IC s production For example a typical D A converter s Rip specification is 15 kQ 33 These film resistors have even better tracking TC than diffused resistors often better than 1 ppm C In addition to the TC you might also be concerned with the shunt capacitance of a resistor Recently back in Chapter 2 I was trying to build a high impedance probe with low shunt capacitance I wanted to put a number of 2 5 MQ resistors in series to make 10 MQ I measured the shunt capacitance of several resistors with our lab s impedance bridge A single Allen Bradley carbon composition resistor had a 0 3 pF capacitance so the effective capacitance of four in series would be down near 0 08 pF not bad Figure 3 3 Then I measured a Beyschlag carbon film resistor Its capacitance was slightly lower 0 26 pF The capacitance of a Dale RN60D was 0 08 pF the capacitance of four in series would be almost unmeasurable It would be an improper generalization to state that certain resistor types films alwa
96. with a TC of 3300 ppm C This type of temperature compensating resistor is often used for correcting the TC of transistor logging cir cuits but it wasn t labelled in an obvious way When we put this resistor in a circuit where a low TC resistor was required it took us a couple of hours of troubleshooting to pinpoint the problem Match the TC to the Application Diffused resistors commonly used in ICs have some strange characteristics Their TC is high around 1600 ppm C and includes a nonlinear or quadratic term Thus the resistance goes up faster at high temperatures than it falls at cold tempera tures These resistors would be useless except for one minor detail They track at the rate of approximately ppm C Since it is very inexpensive to make matched pairs or sets of these resistors in a monolithic integrated circuit their use is popular among IC designers If you re not designing an IC though you probably won t meet up with diffused resistors very often Many ICs such as D A converters and voltage references are made with thin film sichrome or nichrome resistors on the chip Compared with most other resistor types these resistors have the somewhat lower TCs of 50 to 350 ppm C closer ratios better long term stability better TC tracking and less nonlinearity of the 30 3 Getting Down to the Component Level voltage coefficient This last term refers to the nonlinearities in Ohm s Law that occur
97. worry you can drive the primary with a few volts of signal from a function generator preferably in series with a resistor and or a capacitor to prevent saturation and overload and still see what the various windings are doing Two general problems can afflict power transformers The first occurs when you have large filter capacitors and a big high efficiency power transformer When you turn the line power switch on the in rush current occasionally blows the fuse You might install a larger value of fuse but then you must check to make sure that the fuse is not too high to offer protection As an alternative you could specify the trans former to have a little more impedance in the secondary Use smaller wire for the windings or put a small resistor in series with the secondary Another approach often used in TV sets is to install a small negative TC ther mistor in the line power s path The thermistor starts out with a nominal impedance so the surge currents are finite But then the thermistor quickly heats up and its resis tance drops to a negligible value Thus the efficiency of the circuit is quite good after a brief interval If the circuit is a switch mode power supply the control IC should start up in a soft start mode In this mode the IC makes sure the switcher won t draw any extreme currents in an attempt to charge up the output capacitors too quickly However you must use caution when you apply thermistors for in rush
98. you ll almost certainly need some hysteresis and if you are designing a sampled data system you should investigate the techniques of strobing or latching the comparator Using these techniques can insure that there is no direct path from the output to the inputs that lasts for more than just a few nanoseconds Therefore oscillation may be evitable Granted heavy supply bypassing and a properly guarded PC board layout with walls to shield the output from the input may help But you ll probably still need some hysteresis For some specialized applications you can gain advantages by adding AC coupled hysteresis in addition to or instead of the normal DC coupled hysteresis See Figure 9 3 For example in a zero crossing detector if you select the feedback ca pacitor properly you can get zero effective hysteresis at the zero crossover point while retaining some hysteresis at other points on the waveform The trick is to let the capacitor s voltage decay to zero during one half cycle of the waveform But make sure that your comparator with AC coupled hysteresis doesn t oscillate in an unacceptable way if the incoming signal stops Comparators Do Have Noise Figure 9 3 Most data sheets don t talk about the noise of comparators with the exception of the new NSC LM612 and LM615 data sheets but comparators do have noise Depending on which unit you use you may find that each comparator has an indi vidual noise band Wh
99. 1 84 85 2N3904 66 69 73 77 84 109 196 2N4275 78 2N5039 84 data sheet curves 82 photo transistors 74 power transistors 72 81 86 138 140 159 161 167 168 used as a diode 66 67 69 70 167 196 bipolar transistor problen alleged unreliability per Mi HDBK 217 77 base emitter breakdown zenering 77 79 159 161 high current beta not degraded 77 78 ballasting emitter ballasting 82 83 cellular layout 82 current hogging 82 87 88 beta 77 79 105 degradation 77 78 matching 79 too high 78 79 biassing 77 79 bond wires 9 78 capacitances 81 145 collector connections 50 80 85 86 Darlington 58 79 delicate discrete transistors 78 Early voltage 79 ESD damage 78 81 gender confusion 77 176 hrb 79 input overdrive 77 78 104 115 159 161 installed with incorrect orientation 77 186 light sensitive 74 156 melting 77 82 87 negative tempco of VBE 78 oscillations 84 87 108 109 118 output overload 77 105 secondary breakdown 81 82 87 sharing ballasting resistors See ballasting SOA safe operating area 81 82 84 85 FBSOA forward biassed safe operating area 88 thermal runaway 82 thermal limiter circuits 82 105 106 117 turn off circuits 82 159 161 transconductance gm 71 79 192 function of temperature 79 function of collector current 79 VBE 78 tempco of VBE 78 matching of VBE 78 86 voltage gain 71 78 predictions formula 79 transistors JFETs
100. 250 ft of cable Why no he said So I suggested he try that Shortly thereafter he called me back and agreed that the reflections with 250 ft of cable were intolerable without at least some nominal value of resistance at the source He had incorrectly assumed that the worst case occurred with the longest cable It s true that the attenuation was worst with the long length of lossy RG174U cable But it was this attenuation that caused the ringing and reflections to appear damped out With the shorter 250 ft cable a worst case condition existed at a place he hadn t expected to find it So be cautious about where you look for worst case conditions An op amp may exhibit its worst performance at an output voltage other than its maximum negative or positive swing or even other than zero volts or zero output current A regulator s worst case operating conditions may not be at its full rated load current When a regulator s power source is resistive the power dissipation may be higher at three quarters of its rated current than at full current 138 1 Dealing with References and Regulators Once I worked on a regulator that ran okay at 55 C at room temperature and at 125 C but not at some intermediate temperatures That was a nasty one Because some engineers had tested the regulators at hot and cold temperatures and saw no trouble at these extremes I had to work very hard to convince them not to ship these parts I had to
101. 300 000 because unless your signal frequency is lower than 0 1 Hz you cannot take advantage of the high gain Obviously I don t agree with that if you have a step signal the output settles to the precise correct value in less than a millisecond nor second or more The amplifier with gt higher gain just settles to a more precise value it does not take any more time I guess he just doesn t understand how op amps work Especially since he doesn t even want to talk about gain nonlinearity Many amplifiers such as the old OP 07 had low DC gain and poor gain linearity 89 A 8 Gporauiond Ampiifiers he Supreme Activators whereas more modern amplifiers like the NSC OP 07 and the LM607 gain 6 000 000 min have much less nonlinearity than older amplifiers Similarly an op amp may have an offset voltage temperature coetticient specifi cation of 1 uV C but the op amp s drift may actually be 0 33 uvV C at some tem peratures and 1 2 4V C at others Twenty or thirty years ago battles and wars were fought over this kind of specsmanship but these days most engineers agree that you don t need to sweat the small stuff Most applications don t require an offset drift less than 0 98 uV for each and every degree most cases are quite happy when a wV C l op amp drifts less than 49 uV over 50 degrees i Also you gt n t often need to worry about bias current and its temperature coeffi cient or the gain
102. 41 43 45 120 121 127 130 135 160 161 179 192 twisted pair gimmick 20 62 adjustable 19 47 capacitor problems AQL 46 bus inductance See under capacitors for p s bypassing clearing Tick noise due to breakdown 43 Dielectric Absorption See soakage Dissipation Factor 45 drying out of electrolyte 41 fingernail 48 forming up 40 inductance 43 44 labelling or marking 46 large shift vs temperature 42 44 leakage 41 53 120 131 177 long tails 47 piezoelectricity microphonics 45 111 pinholes 42 resonant frequency See induc tance reverse polarity 41 43 55 161 161 164 self heating 41 series resistance too high 41 44 series resistance too low 44 45 series resistance just right 45 soakage Dielectric Absorption 40 45 47 49 131 134 solder connections 46 tolerance 46 46 89 capacitors for power supply bypassing 40 44 45 56 99 113 121 127 135 137 153 155 158 160 164 174 176 191 193 Car Talk 10 CD4001 124 CD4016 CD4066 81 121 123 CD4020 CD4040 120 CD4051 CD4053 133 Chlorofluorocarbons CFCs 23 choreography 126 127 circuit breakers 34 40 AC 34 DC 34 Heinemann Electric 34 MOSFET designs 34 clip leads 23 96 155 clues 3 5 14 47 48 106 108 168 CMOS digital ICs See digital ICs CMOS co workers 4 127 139 Coffman Steve 168 cold soldered joints See solder prob lems with collector connections See under tr
103. 43 A 150W IC Op Amp Simplifies Design of Power Circuits R J Widlar and M Yamatake AN 446 National Semiconductor Corp Santa Clara CA Applications Engineering Staff PowerTech Inc Speed up inductor increases switching speed of high current power transistors Power Electronics May 1989 p 78 Passafiume Samuel J and William J Nicholas Determining a MOSFET s real FBSOA Powertechnics Magazine June 1989 p 48 8 Operational Amplifiers The Supreme Activators External components often determine an op amp s performance and that s why we spent the previous seven chapters discussing them But op amps aren t always abso lutely trouble free Oscillations and noise are two possible areas of difficulty among others After many pages of fiddling around with many different components we finally arrive at the operational amplifier itself And the good news is that half of our op amp troubleshooting problems are already solved Why Because it s the components around op amps that cause many of their problems After all the op amp is popular because external components define its gain and transfer characteristics So if an amplifier s gain is wrong you quickly learn that you should check the resistor tolerances not the op amp If you have an AC amplifier or filter or integrator whose response is wrong you check the capacitors not the op amp If you see an oscillation you check to see i
104. 65 116 LPC660 LPC662 81 190 macromodels 98 205 207 See also computer modelling Magliozzi Tom amp Ray 10 magnifying glass 24 51 manufacturing 1 3 8 9 70 marginal circuits 73 86 87 93 110 118 138 143 168 McCammon Roy 155 159 McKenna s Law 60 a MDACs See under digita to analog converters mechanics aircraft 12 metastability See digital ICs meters analog See analog meters meters digital See digital voll j gt meteis meters grid dip See grid dip me witli ls mistakes repeats of 7 8 26 136 144 MJ3771 85 MM74C74 124 MM74 86 123 187 MM74CHO00 81 Mt Everest xi 54 mu metal shielding 38 multiplexers MUX 132 133 tolerance of loss and power 133 Murphy s Law 2 6 7 12 72 108 National Public Radio 10 National Semiconductor Corp vii 1 7 72 88 91 102 103 107 114 115 142 167 193 Neale Reginald W 166 167 network analyzer 109 a New England Journal of M dicine 1 be No Trouble Found PONTE 9 Noise Gain See under operational amplifiers noise 15 17 18 22 23 25 39 43 47 48 53 56 57 63 71 79 89 94 103 105 107 109 113 126 130 135 137 138 142 145 148 150 155 156 158 159 162 191 194 198 207 1 F noise 104 broad band noise 104 112 113 current noise 105 line frequency 110 111 noise testing 103 popcorn noise 104 voltage noise 79 81 high frequency
105. 90 LM129 LM3239 18 71 135 LM131 LM33 21 21 131 148 149 206 LM133 LM333 105 LM136 LM336 71 LM137 LM337 105 106 178 LM138 LM338 105 137 178 LM139 LM339 111 115 116 LM140 LM340 178 LM149 LM349 102 190 LM150 LM350 37 105 137 178 LM158 LM358 96 115 LM168 LM368 105 LM169 LM369 71 105 135 LM185 LM385 71 LM192 LM392 116 LM193 LM393 115 LM194 LM394 24 78 79 196 LM196 LM396 80 105 178 LM199 LM399 71 135 LM555 120 LM612 113 115 LM615 113 LM709 104 116 LM723 179 181 LM741 18 18 77 90 97 188 LM2575 139 183 183 185 LM2576 139 LM2577 139 LM2578 139 LM2579 139 LM3045 79 LM3046 104 196 LM3086 79 LM3524 139 140 184 185 LM6121 LM6321 117 LM6161 LM6361 96 100 189 LM6325 117 LM6361 See LM6161 LM7800 178 LM7900 179 LMC555 120 LMC660 53 81 96 190 LMC662 81 96 159 190 lock washers 63 170 173 locomotives 3 7 logarithmic amplifier See current meter logarithmic logarithmic current detector See under current meter logarithmic logic analyzer 25 123 140 Index 213 built into PC board 103 confirm on actual circuit 103 formulae for 102 103 non standard pins for offset voltage trim pot 188 190 nonlinearity 29 34 67 89 90 122 loop analog 43 55 92 93 99 100 107 111 116 117 139 141 184 205 loop digital 55 141 loop ground See ground loop Loop John D 170 171 LP3
106. 986 p 1213 Pease R A V F converter ICs handle frequency to voltage needs EDN March 20 1979 p 109 Also available as Appendix C in National Semiconductor Corp Linear Applications Handbook Santa Clara CA 1986 p 1207 Pease R A Understand capacitor soakage to optimize analog systems EDN October 13 1982 p 125 National Semiconductor Corp Linear Databook 2 Santa Clara CA 1986 p 5 5 II Dealing with References and Regulators As the self appointed Czar of Band Gaps I am impelled to continue this book with words of wisdom on voltage references regulators and start up circuits I also pro vide warnings against assumptions about worst case conditions Voltage references and regulators have internal features that make them relatively immune to problems But as with other designs if you ignore the details you ll be headed for Trouble Some desigri i incorporating these parts such as switching power supplies are not for the novice a Many voltage references are based on n band gap circuitry but some of the best references are based i on buried zener diodes If your power supply s output is in the 8 12 V range or higher zener diode references such as LM329 LM399 or LM369 can provide high stability low noise and alow temperature coefficient If your power supply is a lower voltage in the range from 8 down to 1 1 V you can find band gap references that put out a st
107. Add various capacitors across C2 10 to 500 uF Try various caps at C3 0 1 or 100 uF or both Try C2 AND C3 and also increase C1 Try a different type of transistor Check input with scope Check pin 6 with scope add 0 1 or 1 uF mylar to pin 5 Try bigger heat sink Check bolts Check for oscillations Compute power IL x Vinoun etc ete etc etc Back to Electronic Circuits I8 Comments on Troubleshooting Table for LM723 Regulators As you can see there are potentially a lot of things to worry about The LM723 is rarely used these days unless a specific feature requires i t I don t want to scare you but anybody who troubleshoots a number of old style regulators has to understand the circuit thoroughly so he can tell immediately when he looks at a couple voltages whether he is on the right track He has to have the concepts behind the chart built right into his head or it would take forever to troubleshoot a basketful of bad mod ules Okay he or she Figure 14 6 Full Wave Rectifier Table 14 6 Troubleshooting Full wave Rectifiers Indication of Trouble Possible Cause Solution Input amplifier runs badly Anything Output amplifier runs badly Anything Bad diodes Bad AC response Diodes too slow Amplifier too slow Bad DC errors hot Diodes too teaky Apply Vin 0 1 to 10 VDC and troubleshoot it per the procedure above for inverting op amp
108. Amplifiers will oscillate that is The knack of spotting and quashing spurious oscillations is for some fortunate people a well developed art But others have not learned this art well I obviously cannot tell you how to solve every kind of oscillation problem But I will give you some general principles and then notes on what can go wrong with various components including comparators and buffers This information along with a few suggested procedures and recommended instruments will get you off to a good start Here are some of the types of oscillations that can pop up unexpectedly Oscillations at very high frequencies hundreds of megahertz because of a single oscillating transistor Oscillations at dozens of megahertz arising from stray feedback around a comparator Oscillations at hundreds of kilohertz because of an improperly damped op amp loop an unhappy linear voltage regulator IC or inadequately bypassed power supplies Moderate frequency oscillations of a switching regulator loop because of improper loop damping Oscillations at 60 Hz or at 120 Hz or similar line related frequencies Low frequency oscillations coming from physical delays in electromechanical or thermal servo loops As these general descriptions indicate the frequency of an oscillation is a good clue to its source An electric motor loop can t oscillate at 10 MHz and a single transistor can t normally rattle at 100 Hz
109. Bipolar Transistor Pair Sets New Standards for Drift and Noise Application Note AN 222 Linear Applications Databook p 517 National Semiconductor Santa Clara CA 1986 Pease Robert A Low noise composite amp beats monolithics EDN May 5 1980 p 179 Also available as LB 52 in NSC s Linear Applications Databook 1982 1986 1989 etc as A Low Noise Precision Op Amp Pease Robert A Bounding clamping techniques improve on performance EDN November 10 1983 p 277 Pease Bob and Ed Maddox The Subtieties of Settling Time The New Lightning Empiricist Teledyne Philbrick Dedham MA June 1971 Addis John Versatile Broadband Analog IC VLSI Systems Design September 1988 p 18 Pease Robert A How To Get The Right Information From A Datasheet EE Times April 29 1985 p 31 Also available as Appendix F in NSC s General Purpose Linear Devices Databook 1988 1989 etc and as Appendix F in this book 9 Quashing Spurious Oscillations Oscillations are the ubiquitous bugaboos of analog circuit design Not only can you encounter oscillating op amps as described in Chapter 8 but also oscillating transis tors switching regulators optoisolators comparators and buffers And if you think about it latched up circuits are just the opposite of oscillating ones so I included them here too Recall the corollary of Murphy s Law that states Oscillators won t
110. E When you are trying to make precision measurements of resistors you should be aware that even the best ohmmeters even the ones with 4 wire connections and lots of digits on the DVM do not have as good accuracy or resolution as you can get by forcing a current through a stable reference resistor RRer and then through the Ry and comparing the voltages This is especially true for low resistor values See Figure E 3 4 You also have complete control over the amount of current flowing through the Ry Watch Out for Damaged Components Damaged resistors can also be the source of trouble A resistor that s cracked can be noisy or intermittent When resistors are overheated with excess power such as 2 or 3 W ina 1 4 W resistor they tend to fail open they may crack apart but they When Is a Resistor Not just a Resistor 33 R BALLAST don t go to low ohms or to to ashortirouit Thea accuracy or tability of a high value resistor 108 to 10 Oc cal be pac badly degraded i if dirt or fingerprints t touch its body Careful handling and clear all resistors is related to the Seebeck effect the production of an EMF in a circuit composed of two dissimilar metals when their two junctions are at different temperatures In precision circuits you should avoid thermal gradients that could cause a large temperature difference across a critical resistor For example donttistand ls provision resis
111. ERATURE g CALIBRATE POWER SWITCH Figure 2 10 This thermocouple amplifier has inherent cold junction compensation because of the two 24 25 26 halves of Q which run at a 6 1 current ratio Their Vggs are mismatched by 12 mV 40 8 uV C This mismatch exactly cancels out the 40 8 V C of the cold junction For best results you should use four 100 kQ resistors in series for R and two 100 kf resistors in series with two 100 kQ resistors in parallel for Ry all resistors of the same type from the same manufacturer Q and its surrounding components implement a correction for very cold temperatures and are not necessary for thermocouple temperatures above 0 C Credit to Mineo Yamatake for his elegant circuit design dump Then it will soon enter the atmosphere without doing anybody any good I will continue to use up any sprays with CFC based propellants that I already have but when it is time to buy more P H buy environmentally safe ones A magnifying glass or hand lens These devices are useful for inspecting boards wires and components for cracks flaws hairline solder shorts and cold soldered joints An incandescent lamp or flashlight You should be able to see clearly what you are doing and bright lights also help you to inspect boards and components A thermocouple based thermometer If your thermometer is floating and battery powered you can connect the thermocouple to any point in the circuit and meas
112. F if they do get hot You can also run into problems if you tighten the screws on the heat sink too tight or if the heat sink under the device is warped or if it has bumps or burrs or foreign matter on it If you tighten the bolt too much you ll overstress and warp the tab and die attach Overstress may cause the die to pop right off the tab The insulating washer under the power transistor can crack due to overstress or may fail after days or weeks or months Even if you don t have an insulating washer overtorqueing the bolts of plastic packaged power transistors is one of the few ways a user can mistreat and kill these devices Why does the number 10 inch pounds max 5 typ stick in my head Because that s the spec the Thermalloy man gave me for the 6 32 mounting bolts of TO 220 packages For any other package make sure you have the right spec for the torque Don t hire a gorilla to tighten the bolts Apply the 5 Second Rule Your finger is a pretty good heat detector just be careful not to burn it with hi gh voltages or very hot devices A good rule of thumb is the 5 second rule If you can hold your finger on a hot device for 5 seconds the heat sink is about right and the case temperature is about 85 C If a component is hotter than that too hot to touch then dot your finger with saliva and apply it to the hot object for just a fraction of a second If the moisture dries up quickly the case is probably around 100 C if it si
113. IONS SELECTOR 047 uF lt C lt 3800 uF OsRg10 b Figure 2 7 RC boxes based on these schematics extend component ranges beyond those available in off the shelf versions You can house the series RC circuit in a ina X x 2 inch copper clad box Use the smaller plastic film dielectric tuning capacitors or whatever is convenient and a small turn pot Build the circuit in b with tantalum or electrolytic for values of uF and higher capacitors but remember to be careful about their polarities and how you apply them Also you might consider using mylar capacitors for smaller values Sometimes it s very valuable to compare a mylar a tantalum and an aluminum electrolytic capacitor of the same value Use 8 position switches to select R and C values And stay away from wirewound resistors their inductance is too high the body of your scope If you have trouble obtaining an isolation transformer you can use a pair of transformers step down step up back to back Figure 2 8 Or if cost isn t an issue you can use isolated probes These probes let you display small signals that have common mode voltages of hundreds of volts with respect to ground and they won t require you to wear insulated gloves when adjusting your scope A variable autotransformer often called a Variac This instrument lets you change the line voltage and watch its effect on the circuit a very useful trick Warning A variable autotra
114. Law can do to you if you try to slough off your troubleshooting chores we have all seen it happen If you have a bunch of analog circuits that you have to troubleshoot well why don t you just look up the troubleshooting procedures in a book The question is excellent and the answer is very simple Until now almost nothing has been written about the troubleshooting of these circuits The best previous write up that I have found is a couple pages in a book by Jiri Dostal Ref 1 He gives some basic proce dures for looking for trouble in a fairly straightforward little circuit a voltage refer ence regulator As far as Dostal goes he does quite well But he only offers a few pages of troubleshooting advice and there is much to explain beyond what he has written Another book that has several good pages about the philosophy of troubleshooting is by John I Smith Ref 2 Smith explains some of the foibles of wishing you had designed a circuit correctly when you find that it doesn t work right Unfortunately it s out of print Analog Devices sells a Data Converter Handbook Ref 3 and it has a few pages of good ideas and suggestions on what to look for when troubleshooting data converter and analog circuits What s missing though is general information When I started writing about this troubleshooting stuff I realized there was a huge vacuum in this area So I have filled it up and here we are You ll probably use gen
115. M 7 7 174K 56K 174K 174K 10K LMC660 RAP SUGGESTS ADDING DASHED CIRCUITS Figure 13 1 This 20 db high impedance amplifier works at audio frequencies and is a handy troubleshooting aid suggest adding the dashed circuits as useful options 10 11 13 16 Scope probe parts and ground No argument Mt Heisenberg is not 158 13 Letters to Bob Noise from a video terminal I have not seen that problem but such noise may be a serious problem in some cases I don t have a digital computer near my workbench but other people may As I mentioned in Chapter 2 if you bring an AM radio near a computer or keyboard the radio will detect various amounts of graunchy RF noises I too recommend RF bypassing But you point out that bypassing should be against ambient RF I rarely think of bypassing in those terms but you are right Yeah I believe that Some people are capable of building circuits that are the exact opposite of fail safe Yeah I believe that too But data sheets for digital as well as linear ICs do not help because they only indicate quiescent power drain and give no clue about what will change when the outputis swinging Slow or fast Even TTL draws more current when running fast are E I m sure that most Vishay resistors are quite stable I myself keep a group of old wire wounds for that purpose Some of them are so old they are older than the oldest Vishays Consequently Ih
116. M194 LM394 VBE _ 2N3904 Vpp 7 Poutanen t i 2N3904 VcB l aan LM3046 VBE Dr Ne diode pins 1 and 2 7 oe Red LED a Green yellow or super rigt red LED Notes ia i FD200 and FD600 are similar to 5 thelIN4148 c curve Transistors have collector shorted t base ffor curves K L iN All data at 25 degrees C ey va LEDs must be in darkness for accurate Jon j POZE RS EOmmoo wouldn t want to show such a confusing set The IN4148s have a slope of about i 15 mV per cate compared to about 65 mV per decade for some of the Schottky diodes and 60 mV per decade for the transistors and intermediate values for other types of diodes Nobody ever tells you about these widely varied characteristics 00 Note that the transistor VBES all have thes ame slope and are better steeper than most diodes However they ar only good to about 6V of V reverse The LM394 has a built in base emitter clamp diode and thus cannot be reverse biassed lt e Note that the 2N3904 s c b junction curve M has an inferior slope worse conduc tance than the b e junction Consequently it has a higher Vat high currents but worse leakage at low voltages e Note that a red LED may have as little as 1 pA of forward current when biassed with 0 6 V forward But the LEDs must be kept in the dark to avoid photo currents Note I started measuring some of these Vgs with a curve tra
117. M34D Vg 5 Vdc and lLoap 50 pA in the circuit of Figure 2 6 Vie for LM34 and LM34A for 230 F lt Tis 300 F These specifications also apply from 5 F to Tmax in the circuit of Figure 1 Ba F w hnevon wo anion 1O48 packago ie 782F W Keto to ambient and 43F W junction to caso Thoma resistance ofthe TO 02 pachag i 324 F W junction to ambient Ors a Regulation is measured at constant junction temperature using pulse testing with a low duty cycle Changes in output due to heating effects can be Computed by multiplying the intema dissipation by the thermal resistance Note 4 Tested limits are guaranteed and 100 tested in production Note S Design limits ara guaranteed but not 100 production tested over the indicated temperature and Supply voltage ranges These limits are not used to calculate outgoing quality levels Note 6 Specification in BOLDFACE TYPE apply over the tuli rated temperature range td temperate aeprenaed nh on POTOAN te output voltage and 10 VF times the dovc s case temperature a specie condone of votag Current and temperature expressed in F ang ery in tm daviaton of 1o output volage voraus temperaturo curvo fom tha ett Suh ne over the dav rated onprane range Note 9 Quiescent current is defined in the circuit of Figure 1 Note 10 Contact factory for availability of LM34CAZ Note 11 Absolute Maximum Ratings indicate limits beyond which damage to the device may occur DC and AC electrical specific
118. New York NY 1971 References 13 _ Data Converter Handbook Analog Devices Corp P O Box 9106 Norwood MA 02062 1984 4 Bulleid H A V Master Builders of Steam lan Allan Ltd London UK 1963 pp 146 147 Caruso Denise Technology designed by its users The San Francisco Examiner p E 15 Sunday March 18 1990 gt 2 2 of circuit or product you aren both your circuit and your equipme ent a are suppose Choosing the Right Equipment As discussed in Chapter 1 the most important thing you need for effective trouble shooting is your wits In addition to those however you ll normally want to have some equipment This chapter 1 itemizes the equipment that is necessary for most general troubleshooting tasks some you can buy off the shelf and some you can build yourself S Before you begin your iroubleshooting task you 1 should know that the equipment you use has a direct bearing on the time and effort you must spend to get the job done Also know that the equipment you need to do a good job depends on the kind kings on For example a DVM may be unnecessary ifor troubleshooting some problems in ae logic And the availability and accessi _ bility of equipment may present certain obstacles If you only have a mediocre oscil loscope and your company can t go out and buy or rent or borrow a fancy full fea tured scope then you will have to make do Ifyou lac
119. Of course it is unlikely for this error voltage to be a linear function of Vom and that is why I recommend that you look at it with a scope in cross plot X Y mode Too many people make a pretend game that CMRR is constant at all levels that CM error is a linear function of Vem So they just look at two points and assume every other voltage has a linear error and that s just foo silly Even if you want to use some ATE Automatic Test Equipment you will want to look at this error at least three places maybe at four or five voltages Another good reason to use a scope in the X Y mode is so you can use your eyeball to subtract out the noise You certainly can t use an AC voltmeter to detect the CMRR error For example in Figure 8 6 the CM error is fairly stated as 0 2 mV p p not 0 3 mV p p as it might be if you used a meter that counted the noise Anyhow if you have a good amplifier with a CMRR of about 100 dB the CM error will be about 200 uV p p and as this is magnified by 100 you can easily see an output error of 20 mV p p If you have a really good unit with CMRR of 120 or 140 dB you ll want to clip in the R 100b such as 20 Q and then the A noise gain will be 1000 The noise will be magnified by 1000 but so will the error and you can see what E you need to see Now I shall not get embroiled in the question are you trying to see exactly how good the CMRR really is or just if the CMRR is better than the data sheet value
120. ROPYLENE ak A LED ier 0 1 6 180 0 5W 1 10A 20M 2 y 5 TRIAC TEMP NC r ADJUST 20M o RE 4 1k 15V 001 yF MOC 3030 0 01 uF x 17 Hz 20k D TRIANGLE WAVE 15V R2 t Vser 1Ok SUPPLY 390k O 10A A Sh 150k 115V AC Figure 9 2 Stabilizing this heater s slow servo loop and choosing the proper values for Ri Cy and C involved applying a 1 V p p 0 004 Hz Square wave Vser to R and observing the LMI IC s output with a strip chart recorder much Well I like to provide about twice or three times as much hysteresis as the minimum amount it takes to prevent oscillation near the comparator s zero crossing threshold This excess amount of feedback defines a safety margin For more infor mation on safety margins see the box Pease s Principle in Chapter 8 I have never seen this hysteresis safety factor technique outlined in print for comparators so you Can Say you read it here first My suggestion for excess hysteresis is only a rule of thumb Depending on your application you might want to use even more hysteresis For example a comparator in an RC oscillator may operate with 1 2 or 5 V of hysteresis which means you can always use more than my minimum amount of excess hysteresis Also if you have a signal with a few millivolts of noise riding on top of it the comparator that senses the signal will often want to have a hysteresis range that is two or three times greater than the worst case noise Just the
121. Right Touch Comparators are literally very touchy components that is you can drastically alter their performance just by touching the circuit with your finger And because com parators are so touchy you should be prepared for the probability that your safety margin will change for better or worse when you go from a breadboard to a printed Just the Right Touch 113 circuit layout There s no way you can predict how much hysteresis you ll need when your layout changes so you just have to re evaluate the system after you change it For faster comparators such as the LM311 everything gets even touchier and the layout is more critical Yet when several people accused the LM311 of being inher ently oscillatory I showed them that with a good layout the LM311 is capable of amplifying any small signal including its own input noise without oscillating and without any requirement for positive feedback One special precaution with the LM311 is tying the trim pins 5 and 6 normally together to prevent AC feedback from the output pin 7 normally because the trim pins can act as auxiliary inputs The LM311 data sheet in the National Semiconductor Linear Databook has carried a proper set of advice and cautions since 1980 and I recommend this advice for all comparators With comparators that are faster than an LM311 I find that depending on a perfect layout alone to prevent oscillation just isn t practical For these comparators
122. San Diego Each one really worked hard and cared a lot about every word and phrase that we debated and argued and polished and refined I am also grateful to Joyce Gilbert our group s secretary who wound up typing a lot more than she bargained for She believed me when I told her it would only be 50 or 60 pages typed how were we to know how this would grow to 280 pages Note even though Joyce typed everything in here I typed everything too as I find that my creative juices flow best when I am typing on a good word processor I wouldn t have asked her to type anything I wouldn t type However with the price of computers being what they are shrinking and lowering I would never want to re quire anybody to re key this kind of text never again It s not that expensive or diffi cult to type in an ASCII compatible format in the first place I typed my early draft on my old Coleco ADAM with a non compatible cassette memory Joyce re typed everything I wrote with Ashton Tate Multi mate and we sent the ASCII files to EDN back in August and November of 1988 I got the typed files back from EDN and put in dozens and dozens of hours retyping polishing refining and expanding the text I also want to express my appreciation for Wanda Garrett who put up with an awful lot of dumb questions about how to get the word processor running for me If any of my readers is ever going to write a book well think about what you are going to do and
123. So when an engineer complains of an oscillation the first question I have is Oh at what frequency Even though the frequency is often a good clue engineers often fail to even notice what the frequency was This omission tends to make troubleshooting by phone a challenge At very high frequencies 20 1000 MHz the layout of a circuit greatly affects the possibility of oscillation One troubleshooting technique is to slide your finger around the circuit and see if at any point an oscillation improves or worsens Remember knowing how to make an oscillation stronger is not worthless knowl edge that information can provide clues on how to make the oscillation disappear I remember being very impressed when a colleague showed me that some of the earliest IC amplifiers had a tendency to self oscillate at 98 MHz with certain levels of 108 Quashing Spurious Oscillations 109 output voltage Putting a grid dip oscillator nearby caused increases or decreases in the problem when its frequency was near 98 MHz Heathkit makes one of these instruments whose updated name is dip meter see Chapter 2 At that time I didn t have a 100 MHz scope but I could see the rectified envelope of these high frequency oscillations on a 25 MHz scope So if you see a circuit shift its DC level just because you move your finger near a transistor you should become suspicious of high frequency oscillations Of course you will never slide your fing
124. Specified Operating Temp Range Note 2 a Output Voltage 6V to 1 0V Output Current 10mA Tain to Tmax utput Curren m LM34 LM34A 50 F to 300 F Storage Temperature LM34C LM34CA 40 F to 230 F TO 46 Package 76 F to 356 F LM34D 32 F to 212 F TO 92 Package 76 F to 300 F OC Electrical Characteristics note 1 Note 6 LM34A Tested Design Parameter Typical Limit Limit aes Note 4 Note 5 Accuracy Note 7 Ta 77 F 0 4 1 0 F Ta OF 0 6 F Ta Tmax F Ta TMN 0 8 3 0 F Nonineariy Nosy Tun Ta Tum 20 88 erpe ae F Sensor Gain Tain lt Ta lt Tmax 10 0 10 0 9 9 mV F min Average Slope 10 1 mV F max Load Regulation 77 F 0 4 mV mA Note 3 ne lt Ta lt Tmax 03 mV mA OS SIMA Line Regulation Note 3 Ta 77 F t RA 01 0 05 0 01 t0 05 mV V 5V lt Vg lt 30V RA t eos 0 02 0 1 mV V Quiescent Current Vs 5V 77 F 90 75 90 pA Note 9 Vs 5V 131 116 pA Vs 30V 77 F 76 76 pA Vs 30V 132 11 mas pA Change of Quiescent 4V lt Vs lt 30V 77 F 0 5 pA Current Note 3 5V lt Vs lt 30V Tazo pA Temperature Coefficient o Minimum Temperature In cirouit of Figure 1 a Long Term Stability e F Note 1 nona oona a these specifications apply 50 F lt Tqis 300 F for the LM3I4 and LM34A 4F lt Tj 230 F for the LM34C and pvenaga and 32 F lt Tj lt 212 F for the L
125. Then with the DVM in the 1 V range you can spot small deviations in an 11 V signal This measurement is more awkward and inconvenient than a ground referenced measurement with a higher resolution meter would be and this method can cause other problems as well For instance you can end up injecting noise generated by the DVM s A D converter into the sensitive 10 V reference thereby adversely affecting the performance of other circuits In some cases a little RC filtering may minimize this problem see Figure 2 4 but you never can be sure how easy it will be to get the noise to an acceptable level Auxiliary meters It may look silly to see a test setup consisting of two good DVM s two little 3 digit DVMs monitoring a couple of voltage supplies a couple more 3 digit DVMs monitoring current drain and an analog meter monitoring something else But if you don t know exactly what you re looking for and you can borrow equipment using lots of meters is an excellent way to attack a problem even if you do have to wait until 5 15 pm to borrow all that equipment When is an analog volt meter better than a DVM Well the analog voltmeter usually has inferior accuracy and resolution but when you watch an ordinary analog voltmeter your eye can detect a trend or rate of change that may be hard to spot on a DVM especially in the pres ence of noise or jitter As an example if you suddenly connect an ordinary analog volt ohmmeter across a 1
126. V range then the other input can range from 0 36 V without causing any false outputs or causing any harm to the comparator In fact at room temperature the out of range input can go to 0 1 V and still produce the cor rect output But heaven help you if you pull one of the inputs below the 0 1 V level say to 0 5 or 0 7 V In this case if you limit the comparator s input current to less than 5 or 10 mA you won t damage or destroy the comparator But even if no damage occurs the outputs of any or all of the comparators in the IC package could respond falsely Current can flow almost anywhere within the IC s circuitry when the substrate diode which is inherent in the device s input transistor is forward biassed It is this current that causes these false outputs We ll try to be more clear about Vem specs in the future Maybe next time at National Semiconductor we ll phrase the spec sheet s cautions a little more vigor ously In fact Ta da here is the correct phrase from the LM612 data sheet The guaranteed common mode input voltage range for this comparator is V lt Vep S V 2 0 V over the entire temperature range This is the voltage range in which the comparisons must be made If both inputs are within this range of course the output will be at the correct state If one input is within this range and the other input is less than V 32 V even if this is greater than V the output will be at the
127. a good way to improve a circuit s reliability but I ve found that putting in the right parts in the right places works a lot better Refer also to a running commentary and debate on this topic in Chapter 13 If you have any doubt that your anti oscillation fixes are working try heating or cooling the suspected semiconductor device In rare cases passive components may be sufficiently temperatiire s nsitive to be at blame so think about them too Even if a circuit doesn t get better when heated it can get worse when cooled so also take a peek at the circuit while applying some freeze mist My point is that merely stopping an oscillation is not enough You must apply a tough stimulus to the circuit and see whether your circuit is close to oscillation or safely removed from any tendency to oscillate This stricture applies not only to regulators but also to all other devices that need oscillation curing procedures For example if a 47 Q resistor in the base of a transistor cures an oscillation but 24 Q doesn t and 33 Q doesn t and 39 Q still doesn t then 47 Q is a lot more marginal than it seems Maybe a 75 Q resistor would be a better idea just so long as 100 or 120 or 150 Q resistors are still safe In other words even though wild guesses and dumb luck can sometimes cure an oscillation you cannot cure oscillations safely and surely without some thoughtful procedures Furthermore somebody who has an appreciation for the
128. a kid I was really impressed by this toy but later I realized that this logical sequencer was an illusion The ON OFF switch did trigger some kind of a latch to turn the power ON but it did not directly provide the OFF function If it did the hand would stall immediately after it tumed the switch OFF and would not continue back inside the box There was a _ switch to turn the power OFF but it was functionally inside the box triggered by the end of the travel of the hand The point of the story is that we must make sure not to fool anybody especially not ourselves with the start up circuits we design When I design an A D converter I include a shift register sequencer to make sure that every necessary task is completed sequentially at the start of conversion and then again at the end I have no idea how software engineers write valid programs to make sure no false start up sequences occur but I bet some people do it wrong Some designs still implement the micro Toys Illustrate Some Basics 14 processor RESET function with such crude RC timers that the processor gets con fused or faked out and doesn t start correctly I ve heard about those horror stories Sometimes people forget to add the diode to discharge the capacitor and then the reset fails to work after a brief power outage The same diode function as D2 in Figure 11 2 0 9 a on Designers also include start up circuits in linear circuits For example curreri
129. able voltage anywhere i from 0 2 to 5 V with creditable efficiency and economy These band gap references feature as low a tem perature coefficient as you d probably ver be willing to pay for as good as 20 or 10 ppm C They also feature enough r noise so thata dittle filtering can make a big improvement po Fon GY ney AE A good buried zener diode voltage reference is inherently more stable over the long term than is a band gap one good zener designs change only 5 to 10 ppm per month However if you want the best stability possible it s only fair to age stabilize and burn in the references first Also you must screen out the ones that just keep walking away from their initial values by 10 to 20 ppm every week there are always a few sports that are dtiftier than the rest Unfortunately there s no quick and easy test to distinguish between the drifty ones and the stable ones except for taking measurements for many hundreds of hours Regulators Are Almost Foolproof In the last 10 years IC voltage regulators have gotten pretty user friendly Many people use them with no problems at all Still my colleagues and I get at least one call every month about a regulator working poorly The indignant caller complains It s getting hot We ask How big is your heat sink The indignant voice responds What do you mean heat sink I credit all of you readers with enough smarts to recognize th
130. about your series of arti cles A copy of a letter to EDN is enclosed begging for a quality reprint of the entire series I also had to write personally to thank you Letters to Bob 169 You took many things I learned over my 20 or so years in electronics and more I never learned and trimmed that vast information down into the bare essence That s hard to do I ve done both articles and editor work for DECUS Digital Equipment Computer Users Society publications and I know how much work you put into that series Bravo I m probably very lucky My job involves service redesign and enhancement of radiation therapy equipment computers etc The work spans DC to S band microwave analog and digital picoamps to megawatts The hard fun part is at both extremes working with picoamps or the high powered circuits where a not uncommon component failure mode is disappearance I finally evolved my philosophy down to three commandments God came up with 10 but I have enough problems remembering more than one thing at a time so 3 is enough The more tools you can use the better but make sure you know how to use those tools If you don t know the operating procedures capabilities and limitations of your test equipment and know what the equipment does fo the circuit you are testing you re cheating yourself The extreme case is microwave There just taking the cover off the circuit can affect it Get your
131. ad You just about have to use the same basic PC board that is used for the real circuit and then install tiny component jacks that accept 0 018 inch leads such as Amp type 50462 or Interconnection Products type 450 2598 0 1 03 00 If it s inconvenient to go out and shop for these component jacks you can roll your own get some of those strips of sockets the ones that are 25 in a line such as Digikey s Catalog No A208 A209 or similar If strays are not important just snip off a group of jacks as many sockets as you need in a row But if you need low capacitance and very low leakage just use your diagonal nippers to snip away the plastic and use the little jacks one by one You will want to avoid beating these up as they are a little delicate but they are excellent sockets for diodes Rs Cs Qs and other small components with thin leads If you need a little component jack for 0 040 inch diameter pins the Interconnection Products type 450 3729 01 03 00 is good for that and a similar part from Amp is 645 508 1 The main point is you want to affect the stray capacitances and inductances of the real circuit as little as possible so when you stuff it full of good parts it works OK Then when you swap ina bad part it should be obvious what s to blame Here s a final list of some of the most common problems in circuits Swapped resistors installed in the wrong place Resistors of the wrong value wrong code Diodes insta
132. af of printouts Look he said excitedly I ve come up with a simulation of our power plant heating and air conditioning system that will double the plant s efficiency The Chief Engineer studied the printout for a few moments Yes he said but look here And he pointed to the flow diagram This 17 F water is going to be awfully hard on the pumps Your frustration with the menu burden of these wonderful new instruments is right on target One thing I detest intensely and that seems to occur with increasing fre quency is staring blankly at a screen or cursor knowing full well that the reason it Letters to Bob 167 obstinately refuses to do your bidding is almost certainly a consequence of your imperfect understanding of its menus and possibly also of your failure to read the manual all the way through which you ll do the next time you have a spare week Keep up the good work Reginald W Neale Connoisseur of Solder Globs Rochester NY Dear Mr Neale Thanks for the good words But hey water at 17 F isn t such a terrible idea if you allow for a little antifreeze The real problem I foresee is that you can easily get your 17 F water in the winter when you don t need it When you really would like to get some is in the summer and then it s pretty expensive to chill it down that cold Maybe all you need is the plumbing equivalent of the solar powered night light with the 12 000 mile extension
133. ain or threshold degradation may occur So please don t Also power MOSFETs are not quite as rugged as bipolars when it comes to surviving ESD transients A common precaution is to add a little decou pling clamping or current limiting circuitry so that terminals accessible to the out side world can withstand ESD DMOS FETs are so easy to apply that we usually forget about the parasitic bipolar transistor that lurks in parallel with them If dV dt is too large at the drain if the drain junction is avalanched at too high a current and voltage or if the transistor gets too hot the bipolar device turns ON and dies an instant death due to current hogging or an excursion from its safe operating area But I m spoiled rotten I m accustomed to linear ICs which have protection tran sistors built right in so the user rarely has a problem But most of the transistor troubles are left to the IC designer Discrete designs are appropriate and cost effec tive for many applications but the availability of linear ICs especially op amps can simplify your design task considerably at the same time as it improves relia bility Next time we ll discuss the ins and outs and innards of op amps Leonard Charles Is reliability prediction methodology for the birds Power Conversion and Intelligent Motion November 1988 p 4 Pease Robert A Picoammeter calibrator system eases low current measurements EDN March 31 1982 p 1
134. ajor ways for this to happen 1 When there is a diode connected to a delicate input the ambient light in the room can shine in and generate photocurrents With fluorescent lights this is usually at 120 Hz but the higher harmonics can boom in too along with some DC current As soon as you realize this is happening it s fairly easy to troubleshoot this by adding a little darkness to the circuit cover it up with a dark cloth jacket or book You can then localize it and darken it permanently In the case of extremely bright light it can even come in through the insulators in the base of a TO 99 can the little ceramic feed throughs are not really opaque they let a small fraction of the light in Fortunately plastic DIP packages are very opaque these days 2 A power supply can have more 60 or 120 Hz ripple saw tooth shape or pulse shape than you expect This can be caused by bad connections a bad capacitor an open rectifier or a ground loop Again as soon as you recognize that this kind of thing can happen it s easy to search and cure the problem 3 Magnetic flux from a transformer gets coupled into your circuit The two most common sources are a soldering iron close by or a power transformer that is satu rating a little bit spraying flux around This usually has a distinctive shape at 60 Hz with lots of harmonics and is quite position sensitive This too is fairly easy to recognize But if you discover that y
135. al circuits and different logic flow charts So be careful to check things out 555s from different manufacturers can act quite differently i At high speeds the timers don t just respond in a time 0 693 R X C the response time is more like 0 693 R X C Copray TpeLay Most books never mention this fact most data sheets don t either So although you can usually get a fast timer circuit to function to get it to work the way you want it to you still have to be careful These designs are not always trivial and Ref 1 may help you avoid some pitfalls A timer is after all just an aggregation of parts that includes a comparator so many of the techniques you use with comparators work with timers and vice versa Digital ICs Not Purely Digital Although timers are partly digital the more classic digital ICs perform purely logical functions Nevertheless in the hands of a clever linear designer some digital ICs can be very useful for performing analog functions For example CD4066 quad analog switches make excellent low leakage switches and a 74C74 makes an excel lent phase detector for a Phase Locked Loop Ref 2 And not only is the price right so is the power drain Even when ordinary CMOS ICs aren t fast enough you can often substitute a high speed CMOS or 74ALS or 74AS counterpart to get more speed I won t belabor the point instead I ll go straight to the litany of Troubles and Problems that you whether an a
136. also better rejec tion of other noises around your circuit In some high impedance circuits even a 10X probe s capacitance which is typi cally 9 to 15 pF may be unacceptable For these circuits you can buy an active probe with a lower input capacitance of 1 5i to 3 pF 395 to 1800 or you can build your own Figure 2 3 corer Me When you have to work with switching regulators you should have a couple of cur URIET ican lise those current signals are doing Some current proves when you are arching trigger off an event Hip odt only rarely and can store that event and the events that follow it Some storage scopes ate balk y or tricky to apply but it s often worth while to expend the effort to ear how to use them s Digital storage oscilloscopes DSOs let you d the E of triggering and event storage as do the analog type and somet 5 oan aplay ents thiat precede the trigger They are sampled data systems however BO yo Mnustibe sure t apply them correctly Ref 1 Once you learn how to use them though you ll appreciate the special features they offer such as bright CRT displays automatic pulse parameter measurements and the ability to obtain plots of waveforms E s 4 A digital voltmeter DVM Choose o one with at least five digits of resolution such as the HP3455 the HP3456 the Fluke 8810A or the Fluke 8842A Be sure you can lock out the autorange feature S0 that
137. and tests and answers that would take me forever You have your own systems with which you are familiar and your equip ment and your friends Together you can solve problems that nobody else can So guess I l admit that some confidence would be a useful tool for you And if there are specialized techniques that you know well good for you I never told you that know everything But I bet some of the techniques in this book will be useful I will throw in a couple more scenarios for other basic circuits They may not solve every problem but they will indicate the breadth and depth of thinking that may be needed to solve tough circuit problems Examples Single transistor amplifier e Negative regulator with LM337 G Absolute value circuit Switching regulator using 1 M3524 gt e Positive regulator with LLM317 723 type regulator Instrumentation Amplifier Switching regulator using 1 M2575 17 Figure 14 2 Basic Single Transistor Amplifier Indication of Trouble i4 Real Circuits and Real Problems Table 14 2 Troubleshooting Single Transistor Amplifiers Possible Cause 12V C 0 1 f o Vour Ve 10K Q1 2N3904 OR SIMILAR Solution Output at wrong DC level Collector at 12 V Collector at 10 V Collector at 0 7 V Collector at 0 V Base at wrong voltage Base at 3 V Base at 3 V No gain or bad gain Oscillation Low gain non inverting etc
138. and they are in Appendix A in the back Similarly some CMOS devices have many but not all of their functions in common with those of their TTL counterparts For example the 74C74 has the same pinout and 95 of the same functions as the TTL 7474 Both follow mostly the same truth table except that when you pull both the preset and clear inputs low the ETL device s outputs Q and Q both go LOW whereas the CMOS part s outputs both go HIGH If anybody has a complete list of such differences I d love to see a copy In some cases you can buy a buffered gate CD4001 BN an unbuffered gate CD4001 an unbuffered inverter MM74HCU04 or a buffered inverter MM74HC04 Sometimes you can buy one part number and get an unbuffered part from one vendor and a buffered one from another Of course the unbuffered parts are faster with light capacitive loads but the buffered ones are faster with heavy loads So if you have a critical application be aware that substituting different vendors parts Can mess up your circuit Be careful when interfacing from linear ICs into di g ital ones For example an LM324 running on a single 5 V supply doesn t have a lot of margin to drive CMOS inputs but an op amp running on 5 or 10 V would need some kind of attenuation or resistive protection to avoid abusing the logic device inputs Figure 10 2 Likewise it s considered bad form to overdrive the inputs of digital ICs just be cause they are pro
139. ansis tors bipolar Collins Jack with David White 25 COMPAQ 173 comparators 56 108 111 116 119 122 123 190 194 195 207 common mode range definition of 115 offset voltage test 194 195 comparators problems with common mode range 114 116 common mode slew problem 115 differential over drive 115 false operation when overdriven negative 115 layout 111 113 linear preamplifier ahead of com parator 114 need for minimum hysteresis 111 113 AC coupled hysteresis 113 113 194 maximum hysteresis 112 safety margin in hysteresis 112 noise of comparators 113 oscillations 111 114 194 195 phase shift 111 116 shielding guarding See under layout strobe or latch modes 113 test for Vog 113 114 194 195 use of amplifier as comparator 116 use of comparator as amplifier 116 component jacks 185 components engineer 32 34 43 70 78 components Used or second hand or recycled 48 computer modelling 98 144 145 203 207 airborne computer 145 analog modelling See under analog computer analogues computer aided design CAD 205 207 SPICE definition of 203 comments on SPICE 111 144 146 203 207 convergence 144 145 204 205 spurious oscillations 109 111 144 205 207 typical computer errors 111 128 138 144 145 148 155 166 172 203 207 typical human errors 144 computer ignition 172 computers analog See analog computers computers digital 1 22 59 145 144 147 158 172 173
140. arly evident I pointed out that if we tried to force what Bob had given us into a more traditional mold it would lose a significant part of its value Among the reasons that Bob is so successful are who he is and how he ap proaches problems There was no better way than through his style to convey the essence of Bob s personality to the readers One of EDN s rules is that we don t use rhetorical questions readers may an swer them in unexpected ways ways that can play havoc with the point the writer is trying to make The staff jokes that we ration rhetorical questions According to the legend each issue our managing editor Joan Morrow Lynch grants one editor who requests it the right to ask a rhetorical question She does so on a first come first served basis but can only deny the privilege to anyone whose recent work has posed too many of the queries After reading what eventually became the first installment of Troubleshooting Analog Circuits I observed that this one article would more than use up EDN s entire annual allotment of rhetorical questions But why not risk it I asked Bob begins solving problems by asking questions Something about the series that to my knowledge is unique is that it approaches the subject of troubleshooting from a design engineer s perspective EDN readers are designers Pease is an accomplished designer Yet he is one who not only doesn t see troubleshooting as beneath
141. at some low priced parts have not even been sample tested Variable Capacitors May Have Finite Rotational Lives Variable capacitors are usually made of low K material with characteristics similar to those of COG capacitors Their electrical performance is excellent The dielectric doesn t cause much trouble but the metal sliding contacts or electrodes are on some models very thin after only a small number of rotations hundreds or even dozens the metal may wear out and fail to connect to the capacitance In general capacitors are very reliable components and if you don t fry them with heat or zap them mercilessly the small signal ones will last forever and the elec trolytic ones will last for many years Old oil filled capacitors aren t quite that reli able and have probably been replaced already at least they should have been re placed The only way you can have an unreliable capacitor is to use a type that is unsuitable for the task And that s the engineer s fault not the capacitor s fault Still some troubleshooting may be required and if you recognize the clues that distinguish different types of capacitors you ve taken a step in the right direction First Try Adding a Second What procedures are best for troubleshooting capacitors I use two basic procedures the first of which is the add it on approach Most circuits are not hopelessly critical about capacitor values as long as the capacitors valu
142. at you can t put a whole lot of power into a little regulator unless you secure it to a sufficient heat sink or heat fins Then there really aren t too many things that are likely to go wrong because voltage regulators have just about every feature for protection against the world s assaults You ll have problems with regulators when you don t provide the required speci fied output bypassing Most negative regulators and some other types such as low dropout regulators require an electrolytic bypass capacitor from the output to ground If you insert a tantalum capacitor you may be able to get away with a value 136 1 Dealing with References and Regulators Figure 11 1 When you re the Czar of Band Gaps people look up to you of 1 or 2 pF if you use an aluminum electrolytic Capacitor you can get away with 20 to 100 uF or whatever the data sheet spells out But in all cases on all the parts I know an electrolytic capacitor will work and a film or ceramic capacitor won t work its series resistance is just too small Now if you put a Q resistor in series with a pF ceramic capacitor the filtering will probably be adequate around room s temperature the loss factor is then similar to a tantalum capacitor But if you take it to 40 or 100 C the ceramic capacitor s value will shrink badly refer to Chapter 4 on capacitors and the regulator will be unhappy again It may start oscillating or it might just start ri
143. ate An LF351 or one fourth of an LF347 will re spond somewhat faster So if you want to use an op amp as a comparator you d better need merely a slow comparator Note however that one LM358 plus one LM392 will give you effectively three fourths of an LM324 plus one fourth of an LM339 and the space taken by the two 8 pin mini DIPs would be only 4 more than that taken by a single 14 pin DIP But even so some people do use op amps as slow precision comparators Even though op amps are generally not characterized as comparators you can engineer such a circuit successfully For example the LM709 minus its compensation capaci tors is a surprisingly competent fairly quick comparator But please don t overdrive and damage the inputs Conversely I am occasionally asked Can I put some damping capacitors on an LM339 and use it as a unity gain follower The general answer is NO because the LM339 s phase lags are too squirrelly to be controlled by any possible compensation scheme But I have used the slower LP339 and LP365 successfully this way as a slow inverter or slow follower Even Buffered Circuits Can Oscillate Any circuit that adds current gain can oscillate even a buffer Let s agree that a buffer is some kind of linear amplifier that has a lot of current gain Some have a voltage gain around 0 90 or 0 95 Others have gains as high as 10 or 20 because their outputs must swing 50 or 100 V p p or more Even emitter fol
144. ations do not apply when Operating the device beyond its rated Operating conditions see Note 1 F How to Get the Right Information From a Data Sheet A Point By Point Look Let s look a little more closely at the data sheet of the Na tional Semiconductor LM34 which happens to be a temper ature sensor Note 1 lists the nominal test conditions and test circuits in which all the characteristics are defined Some additional test conditions are listed in the column Conditions but Note 1 helps minimize the clutter Note 2 gives the thermal impedance which may also be shown in a chart or table Note 3 wams that an output impedance test if done with a long pulse could cause significant self heating and thus error Note 6 is intended to show which Specs apply at all rated temperatures Note 7 is the definition of the Accuracy spec and Note 8 the definition for non linearity Note 9 states in what test circuit the quiescent current is defined Note 10 indicates that one model of the famity may not be available at the time of printing but happens to be available now and Note 11 is the definition of Absolute Max Ratings Note the 4 seconds soldering time is a new standard for plastic packages Note the wording of Note 11 has been revised this is the best wording we can devise and we will use it on all future datasheets APPLICATIONS Another important part of the data sheet is
145. ator clip Do not put the plus input pin into the device s socket Circuit b lets you test an op amp s differential input capacitance for this circuit Cin differential Vp p X Crota V 1 p p Vaip py where Crotal Cin common mode C cable Cscope in 100 pF I ve measured some input capacitances and find the circuits of Figure 8 8 to be quite useful Input capacitance data is nominally of interest only for high impedance high speed buffers or for filters where you want to make sure that the second source device has the same capacitance as the op amps that are already working okay Recognize False Error Characteristics Sometimes an op amp may exhibit an error that looks like a bad problem but isn t For example if you have your op amp s output ramping at 0 3 V usec you might be surprised when you discover that the inverting input a summing point is nor at ground Instead it may be 15 or 30 or 100 mV away from ground How can the offset voltage be so bad if the spec is only 2 or 4 mV Why isn t the inverting input at the virtual ground that the books teach us The virtual ground theory is applicable at DC and low frequencies but if the Output is moving at a moderate or fast speed then expecting the summing point to be exactly at ground is unreasonable In this example dV oudt equals 27 times the umty gam frequency times the input voltage So 15 mV of Vin IS Quite reasonable for a medium bandw
146. ave confidence that they have good long term stability which I find more important than temperature coefficient equipment for calibration is a serious issue In our group nt disappear for calibration just whien we needed tthe equip lithe probl m by requiring that engineers put hen it was due to be calibrated If they don t tealiti leads jotend to disappear We have a budget for those TS het items every year so we avoid running out oe nent M the only one who can affect a meagurement by looking atati 0 py ces unk of coaxial hose on an op amp s output can sure You are right that adding a chi make it unhappy more oftenithan doing so makes it happy You are right to complain that foolish people p where it saysI can tdpit they protest Bd Note The lere S no poit in pitti ig Up signs in the desertisaying Thou halt n t eat rocks Good statement about p obe comi tibility Sometimes you turn the adjustment all the way to the end anc be Still utralizeiout o Good point T6 hg iron by ing about one inch away Mi ning ave inftansd receptors in their lips If you close your eyes and slowly raise the back of your hand past your lips you should be able to sense the presence of your hand Use the back of your hand because the callouses on the paltn of your hand block heat radiation You point out that drafting depart
147. ave transistor troubles Robust design methods and proper assumptions regarding their performance characteristics will steer you past the shoals of transistor vexation and the rocks of transistor disasters Transistors are wonderful they re so powerful and versatile With a handful of transistors you can build almost any kind of high performance circuit a fast op amp a video buffer or a uniqu Jogic circuit On th other hanid transistors are uniquely adept at causing trouble For example a simple amplifier probably won t survive if you short the input to the power supplies or the output to ground Fortunately most op amps include forgiving features so that they can survive these conditions When the A741 and the LM101 op amps were designed they included xtra transistors to ensure that their i inputs and outputs would survive such abuse But ani individual transistor is vulherable to damage by excessive forward or reverse current atits inpuit a and almost every transistor is capable of melting So it s up to us ithe engineers to desigti transistor circuits so that the transis tors do not blow up and we must troubleshoot these circuits when and if they do A simple and sometitive isosobvious problem i is installing a transistor incor rectly Because trarisistors have three terminals the possibility of a wrong connection is considerably greater than with a mere diode Small signal trans
148. ay The scope didn t show anything abnormal but I suspected that something was causing the circuit to oscillate Now one very important detail My new house was two blocks from the top of a hill covered with an antenna farm including an AM broadcast station at 970 kHz The bandwidth of my stereo s power amp was well over MHz I discovered that if I hung a 0 1 uF capacitor ftom both sides of the power line entrance to chassis ground everything was fine EMii was coming down the power line causing the circuit to break into destructive oscillation I couldn t see the oscillation on the scope because the scope s ground lead shunted the EMI to power line ground I learned that Identical circuits aren t necessarily Marginal designs that work on the bench may fail when put into service Look for olives anywhere Thanks again for some great articles Steve Coffman i NovaTest Beaverton OR Dear Mr Coffman I think you forgot a fourth point If some bandwidth is goodiand some more bandwidth is better then lots and lots of bandwidth is not best it va disaster Giving aipower amplifier lots and lots of bandwidth is one good way to buy trouble But T find it interesting that the 970 kHz EMI came in on the power line and not on the speaker cables RAP Dear Bob Last month the family and I took in the Moscow Circus When the show was over I just sat there wishing it wasn t over I feel the same way
149. ays ia necessarily wrong For example one approach might be The original designer of the product writes the data sheet inside his head at the same time the product is designed The concept here is if one can t find the proper ingredients for a data sheet godd applications convenient features for the user and nicely tested specifications as the part is being designed then are completed Thus the collection of raw materials for a y good data sheet is an integral part of the design of a prod uct The actual assembly of these materiais is an art which can take place later days The marketing people insist that the data sheet must seem to have been thrown together under panic conditions ie SLA thoy Dave so many rough spots Users should be o ee io sltometives By not asking the right question a mis i i ductor Corp Santa Clara Calit for eleven years He has a designed numerous op amps data converters voltage reg i ators and analog circuit functions WHEN TO WRITE DATA SHEETS A new product becomes available The applications engi neers start evaluating their application circuits and the test engineers examine thelr production test equipment But how can the users evaluate the new device They have to have a data sheet which is stil in the process of being written Every week as the data sheet writer tries to polish and refine the incipient data sheet
150. big feed back capacitor across the feedback paths of these Op amps would be exactly the wrong thing to do although in some cases 1 2 or 1 pf may be helpful Recently I observed that a number of National Semiconductor op amp data sheets were advising feedback capacitor values of C Cir Rin R But if you had an ordinary op amp whose Cin was 5 pF and an inverter with a gain of 0 1 with Rp 1 MQ and Rin 10 MQ this equation would tell you to use a Cp of 50 pF and accept a frequency response of 3 kHz That would be absurd If you actually build this circuit you ll find that it works well with Cp 1 5 pF which gives the inverter a bandwidth of 100 kHz So we at NSC have just agreed to deep six that equation We have a couple new formulas which we ve checked carefully and we have found that you can get considerably improved bandwidth and excellent stability For high values of gain and of Rp use the following equation GBW x R Oscillations Do Occasionally Accompany Op Amps 103 where GBW is the gain bandwidth product In those cases in which the gain or impedance is low such as where 1 R R lt 2 GBW x R X C use the following equation t C in C 2 2R R I won t bore you with the math but these equations did come from real analytical approaches that have been around for 20 years I championed them back at Philbrick Researches The value of Cp that you compute is not t
151. bii K Researches the company s motto w Og ay is the model way In thos days we sold some analog compute igh that part of the business w ing and the popularity of the opia But tof ty line tha nalog compu tation wi rc etre ics busin ing and someday T willexpound on that ant with Integfated Circuit Einphasis is a rather d powerful tod la alinios erybody finds it useful to some extent I remember when my old boss Tim Isbell showed me how to use it and then we Spent half a day horsing around because it said we had a 72 V forward voltage across aidiod was no current through the diode I will emphasize Wof thi ms with SPICE oa gine i analog compitation j But SPICE ist its answers asithey trust most ould have evaporated Ihave come very 8 when a personiclaims thats to him Conversel i Yoy Couldn t use the slide rule unless you already Knew appriy imately what the answer was It s not like a calcu lator where the decimal place is provided on a platter you have to provide your own decimal place In other words youiare forced to be a pretty good engineer before you even pick up your slide rule or your analog computer But people who use SPICE are often buffaloed or fooled by any absurd Kind of answer So trusting your computer seems to be one of the new trends which I want to see quashed It s too
152. ble to talk to technical people they explained why these transistors were not available they admitted that I was not dreaming but that the parts had been discon tinued recently These engineers at some of the major power transistor manufacturers were quite helpful as they explained that newer geometries helped planar power tran sistors approach the safe area of the other older types withou sacrificing the planar advantages of speed Also power MOSFETs had even wide amounts of SOA and their prices have been dropping and they were able to take over many new tasks where the planars did not have enough SOA So the puzzle all fits together There is still one tricky problem Originally the old 2N3771 was a single diffused part If you wanted to buy an epi base part that was the MJ3771 But now if you order a 2N3771 you get the epi base part which does meet and exceed the JEDEC 2N3771 specs It just exceeds them a lot more than you would expect like the current gain bandwidth is 10 or 20 x higher So if you try to replace an old 2N3771 with a new 2N3771 please be aware that they are probably not very similar at all 86 7 identifying and Avoiding Transistor Problems BASE EMITTER BASE MESA ETCHING Figure 7 6 Inthe olds a diffused st ype dopa eich alread into the front hob fort Kou jistou of challenge even the most experi o consid r your oper ing condi jas beta and Vo you ll match and figure out
153. cer I realized later that the V s were wrong the curve tracer was badly out of calibration I black flagged it 196 22 oz gi 1 Ph OL go 90 to Zo 0 TTT TTT TTI Ter bs besa ttt ttt rr T ry eV Ae ttt TAA T TTT Or ae ttt eT Tt ae suererauuuerseer 2274 ALA DA LATATA LS A LAA TTT ATT AAA A T TT C CT ES A for various diodes of V versus Figure E 1 Semilog plot 198 E Ve vs Ip on Various Diodes right away Then I tried measuring the Vp of a diode with a battery powered DVM in series with the diode I got ridiculous answers such as 30 nA of leakage on a good transistor at 100 mV As soon as I plotted the data I realized I was on the verge of being fooled Plots are valuable because you can see the shape of a curve and if it doesn t make sense you can spot it It turns out the DVM was pumping enough AC noise current into the diode to cause rectification and false values of current Remember I warned you jabout that back in Chapter 2 When I then put 0 47 pF across the diode I got valid data You should be aware that you as well as I can easily take bad data it easier than taking good data You just have to be suspicious and stop and got bak and d make sure to throw out the bad data and then get valid data 7 ZA National APPENDIX F gt How to Get the Right Information From a Data Sheet Not All Data Sheets Are Created Alike and False Assumptions
154. cessively hot Pa High temperature per se does not cause a power transistor to fail But if the drive circuitry was designed to turn a transistor ON and only a base emitter resistor is available to turn it OFF then at a very high temperature the transistor will turn itself ON and there will be no adequate way to turn it OFF Then it may go into secondary breakdown and overheat and fail However overheating does not by itself cause failure I once applied a soldering iron to a 3 terminal voltage regulator I hung it from the tip of the soldering iron and then ran off to answer the phone When I BASE a Figure 7 3 Apply the 5 Second Rule 83 COLLECTOR EMITTERS BALLAST RESISTORS EMITTER b Ballast resistors also known as sharing resistors are often connected to the emitters of a number of paralleled transistors a to help the transistors share current and power In an integrated circuit b the ballast resistors are often integrated with adjacent emitters Photo of National Semiconductor Corp s LM138 came back the next day I discovered that the TO 3 package was still quite hot 300 C which is normally recommended for only 10 seconds When I cooled it off the regulator ran fine and met spec So the old dictum that high temperature will necessarily degrade reliability is not always true Still it s a good practice to not get your power transistors that hot and to have a base drive that can pull the base OF
155. cisely you might start failing parts that are actually meeting their specs For example let s say we want to test the load regulation of an LM323 a5 V regulator when V is held at exactly 8 00 V and the load changes from 5 mA to 3 00 A Fig 5 4 In this circuit there are four pairs of Kelvin connections at work The first pair is located at the power supply s output This programmable supply s remote sense terminals permit it to maintain an accurate 8 00 V output right up to the pin of the DUT This is commonly called remote sense when you are in the power supply business but actually it represents a Kelvin connection This is important because if the 8 00 V supply dropped to 7 9 or 7 8 that would be an unfair test The second Kelvin connection in Figure 5 4 is located at the output of the DUT In order for you to observe the changes in Vout as you apply various loads the Kelvin contacts provide Force leads for the 3 A of output current They also provide Sense leads so you can observe the DUT s output with a high impedance voltmeter Note that there are two Sense and two Force connections to the ground pin of the DUT You don t really need all four contacts you can tie both Force leads together and also both Sense leads together You can do that because there is no significant current flowing in the Sense lead and in the Force lead we don t care how much current flows nor do we care exactly what the voltage drop is T
156. connection to open per manently thus leadi i ification notes on The Sout off 4 Setupa storage scope ot in acanisition system to trap and save the situa tion at the instant of the failure Depending on the nature of the instrument you may be able to store the data before the event s trigger or after or both This may be espe cially useful in self destructive cases 5 Get one or more buddies to help you analyze the situation Friends can help propose a failure mode a scenario or a n w test that may give a clue 6 As the problem may be extremely difficult use extreme measures to spot it Beg or borrow special equipment Make duplicates of the circuit or equipment that is failing in hopes of finding more examples of the failure In some cases you are Justified in slightly abusing the equipment in hopes of turning the intermittent problem into an all the time problem which is often easier to solve 143 144 12 Roundup of Floobydust Sugar and Spice and Nothing Nice In case you haven t guessed I m not a big fan of digital computers and simulation When a computer tries to simulate an analog circuit sometimes it does a good Job but when it doesn t things get very sticky Part of the problem is that some people put excessive confidence and belief in anything a computer says Fortunately my bosses are very skeptical people and they agree that we must be cautious when a computer makes ou
157. cooled there was no incentive for the materia to flow back into the gap thus the board was loose A spring washer be it a star or a Belleville type will eventually loosen to the point that the desired electrical connection is poor if not lost altogether It s better to solder if you can If a screw is needed make the pad very large and the washer under the screw wide in order to spread the load and maximize the joint s life Recently I discovered a potential problem with surface mount capacitors when soldered onto a PC board The capacitance can be increased beyond the specifica tions of the capacitor if any flux is left hidden under the capacitor If all the flux is not washed away it can cause the capacitance to appear out of spec Washing with solvent and then washing in the dishwasher can solve that problem Keep the good practical articles coming and we will all benefit Richard T Lamoureux Hawthorne CA RAP s reply Well I believe you are correct that you are more discerning and have more experi ence in these areas than I do Thank you for the tips You are saying that any bolt on a PC board is likely to loosen unless in an air conditioned constant temperature office with no significant self heating H mmm Thanks for the tips RAP Dear Bob I really enjoyed your recent series on troubleshooting Although I m not much of an analog designer I have gained experience over the years in digital system desi
158. cord RAP Dear Bob I m really confused by your apology on page 34 of the November 23 1989 issue of EDN I understand about diode connected transistors we use them often However we connect the base to the emitter and thereby use the base collector junc tion The breakdown voltage of such a connection is typically about the same as the transistor s maximum Vg rating If we use your method the breakdown rating will be only 5 to 7 V typ according to data sheets I usually want more John Paul Hoffman Caterpillar Inc Peoria IL Dear Mr Hoffman You re right the collector base junction can handle more voltage than the base emitter junction but it s also slower RAP Dear Bob A few years ago when I first moved into a new house and hooked up my stereo the left channel decided to fry the output transistors The power amp had heavy feed 168 13 Letters to Bob back and was a standard design running class A or AB It had a tweak for setting the standing current in the output stage I found that as I monitored and increased the standing current in the left channel the left stage would run away at a standing cur rent value which the right channel could handle with no problem Now I was confused I had two identical circuits one of which was fine and the other of which wouldn t tweak up to spec I noticed that if I was probing certain nodes in the left channel with my scope the left channel wouldn t run aw
159. d keep a breadboard with sockets around because the inductor is such a tricky component 184 Vin 8V O Vin RETURN O 0 01pF MOUNTED ON STAVER HEATSINK No V5 1 Q1 BD344 Q2 2N5023 14 Real Circuits and Real Problems L1 gt 40 TURNS No 22 WIRE ON FERROXCUBE No K300502 TOROID CORE Figure 14 9 LM3524 Switching Regulator Indication of Trouble Possible Cause _ Table 14 9 Troubleshooting LM3524 Switching Regulators SIMPLIFIED Solution Something overheats Clock frequency bad Bad loop stability Bad current limit Output voltage tolerance is bad Transistor overheats Nothing works Can t drive rated load ete ete ete ete ete etc Frequency too high Bad Di rectifier Bad transistors Bad capacitor Bad LM3524 Bad capacitor Bad resistor Bad R C damper Bad C6 Bad inductor Bad Reense Bad resistors Bad reference Resistors bad Bad inductor Solder shorts Something bad Bad transistor Bad input voltage Bad 3524 Measure F at pin 7 Check D1 Check transistors Check capacitor Check LM3524 Check Ct at pin 7 Check Rt at pin 6 Check R C at pin 9 Swap a good cap Check L1 Check Rense at pins 4 5 Check RI 2 4 5 Check V at pins 16 2 1 Check R8 9 10 Check LI Look for solder shorts Check everything Check Q1 Q2 Swap in good transistors Check V with scope Study all data swap in a
160. d other power ICs have an on chip temperature limiter Thermal shutdown circuits cai 106 8 Operational Amplifiers The Supreme Activators improve reliability If a heavy overload is applied for a long time or there is no heat sink or the ambient is just too hot these circuits detect when the chip s temperature exceeds 150 C and then turn off the output The thermal limiter circuit in the LM117 and other early power ICs sometimes just decreased the Output current to a sate DC value to hold the die temperature to around 160 C In other cases where the load is lighter and the thermal gradient transients are different these thermal limiters oscillate ON and OFF with a duty cycle that ensures the 160 C chip temperature As I was about to design the LM137 I looked back and decided the latter characteristic was preferable so I designed about 5 C of thermal hysteresis into the thermal limit circuit That way the circuit makes a strong attempt to restart its heavy load with a repetition rate of about 100 Hz If the regulator makes only a feeble attempt it may be unable to start some legal Joads So we actually designed an oscillation into this thermal limit circuit but we never bothered to mention itionithe data sheet H mmm we shouldn t be so sloppy I apologize I ll do betternext time This situation has a bearing on one of my pet peeves Bad data sheets Iiget really cross about bad ones and I really do try hard and wor
161. d to the digital ground nd another link to the case or chassis This technique works well for me and Iireoommend i it Itisa little known ifacrthat some coaxial cable can degrade just sitting on the shelf Well that s trie butt degrades faster if the shelf is sitting in the sunshine or out in the rain Some specialty types of cable whose codes and specifications are nominally similar can have an outer jacket that is not guaranteed to have good chem ical stability The jacket may be especially resistant to some chemicals but less resis tant to others Specifically i inthe 1950s there was a lot of military surplus cable similar to RG 58 and RG 74 that did not have good Stability As the outer jacket degraded the inner insulator was chemically degraded and the cable s UHF attenua tion was degraded In other cases the outer shield was chemically attacked and cor roded and its conductivity g ot worse and its UHF attenuation was also impaired Most of that old cable has died and you can t even find it in junk piles any more But there are still specialty cables being made and sold now that do not last as well or age as gracefully as you would expect a good wire to do If you select a cable to be espe cially resistant to one kind of chemical it may be less resistant than normal to the attack of other ordinary chemicals So you should be aware that even in something as simple as a wire there may be more problems tha
162. dditional antireversal recti fiers to your circuit would probably be a good idea Now in what cases should designers not incorporate your series diodes without adding parallel antireversal diodes The answer is low voltage high current or well regulated applications Of course you are correct that the rectifiers must be able to carry the short circuit current of the regulator Fortunately 1N5400s which carry 3 A are only 19 each Best regards and thank you for your comments You are wise to question au thority RAP Czar of Floobydust PS Whoops Let s say you put diodes in series with your path with a couple bat teries for supplies Now if you short out the 14 V busses how can you be sure that your rectifier won t get ruined Maybe you need to put a fuse in series with each diode Maybe I ought to put a fuse in series with my supply busses It just goes to show things are not simple any more I recently invented a circuit Ref 1 to fulfill the request of a customer who didn t want the disadvantages of either approach neither the series losses nor the shunt crowbar problem In Figure 13 5 the FET turns ON when the battery s polarity is correct but turns OFF when the battery is reversed The Ron of inexpensive FETs is very low these days so the series losses can be a LOT better than the 0 5 V of a diode Yes this circuit looks funny Yes it is correct and yes it works well Dear Bob To make component
163. ded because with drives the gate inputs excessively as the waveforms of b indicate v a b 0 1 uF 0 1 pF T NOTE ALL GATES ARE CMOS Figure 10 4 The addition of resistors to the circuit of Figure 10 3 a helps reduce overdrive but the addition of diode clamps in the shunt leg of the attenuators b is even more effective If you have two 2 input NAND gates available the circuit of c is the best implementation the values shown it over 125 A Time to Ask Probing Questions 126 10 The Analog Digital Boundary parts if you overdrive the inputs much below ground Refer to the letter by Mr J Koontz in Chapter 13 A number of years ago I was watching the negative transition of an ordinary TTL gate and I was especially concerned by the way it was overshooting to 0 4 V I set up an attenuator with 1 pF in the input leg Figure 10 5 and was astounded to see that if I looked at the waveform with an ordinary 1 1 pF probe the overshoot occurred but if I disconnected the probe from the gate output and connected it to the attenuator output the overshoot went away So even if you use a fairly high impedance probe you should always be prepared for the possibility that by looking at a signal you can seriously affect it even if what you re looking at is as mundane and supposedly robust as a TTL output Consequently you should be prepared to build your own special purpose probes so you can see
164. digit 149 high input impedance 15 17 36 ohmmeters 17 32 33 52 53 149 155 171 digital logic See digital ICs digital loop See loop digital digital to analog converter DAC 126 128 problems with AC gain errors 128 computer models inadequate 128 DC gain errors vs Vog 128 de glitchers 128 multiplying DACs MDACs 128 glitches 128 noise rejection on bit lines 126 128 130 on power supplies 126 127 diode clamps 37 67 69 118 119 124 133 156 159 diodes 20 48 55 65 74 119 124 140 141 150 163 164 185 196 IN87 196 IN645 65 196 IN914 65 70 196 See also 1IN4148 4 IN4001 1N4002 67 196 1N4148 66 67 68 i IN914 0 1N5400 164 1 anti reversal rectifiers See under power supplies forward voltage vs Ip 196 197 types of i computer diodes 61 69 oy i transistors used as diodes fast tum off IT fast turn on 67 68 FETs used as diodes 66 germanium 65 ae ni gold doped 67 00 00 5 04 whe hi tefieiency rectifiers 65 67 r is diodes 69 196 1 direction 70 185 bad conductance even at zero bias 69 69 70 ve conductance unpredictable at at Various cirrents 65 196 197 vi on surge of charge failure modes 70 improvements 68 inductance 69 eppi ae i schativiey 70 73 110 156 long ga after turn off 67 G i a ot ts pei
165. dinary DIPs So when you think you can pack even more of a good thing onto a board by going to surface mount Small Outline packages you may also pack in more trouble There is no specification on this on any data sheet So a SPICE analysis has no way to warn you about this potential problem Even a bread board does not necessarily tell you about this The actual prototype units on the real PC boards must be checked out These days just about every manufacturer s monolithic op amps will survive a short from the output to ground Hybrids are often unprotected But it s not always clear whether an op amp will survive a short circuit to the Positive or negative supply or if so for how long You may have to ask the manufacturer and you can expect some kind of negative answer You ll be told to avoid overheating the device above its absolute maximum junction temperature Even if an amplifier or regulator does recover fairly quickly from current limit nobody will guarantee that it won t oscillate when in current limit Nor will the manufacturer have much knowledge about how the circuit recovers from the thermal gradients caused by current limit If an op dmp survives a high power overload it s not fair to ask the device to recover its full accu racy very quickly The most you really can ask for is that it survives with no degrada non of rehability that s the standard Some op amps such as LMJ2 and LM 0 and most voltage regulators Gan
166. dings both bifilar and twisted pairs have much better mag netic coupling and less leakage inductance than do well separated primary and sec ondary windings As the magnetic coupling improves the capacitance between wind ings increases but high capacitance between windings is often an undesirable effect in a transformer An experienced transformer designer weighs all the tradeoffs and knows many design tricks for example the use of special pi windings and Litz wire Mostly you should know that these special techniques are powerful if you ask the transformer designers the right questions they can do amazing tricks I recently read about an engineer who designed an elegant shield made of mu metal However the shield was difficult to install so the technician had to tap on it with a hammer When the engineer operated the circuit the shielding seemed nonex istent as if the shield were made of cardboard After a lot of studying the engineer realized that the mu metal which costs about 2 per 15 Square inches the same as a 2 bill had been turned into perfectly worthless material by the pounding and ham mering In retrospect the engineer had to admit that the mu metal when purchased was prominently labelled with a caution against folding bending or hammering So remember in any area of electronics there are problems with inductors and magnetic materials that can give you gray hair References 39 Consider the Effects of Magne
167. diodes were 1N914s and their leakage currents were increas ing from 10 nA at room temperature to about 8 2A at the high temperature And remember that the conductance of a diode at zero voltage is approximately 20 to 30 mS MA X h EAKAGE That means each of the two diodes really measured only 6 kQ Because the impedance at each input was only 6 kQ the op amp s gain fell by a 70 6 Understanding Diodes and Their Problems factor of four even though the diodes may have only been forward or reverse biassed by a millivolt When we substituted collector base junctions of transistors for the diodes the gain went back up where it belonged Thus you cannot safely assume that the impedance of a diode at zero bias is high if the junction s saturation current is large For example at 25 C a typical IN914 will leak 200 to 400 pA even with only 1 mV across it Therefore a 1N914 can prove unsuitable as a clamp or protection diode even at room temperature despite having virtually no voltage biassed across it in even simple applications such as a clamp across the inputs of a FET input op amp How can diodes fail Well if you were expecting a diode to turn ON and OFF but instead it does something unexpected of the sort I have been mentioning that unexpected behavior may not be a failure but it could sure cause trouble Further you can kill a diode by applying excessive reverse voltage without lim iting the current or by feeding it excessiv
168. dy thing to have is a 60 Hz passive twin T notch filter in a small Pomona box with dual banana plugs for input and output Another handy thing to have is a 20 dB high impedance amplifier in another Pomona box The circuit in Figure 13 1 works at audio frequencies The CMRR of an op amp is not a constant function of the common mode voltage This inconsistency often dominates nonlinearity in noninverting circuits vee oss ERE gt pe pens eS ere i Letters to Bob 157 27 Getting some engineers to hold design reviews is hard By law our drafting depart ment will not start a PC board design until the designer hosts a review The drafting supervisor can easily enforce this policy because he does not report to the first level engineering managers The moral indignation of the designer s peers informs the quality of the review gt 28 Those partial but detailed schematics National Semiconductor sometimes places on data sheets provide valuable insight into how a part may be acting in unusuat circum stances Please encourage National to continue the practice Roy McCammon 3M Dynatel Austin TX Hello to Mr McCammon Well I thank you for your comments many of which are excellent I will comment on some of them individually but as a collection they are the best new ideas that anybody has given me 1 22 pF 174K 1M 100K 174K 2 UF 1 AAA 0 Q O 0 600 2 900 Pee een 0 122uF i
169. e a stibstitution i is made the replacement part is a good place to start looking for trouble A capacitor with higher than planned for ESR can cause a feedback loop to oscillate for example when a capacitor without extended foil construction is substituted for one with such construction Substitution of capaci tors with higher ESR than the designer intended can also cause filters to fail to prop a b BODY OF BODY OF CAPACITOR LEAD LEAD EXTENDED FOIL EXTENDED FOIL LEAD FILM FILM a When the tabs connect to one end of a long foil some elements of the capacitor will be 10 or 20 feet away from the leads The series Rs and Ls are poor This construction was adequate for low fidelity audio circuits but is uncommon these days b When the exposed edges of the extended foils are crimped together no element of the capacitor is more than an inch or two from the leads and connections Most film capacitors are made with extended foils these days 44 4 Getting Down to the Component Level Capacitor Problems erly attenuate ripple Another consequence of excessive ESR is the overheating and failing of capacitors capacitors may be passive components but they are not trivial Not only does extended foil construction lower a capacitor s ESR it also lowers the component s inductance As a friend Martin Giles pointed out after reading a draft of my troubleshooting text Pease you understand
170. e good for any serious work So if you insist on using these slabs of trouble you can t say f didnt warn you 13 Letters to Bob My series in EDN on troubleshooting generated myriad letters from readers Because so many of the letters contained worthwhile troubleshooting tips and amusing per sonal anecdotes we decided to collect some of the best letters into a chapter along with my replies and interjections The tips just keep on coming Dear Bob Here are some tips and gotchas A significant source of noise in my lab is the ever present video display terminal It couples especially well to audio frequency transformers head off a lot of trouble by providing RF bypass on audio and DC circuits Their audio and low frequency only inputs can pick up AM radio Having music come out of a speaker that is supposed to be a monitor on a telephone circuit is very bad form My computer brethren frequently fail to consider what happens during reset I saw a thermal printer catch on fire once when its internal uP was reset The reset 3 stated the printhead s drivers which allowed all of them to turn on continuously Later a software bug turned them on continuously again I finally made the printhead com puter proof by capacitively coupling the drivers so that the uP had to produce a con tinuous sequence of pulses to keep the heads turned on My inexperienced brethren frequently forget to calculate total power sup
171. e forward current When a diode fails it tends to short out becoming a small blob of muddy silicon rather than an open cir cuit I did once see a batch of 1N4148s that acted like thermostats and went open circuit at 75 C but such cases are rare these days One of the best ways to kill a diode is to ask it to charge up too big a capacitor during circuit turn on Most rectifiers have maximum ratings for how much current they can pass on a repetitive and on a nonrecurring basis I ve always been favorably impressed by the big Motorola Phoenix AZ books with all the curves of safe areas for forward current as a function of pulse time and repetition rate These curves aren t easy to figure out at first but after a while they re fairly handy tools Manufacturers can play tricks on you other than changing processes If you expect a diode to have its arrow pointing toward the painted band sometimes called the cathode by the snobbish and the manufacturer put the painted band on the wrong end your circuit won t work very well Fortunately reverse marked diodes are pretty rare these days But just this moming I heard an engineer call the pointed end of the diode an anode which led to confusion and destruction Sigh Once I built a precision test box that worked right away and gave exactly the right readings until I picked up the box to look at some waveforms Then the leakage test shifted way off zero Every time I lifted up the box
172. e right thing is to engi neer the right amount of speed for your circuit High speed fast recovery and ultra fast diodes are available The Schottky rectifiers are even faster but not available at high voltage breakdowns When you start designing switching regulators at these speeds you really must know what you are doing Or at least wear safety goggles so you don t get hurt when the circuit blows up Turn Em Off Turn Em On Computer diodes like the 1N914 are popular because they turn OFF quickly in just a few nanoseconds much faster than low leakage diodes What isn t well known is that these faster diodes not only turn OFF fast they usually turn ON fast For example when you feed a current of 1 0 mA toward the anode of a 1N914 in parallel with a 40 pF capacitance 20 pF of stray capacitance plus a scope probe or something similar the 1N914 usually turns ON in less than 1 ns Thus the Ve has only a fe millivolts of overshoot But with some diodes even 1N914s or IN4148s from some manufacturers the forward voltage may continue to ramp up past the expected DC level for 10 to 20 ns before the diode turns ON this overshoot of 50 to 200 mV is quite surprising Figure 6 2 Even more astonishing the Vp overshoot may get worse at low repeti tion rates but can disappear at hi gh repetition rates Figure 6 2b d I spent several hours once discovering this particular peculiarity when a frequency to voltage conve
173. e superimposed 20 kHz waveforms are all invariant with frequency except for the bad 1N4148 sions to improve the product One man s improvement is another man s poison Thus you must always be alert for production changes that may cause problems When manufacturers change the diffusions or the process or the masks they may think that the changes are minor but these changes could have a major effect on your circuit Many circuits obviously require a diode that can turn ON and catch or clamp a voltage moving much faster than 20 V s Therefore if you want any consistency in a circuit with fast pulse detectors for example you ll need to qualify and approve only manufacturers whose diodes turn ON consistently So as with any other unspec ified characteristic be sure to protect yourself against bad parts by first evaluating and testing and then specifying the performance you need Also if you want to see Other Strange Things That Diodes Can Do to You 69 wee wee ww ee oe a Figure 6 3 Even though the diodes in the first stage of this op amp are forward or reverse biassed by only a millivolt the impedance of these diodes is much lower than the output impedance of the first stage or the input impedance of the second stage at high temperatures Thus the op amp s gain drops disastrously fast turn on of a diode circuit with low overshoot you must keep the inductance of the layout small It only takes a
174. e temperature sensor is strictly a function of the mechanical and thermal mounting of those components This relationship would hardly be amenable to computer modelling or analysis Comparators Can Misbehave Saying that a comparator is just an op amp with all the damping capacitors left out that is an oversimplification Comparators have a lot of voltage gain and quite a bit of phase shift at high frequencies hence oscillation is always a possibility In fact most comparator problems involve oscillation Slow comparators such as the familiar M339 are fairly well behaved And if you design a PC board layout so that the comparator s outputs and all other large fast noisy signals are kept away from the comparator s inputs you can often get a good clean output without oscillation However even at slow speeds an LM339 can oscil late if vou impress a slowly shifting voltage ramp on its differential inputs Things can get even messier if the input signals sources have a high impedance gt gt 10 kQ or if the PC board layout doesn t provide guarding In general then for every comparator application vou should provide a little hysteresis or positive feedback from the output back to the positive input How 9 Quashing Spurious Oscillations bid O 200 F THERMAL CUTOUT 500W HEATER BLOWER me je gt POLE LLL Eee AIR FLOW T AIR FLOW 82 a V 0 75V AT 75 C 1 aF T O 15V C 7 C 2 uF 0 33 uF MYLAR POLYP
175. e to fix things It s necessary to pass this type of wisdom along to each generation to keep common sense in the field of engineering development o However as with all us old timers you slipped on a few points to wit Eyelets are out especially in multilayer boards They are still OK for 2 sided experimental boards and single sided boards where changes in components are desired In the latter case an eyelet socket is applied to the finished circuit to hold everything stable Using an eyelet in a multilayer board will distort the plated barrel frequently causing separation from the inner layers of copper This shows up only as a temperature induced intermittent problem during testing It will become a hard failure only when it gets to the customer or is in the field Good plating shops can create excellent plated through holes at a reasonable cost today so give them a try The use of lock washers on printed wiring boards is fraught with danger We spent a lot of money at one company in trying to find out why our screws kept backing out of their holes causing their boards to come loose in their mounts The problems happened during a thermal vibration test Running the same boards through the vi bration test without the thermal test did not loosen the screws Thermal cycling was the culprit It caused the board to expand and near the glass transition temperature 125 C it caused the board to deform to relieve the stress When the assembly
176. e touchy Ones that do not mention this fact in their data sheets I rarely work with wrapped wire stuff but I bet a lot of people get fooled by bad daisy chaining of power supply runs and lousy power supply bypassing whether for linear or digital ICs I rarely find batteries necessary but in extreme cases they are useful I rarely find notching out 60 or 120 Hz interference necessary I usually subtract the 60 Hz noises visually from a scope trace Yes portable preamps are often useful Just as I was saying back in Chapter 8 it s silly to assume that the CM error is linear Design reviews are a good idea But even if the circuit design is perfect I find the lay out to be pretty critical So a beer check by all your buddies is awfully important too RAP Dear Bob AS a practicing technician for many years I want to comment on one or two things I read in your series and perhaps pass on an experience or two On page 130 of the August 17 1989 article new Chapter 7 you mention the pos sibly harmful side effects of drawing base current out of a transistor Most if not all of the switched mode power supplies I have come across appear to do just that to switch the transistor off more quickly by removing carriers from the base This technique appears to work well in practice I have used the technique in many of the inverters I have designed and on the face of it at least there doesn t appear to be any component
177. e under oscilloscopes paperwork 8 126 130 139 Paralysis by Analysis 6 214 passive components 26 49 patents xi 78 PC Boards See printed circuit boards Pease s Principle 99 112 137 175 178 phase detector See under digital ICs phase shift 89 93 99 101 102 106 111 116 128 143 151 picoammeter See under current meter Philbrick See Teledyne Philbrick philosophy 1 12 99 118 169 171 Photo couplers See Opto isolators plans logical 5 12 151 175 pliers 9 23 Popsicle stick with capacitors 164 165 potentiometers See resistors adjustable potting epoxy 54 power dissipation 137 178 power supplies 1 2 17 18 18 20 22 34 37 39 41 57 58 89 106 110 111 118 124 139 163 See also under voltage regulators power supply reversal 118 119 163 164 163 164 anti reversal rectifiers on each PC board 118 119 163 164 anti reversal rectifiers at each power supply 118 119 163 164 improve reliability by leaving out parts 118 power saving circuit with MOSFET 164 164 series diodes 163 163 power transformers See under trans formers preamplifier seismic x printed circuit boards PC boards 50 59 111 113 coupling capacitors 56 eyelets 55 56 170 foil 50 51 55 57 ground plane 56 57 162 layout 55 58 materials 50 51 51 Fish plate See phenolic FR 4 fire retardant 51 G 10 See glass epoxy glass epoxy 50 51 51 phenolic 50 51 51 polyimide 51 Tefl
178. e working on sim Troubleshooting by Phone A Tough Challenge 9 ilar projects Sometimes I hang a copy on the wall to warn all my friends Some times I send a copy to the manufacturer of a component that was involved If you communicate properly you can work to avoid similar problems in the future Then there are other things you can do in the course of your investigation When you find a bad component don t just throw it in a wastebasket Sometimes people call me and say Your ICs have been giving me this failure problem for quite a while I ask Can you send me some of the allegedly bad parts And they reply Naw we always throw them in the wastebasket Please don t do that because often the ability to troubleshoot a component depends on having several of them to study Sometimes it s even a case of NTF No Trouble Found That happens more often than not So if you tell me Pease your lousy op amps are failing in my circuit and there s actually nothing wrong with the op amps but it s really a misap plication problem I can t help you very well if the parts all went in the trash Please save them at least for a while Label them too Another thing you can do with these bad parts is to open them up and see what you can see inside Sometimes on a metal can IC after a few minutes with a hacksaw it s just as plain as day For example your technician says This op amp failed al
179. eally big digital ICs can get along without this chore ography technique maybe they have other mnemonic tools but this one works for me I first developed this approach the time I designed a 12 bit monolithic ADC the industry s first back in 1975 I had this big choreography about 33 inches square and the circuit worked the first time because the choreography helped me avoid goofing up any digital signals Right now I m working on a system with one chore ography in nanoseconds and tenths of nanoseconds linked to a second one scaled in microseconds and a third one scaled in seconds I hope I don t get lost Of course this tool is partly for design but it s also a tool for troubleshooting and for planning so you can avoid trouble in the first place D A Converters Are Generally Docile D A converters are pretty simple machines and they can usually give excellent re sults with few problems If the manufacturer designed it correctly and you are not misapplying it a DAC usually won t cause you much grief One area where DACs can cause trouble however is with noise Most DACs are not characterized or guaranteed to reject high frequency noise and jumps on the sre ae et ey tiie Loma bania Ce ge ee E T D A Converters Are Generally Docile 127 Figure 10 5 An ordinary high impedance THIS PROBE CAUSES TTL GATE TO OVERSHOOT AT HIGH TO LOW TRANSITION probe can cause TTL outputs to appear to overshoot when you
180. ear pertiaps within I or 2 least significant digits Other DVMS claim to have the advantage of higher conversion speed this higher speed may be of no nyse to tothe bench engineer butit is usable when the DVM is part ofan automated idata aoquisition system These faster instruments usually use a successive approximation or recirculating remainder conversion scheme both of which are not inherently linear but depend on well trimmed components for linearity I have seen several DVMs that cost more than 1000 and were prejudiced against certain readings One didn t like to convert 15 mV it preferred to indicate 14 or 16 One time I got a call from an engineer at one of the major instrument companies He wondered why the Voltage to Frequency converter he made with an NSC LM331 was showing him poor linearity worse than the guaranteed spec of 0 01 I told him that was strange because if it was true it was the first LM331 to have poor lin Digital Meters Not So Bad and Sometimes Better Than That 149 earity from the first couple million we had produced I advised him to check the capacitors and the op amp waveform and to call me back because if he had a part that didn t meet specs I wanted to get my hands on it The next day he called me back feeling very sheepish and embarrassed He ad mitted he had been using a prototype DVM designed by his company and because it was a prototype it was not exactly under calibration control It wa
181. ease not at all easy to guess Sometimes if you replace the high K ceramic with an NPO or a film capac itor it will solve the problem Recently we ran a picoammeter and when the power supply lead ran near the summing point there was a certain amount of charge Q C X V When the wire was vibrated at line frequency a 60 Hz current I V X dC dT flowed into the input The current stopped when we guarded the 5 V bus away from the input and we also added shock mounting for the whole assembly to keep out all vibrations There are probably a few other ways to get 60 Hz noises into a circuit so you must be prepared to exercise ingenuity to search for nasty coupling modes But if the os cillation is at exactly line frequency and it synchronizes with the line synch mode of your scope then it is certainly not a real oscillation Now I have seen 59 Hz oscil lations that would fool you into thinking they were at 60 Hz but that is quite rare It just goes to show that there are many noises to keep you on your toes Some are os cillations and some are oscillations You can best analyze the design of a slow servo mechanism such as that in Figure 9 2 with a strip chart recorder because the response of the loop is so slow A storage scope might be OK but a strip chart recorder works better for me You might wish to analyze such a servo loop with a computer simulation such as SPICE but the thermal response from the heater to th
182. easy to find weeks later that the computer told you a lie Originally published in Electronic Design November and December 1990 oS Shave a circuit that shows bad conve 204 G More on Spice boards but I like them because they often offer a greater insight and understanding as to what s really going on so if you survive their problems you are smart enough to keep out of other kinds of trouble But that s just a bunch of philosophical stuff The thing that makes me nervous about SPICE is that it was largely designed by a group of grad students Laurence Nagel and others at Berkeley back in 73 Now when you find a problem a discrepancy a glitch a flaw an error that seems to be built into SPICE can you go back to the people who designed it Hardly There is no continuity There are some people who claim to support SPICE but I don t always agree with their statements My biggest gripe with SPICE is its lack of convergence The ordinary SPICE 2G6 has all sorts of problems even if you don t use FETs We find that FETs usually make the convergence situation really unhappy For example one time I had a mod erate sized circuit with about 33 bipolar transistors and it didn t converge well Then all of a sudden one day it started to converge beautifully and quickly I was so im pressed I backed up to find the scene of the crime I tried to duplicate all the changes I had made since I last had problem
183. ecessarily cause harm But and the readers should be aware that sometimes ss they may cause harm so don t hack around if you don thave to As for tantalum capacitors Tve en very few fail with no o provocation I ve used a lot of cheap tantalum capabitons a and they must have been more reliable than I de served To ensure that they survive reversal I suggest the arrangement in Figure 13 3 Using a current probe to find short circuits is a viable technique but I don t under stand how you can clip a current probe around a board trace I find that my DC mi such DC shorts I I had to troubleshoot a lot of boards I m sure one eof your audio output milliwatt detectors w uld be terribly useful Although I ve never seen a problem with floating TTL gates you are correct to caution against circuits like the one in Figure 13 2 RAP Dear Bob As an old hand with 30 years in the business have run into some anomalies you did not mention LS logic is totally unforgiving of negative undershoots at its inputs The worst chips have seen are 74LS86s which hang up for microseconds totally confusing other circuitry The second worst is the 74LS75 which can go into either logic state after a negative undershoot but will recover upon the next clock pulse l have seen circuits with a 7470 73 76 107 109 110 or 111 that remember 162 13 Letters to Bob highs on the Preset and Reset l
184. ecisely the wrong time on the clock s rising edge making the output pulse narrower or wider than normal You can solve the problem by using two flip flops with the clocks separated by a delay counter makes a carry from 01 l l to 11000 for a few nanoseconds the output code will be 0000 and the decoder can spitiout anatrow pulse of perhaps 6 8 ns in duration corresponding to 0000 Even if yoiiare observing with a good scope such a pulse can be just narrow enough to escape detection If the decoder were merely feeding an LED display you would never see the sub microsecond light pulse but if the de coded output goes to a digital counter a false count can occur In digital systems engineers often use logic analyzers storage scopes and scopes with very broad band widths to look for glitches or runt pulses and the conditions that cause them In analog systems you may not have a logic analyzer but these nasty narrow pulses often do exist and you have to think about them and be prepared to look for them Another thing to know about digital ICs is that many CMOS ICs have the same pinouts as TTL parts For example the 74193 TALS 193 and 74C 193 have the same pinouts On the other hand some of the older CMOS parts have pinouts that differ from those of similarly numbered TTL devices The 74C86 s pinout is the same as the 741 86 s but differs from the 7486 s Beware have made up a list of all these ICs with nonstandard pinouts
185. en differential input signal enters this band slowly from either side the output can get very noisy sometimes rail to rail because of amplified noise or oscillation The osci lationi an continue even if the input voltage goes back outside the range where the circuit started oscillating Consequently you could easily set up your own test in which your data for offset voltage Vog doesn t agree with the manufacturer s measured or guaranteed values Indeed it can be tricky to design a test that does agree This zero crossing detector has no DC hysteresis but 50 mV of AC coupled hysteresis 100K 0 001uF 114 9 Quashing Spurious Oscillations Figure 9 4 When oscillations get nasty you need a scope with pinpoint triggering to help you study the problem For my tests of comparator Vog I usually set up a classic op amp oscillator into which I build a specific amount of hysteresis and a definite amount of capacitance so that the unit will oscillate at a moderate controlled frequency If you re curious refer to Appendix D which is not trivial Another way to avoid Vos trouble with comparators is to use a monolithic dual transistor as a differential amplifier preamplifier stage ahead of the comparator This preamp can add gain and precision while decreasing the stray feedback from the out put to the input signal Refer to the example of a fairly slow precision comparator in LB 32 Ref 1 Common Mode Excursions Unpredic
186. en called 8 lead TO 5 or 8 pin mini dip or SO 8 all have the same pinout as far as I have ever seen Examples are LM158 LM833 LM1558 LM6218 LF412 LF442 LF453 LMC662 LPC662 etc etc There are no exceptions that I know of tell me if I am wrong in this field The old dual LM747 in its 10 pin package or the dual LM709 in 14 pin packages or any dual in any package with more than 8 pins they do not count Notes on Quad Amplifiers All quad operational amplifiers in 14 pin DIP packages all have the same pinout EXCEPT FOR THE LM4136 The standard ones include LM124 LM324 LP324 LM349 LM837 LF347 LF444 LMC660 LPC660 etc There are very few exceptions in this field Note Quad comparators such as LM339 do not have the same pins as quad op amps Even the power supplies are different Quad Norton amplifiers such as LM2900 are also nonstandard Figure C i Appendix C Understanding and Reducing Noise _ Voltage on 3 Terminal Voltage Regulators Erroll H Dietz Senior Technician National Semiconductor Corporation The usual approach to reducing noise on 3 terminal voltage regulators has been to simply place capacitors on the output and on the adjust pin for adjustable regulators As it turns out the addition of output capacitance on most voltage regulators may reduce the noise over a broad frequency range but may increase noise within a narrow frequency range Since the output impedance of most 3 termi
187. en the one who did the most editing In addition to my self the technical editors were Senior Editor Charles H Small and Anne Watson Swager now EDN s East Coast regional Editor in Wynnewood Pennsylvania The nontechnical editing of every article in the series was done by Associate Editor Julie Anne Schofield EDN s Art Department directed by Ken Racicot handled the Foreword ix graphic work in Newton In addition many of the photos were taken by Bob s col leagues at National Semiconductor Despite its length this list of contributors isn t complete A magazine is the work of many people and any list inevitably omits someone What should be apparent from the above is that working on the series was fun When he first interviewed me for this job at EDN Roy Forsberg now publisher of another Cahners magazine Test and Measurement World remarked that a technical editor s job was the best job in all of electronics At the time I passed off Roy s com ment as somewhat self serving But working on the Pease series dispelled any doubt I might have had about how much fun the job can be Working on the series was a great experience As important as Bob s tips on troubleshooting are I hope the se ries and now the book provide more I hope they communicate the exhilaration felt by all of us who were involved with the project Dan Strassberg Associate Editor EDN Magazine Acknowledgments I would like to dedicate this book
188. ences bandwidth 14 15 22 84 85 87 95 98 102 103 123 168 See frequency response or step response bandwidth too much 87 88 168 205 bankrupt 8 batteries rechargeable 18 74 76 books about batteries 76 deep discharge 75 disposal recycling 75 lead acid 75 76 float voltage 75 tempco of charging voltage 75 NiCads Nickel Cadmium 18 74 16 shorted cells 75 9 volt 18 battery charger 37 38 74 137 battery powered equipment 18 24 25 34 74 150 156 159 163 164 164 198 beads See under inductors bedside medicine 12 beer check 4 4 159 beta 77 79 86 105 bias current input current of opamp See under operational ampli fier biassing 43 65 74 77 79 96 103 115 116 141 144 161 164 168 176 196 bias current input current of op amp See operational amplifiers binary 5 6 bond wires 9 78 80 192 bond of die 165 books Analog Devices Data Converter Handbook 2 13 90 107 Bulleid H A V 3 13 Dostal Jiri 2 3 2 Frederiksen Tom M 103 107 Kidder Tracy 59 64 Sinclair lan 33 Smith John L 2 1 Williams James M Analog Circuit Design Art Science and Personality book on Switch mode power sup plies vii x 1 books on batteries 76 National Semiconductor Databooks 107 113 119 134 140 154 Borger Frank 168 169 Bowman Scott 153 breadboards 6 39 84 103 105 112 153 178 182 183 185 203 205 207 buffers analog 96 97 116 117 150 def
189. engineer Ref 9 points out that thermal tails can be a major source of error in fast signal amplifiers and that innovative circuit design can minimize those overdrive recovery errors If you have ever studied the gain errors of older OP 07 type amplifiers you have probably recognized that these errors and nonlinearities were caused by thermal errors which were related to a bad layout These days most OP 07s have a better layout and the thermal distortions have been banished Another characteristic that is not specified or discussed is the change of offset voltage vs stress This is most noticeable on BIFET amplifiers as the FETs are much more sensitive to stresses in the silicon die than bipolar transistors are When you install and solder a plastic DIP op amp in a PC board and then warp the board you can monitor the Vog and watch it shift Some amplifiers are better than others of similar types It has a lot to do with the layout and also with the die attach If you need the lowest offset watch out for this If the board is vibrated the AC warping and stress can cause microphonic AC noises too CER DIP amplifiers have a stronger ceramic base and have a little less of a problem Here s where it gets really wild Buy your BIFET amplifiers in an SO Small Outline surface mount package The smaller plastic package is able to take up even less of the stress and the die gets warped even more and the change of Vos gets even worse than in or
190. er 59 stainless steel solder 59 tin lead solder ordinary solder 58 59 problems with solder 153 cold soldered joints 8 24 58 59 186 joints not soldered 59 186 solder shorts 21 24 50 186 unsuitable soldering iron 51 wave soldering 46 59 wrong type of solder 59 See also under solder wick solder sucker solder mask 50 51 55 solder sucker 23 solder wick 23 soldered joints 57 soldering iron 22 23 32 51 82 110 150 158 159 solderless breadboards 153 spare parts 20 25 137 172 spectrum analyzer 25 SPICE See computer modelling spreadsheets 166 spring loading 48 start up circuits 7 140 142 analog 7 141 142 Czar of Start up circuits 7 digital 140 141 production tests for start up 141 142 steam engines See locomotives step response 15 15 16 89 93 94 10 411 115 116 122 131 133 139 191 194 207 See also under bandwidth and frequency response storage oscilloscope See under oscilo scopes strip chart recorder 111 112 Sturgeon Bill 165 166 substitution boxes See under resistors capacitors R C boxes substitution technique See under troubleshooting techniques Sunday School 12 superheater 3 switch mode regulators 2 18 22 41 66 67 73 87 138 141 59 183 184 See also under voltage regulators linear 1M2575 2576 2577 2578 2579 simple 215 switchers TM 138 139 183 consulting engineer need for 139 current limiting 139 problems
191. er don t think that you must neces sarily be wrong You can usually get an opinion from another instrument to help prove where the truth lies Don t automatically believe that a piece of data must be correct just because it s digital And be sure to hold onto the user s manual that comes with the instrument It can tell you where the guaranteed error band of the DVM gets relatively bad such as for very low resistances for very high resistances for low AC voltages and for low or high frequencies Most digital voltmeters have a very high input impedance 10 000 MQ typ for small signals However if you let the DVM autorange at some level the meter will automatically change to a higher range where the input impedance becomes 10 MQ Some DVMs change at 2 V or 3 V others at 10 or 12 or 15 V and yet others at 20 V As I mentioned in the chapter on equipment I like to work with the DVMs that stay high impedance up to at least 15 V But the important thing is to know the voltage at which the impedance changes A friend reminded me that his technician had recently taken a week s worth of data that had to be retaken because he neglected to allow for the change of impedance I think I ll go around our lab and put labels on each DVM Still DVMs are very powerful and useful instruments often with excellent accu racy and tremendous linearity and resolution often as good as ppm I ve counted some of these ultralinear met
192. er around in a circuit with high or lethal voltages One of the easiest ways to inadvertently cause a very high frequency oscillation is to run an emitter follower transistor even a nice docile type such as a 2N3904 at an emitter current of 5 or 10 mA In such a case you can easily get an oscillation at a few hundred megahertz So although a good 100 MHz scope cannot spot this kind of oscillation the resulting radiated noise can cause other circuits to go berserk and can cause an entire system to fail tests for radiated electromagnetic noise For example when the first personal computers were being designed designers needed a RESET function for their processor Several designers decided quite independently to use the simplest cheapest possible RESET circuit as shown in Figure 9 1 When they finished their designs and sent the prototype computers to be approved by the FCC these designs all failed badly Why Because the little tran sistor would run at over 10 mA and with a bypass capacitor at its base the transistor would oscillate at a very high frequency The frequency was so high that none of the designers noticed it but as the transistor sprayed around a lot of RF energy up at a couple hundred megahertz the FCC exarhiners noticed it causing the computers to fail the tests for radiated RFI They all had to go back and fix it How For such an emitter follower a 50 or 100 Q carbon resistor directly in series with the base of t
193. er times it has other functions No comment Nonstandard Pinouts ee e LM709 o UMIOILLM30I LM107 LM307 108 LM308 e LM118 LM318 0 Y LMI LM LM10 oe 6 S002 eee Vs e LM741 LM143 100 KO LM776 100 KQ e LM4250 100 kQ e LF351 LF411 LF441 Figure B I Type Pins and 5 Trim to Vg 188 B Operational Amplifiers with Nonstandard Pinouts 189 LF451 in SO 8 package LF13741 LH0022 LH0042 LH0052 TYPE ll 25 k 0 pot to V from pins and 5 LF156 LF356 LF155 LF355 LF157 LF357 LF400 10 kQ Figure B 2 Type Il Pins and 5 Trim to V TYPE tll 10 kO pot to V from pins and 8 LM11 100 kQ LM112 100 kQ LM607 LM627 LM637 LM725 OP 07 20 kQ Vs Figure B 3 Type lil Pins and 8 Trim to Vs TYPE IV 100 k0 pot to V from pins and 8 LM6161 LM6361 LM6162 LM6362 LM6164 LM6364 1 M6165 M6365 Figure B 4 Type lV Pins i and Trim to W 190 B Operational Amplifiers with Nonstandard Pinouts Now it is true that I have only listed NSC parts in here Maybe someday we will list all the parts in the world but I haven t the time to do that now I Il leave enough space for you to put your notes alongside these for the sake of completeness Note in no case can I vouch for which direction you turn the pot to increase Vos That s for you to determine 7 Notes On Dual Amplifiers All DUAL operational amplifiers in 8 lead TO 99 packages oft
194. erage user of IC regulator circuits However this inductive output impedance coupled with an output capacitor to ground can produce a noise peak within a narrow frequency range This noise peak coincides with the resonant frequency of the inductive output impedance of the regulator and the load capaci tance on the output Figure C 2 shows typical noise peaks produced by an LM317 and various capacitive loads The frequency range of the noise spike does not extend much above 100 kHz nor below 10 kHz due to ohmic losses in the added output capacitance and the inductance of the regulator The frequency is predictable according to 1 2 7 LC The magnitude of this noise spike will vary with the Q of the resonant circuit which is mainly dominated by the series resistance of the output capacitor and is proportional to the gain of the reference voltage For example a good uF polypropylene capacitor with an ESR of 20 mQ at 30 kHz will have a noise peak three times greater than that of the same value tantalum capacitor which has an ESR of 1 or 2 Q The noise peak is also reflected back to the input of the regulator and is about 20 dB down from the output level It is a little known fact that the output impedance of 3 terminal regulators can vary greatly with load current and the programmed output voltage which in turn varies the noise peak resonant frequency As load current increases the gm of the regulator s output transistor will also increase This i
195. eral purpose test equipment What equipment can you buy for troubleshooting I ll cover that subject in considerable detail in the next chapter For now let me observe that if you have several million dollars worth of circuits to troubleshoot you should consider buying a 100 000 tester Of course for that price you only get a machine at the low end of the line And after you buy the machine you have to invest a lot of time in fixturing and software before it can help you Yes you can buy a 90 tester that helps locate short circuits on a PC board but in the price range between 90 and 100 000 there isn t a lot of specialized trouble shooting equipment available If you want an oscilloscope you have to buy a gen eral purpose oscilloscope if you want a DVM it will be a general purpose DVM 1 must say I recently re read Mr Dostal s book and it is stil just about the best technical book on operational amplifiers It s more complete more technical but less intuitive than Tom Frederiksen s Intuitive IC Op Amps Of course for 113 it ought to be pretty good It is getting a little old and dated and I hope he plans to update it with a new revision soon Experts Have No Monopoly on Good Advice 3 Now it s true that some scopes and some DVMs are more suitable for trouble shooting than others and I will discuss the differences in the next chapter but to a large extent you have to depend on your wits Your wits Ah very
196. ers as my friends for many years I really do like ma chines such as the HP3455 HP3456 and HP3457 that are inherently repeatably linear as some of these DVMs are absolutely first class One picky little detail Even the best DVM is still subject to the adage Heat is the enemy of precision For example some DVMs have a few extra microvolts of warm up drift but only when you stand the box on its end or side Some of them have a few microvolts of thermal wobble and wander when connected to a zero volt signa shorted leads but only when you use banana plugs or heavy gauge 16 18 or 20 150 12 Roundup of Floobydust gauge leads not when you use fine wire 26 or 28 gauge The fine wired leads apparently do not draw as much heat from the front panel binding posts So even the best DVM auto zero circuit cannot correct for drifts outside its domain Most engineers know that DVMs add a resistive 10 MQ load to your circuit and a capacitive load 50 to 1000 pF that may cause your circuit to oscillate But what s not as well known is that even the better DVMs may pump noise back through their input terminals and spray a little clock noise around your lab So if you have a sensi tive circuit that seems to be picking up a lot of noise from somewhere turn off your DVM for a few seconds to see if the DVM is the culprit If that s not it turn off the function generator or the soldering iron If it is the DVM s fault you ma
197. es The BiFET process made it feasible to make JFETs along with bipolars on a monolithic circuit It s true that the character istic of the best BiIFET inputs are still slightly inferior to the best bipolar ones in terms Of Vos temperature coefficient long term stability and voltage noise But these BiFET characteristics keep improving because f improved processing and innovative circuit design As a result BIFETs are quit close to bipolar transistors in terms of voltage accuracy and offer the advantage of low input currents at room temperature VN trademark of Nanonal Semuconductar Corporation Figure 7 1 80 7 Identifying and Avoiding Transistor Problems Using equations to analyze circuits can sometimes help you define a problem But if the equa tions are inapplicable they do a lot more harm than good Photo copyright Peggi Willis JFETs can have a larger gate current when current flows through the source than when no current flows which is called Toss When I discovered this and discussed it with Joel Cohen at Crystalonics back 20 years ago we called it the Pease Cohen Effect I thought it was caused by imperfect ohmic contacts but other engineers showed that it was actually caused by impact ionization or hot carriers Either way the gate current has a tendency to increase as a linear function of source current with an exponential dependence on high drain source voltages I recall working on a hybrid
198. es are large enough So if there is a0 01 pF capacitor that I suspect of not doing its job I just slap another 0 01 uF capacitor across it If the ripple or the capacitor s effect changes by a factor of two the original capacitor was probably doing its job and something else must be causirg the problem But if I observe little or no change or a change of a factor of three five or ten I suspect that capacitor s value was not what it was supposed to be THEN I pull the capacitor out and measure it Of course the capacitor substitution boxes I mentioned in the section on test equipment part 8 of Chapter 2 can be valuable here they let me fool around with different values But in critical circuits the lead length of the wires going to the substitution box can cause crosstalk oscillation or noise pickup so I may have to just touch in a single capacitor to a circuit Suppose for example that I have a polyester coupling capacitor that seems to be adding a big slow long tail to my circuit s response I don t expect the performance with the polyester capacitor to be perfect but a tail like this one is ridiculous Note when a capacitor s voltage 1s supposed to settle but there is actu ally a long tail that is just another way of saying that the capacitor has poor dielec tric absorbtion or soakage It s the same thing with different aspects So I lift up one end of the polyester capacitor and install a polyprop
199. es to be made to each new engine in hopes of solving the problem Teething troubles bring modifications and each engine can carry a different set of modifications The manufacturing managers shuddered as these modified drawings seemed to pour in from Derby Ed site of the design fa cility the Drawing Office continually upsetting progress in the works Lots of fun for the manufacturing guys eh In the end the problem took a long time to find because it was on the list of things that couldn t go wrong Allow me to quote the deliciously horrifying words from the text Teething trou bles always present these two difficulties that many of the clues are very subjective and that the confidence trick applies By the latter I mean when a certain factor is exonerated as trouble free based on a sound premise and everyone therefore looks elsewhere for the trouble whereas in fact the premise is not sound and the exoner ated factor is guilty In Stanier s case this factor was low super heat So convinced was he that a low degree of super heat was adequate that the important change to increased superheater area was delayed far longer than necessary There were some very sound men in the Experimental Section of the Derby Loco Drawing Office at that time but they were young and their voice was only dimly heard Some of their quite painstaking superheater test results were disbelieved But of course nothing like that ever
200. ese metallized capacitors would recover from pinhole flaws not just once but several times However at low voltages the energy stored in the capacitors would often prove insufficient to clear a fault Thus the capacitors reliability at low voltages was often markedly worse than it was at their rated voltage You could safely use a cheap compact metallized polyester capacitor in a 100 V TV circuit but not in a 2 V circuit Fortunately there are now classes of metallized polycarbonate metallized polyester and metallized polypropy lene capacitors that are reliable and highly suitable for use at both low and high volt ages I was reading one of these data sheets the other day and it said that at low volt ages any pin hole fault is cleared by means of oxidation of the ultra thin metal film When the old metallized polyester capacitors began to become unreliable in a TV t pade 1 polyester capacitor The foil ANS 4 1 Extended Foil Offers Extensive Advantages 43 set the clearing of the shorts would make the signals very noisy Likewise when used as audio coupling capacitors dry tantalum capacitors would sometimes make a lot of noise as they cleared their leaky spots These parts have therefore become unpopular for audio coupling Similarly you might use an electrolytic capacitor with a small reverse voltage perhaps 0 5 V with no harm or problems BUT a friend
201. esign that was amenable to this modification My design was a high speed integrating converter with an input voltage to current converter that just happened to be capable of rejecting wideband noise and DC offsets between grounds The general lesson is that any ADC system is nontrivial and should be engineered by actually plugging in some converter circuits Paper designs usually don t hold water in ADC systems To beat the requirement that every ADC have its own set of power supplies dedi cated exclusively to powering just the single converter you may want to bring power to your PC boards in unregulated or crudely regulated form and then put a small reg ulator right near each ADC These small regulators whether LM320L15 pA78L05 LM317L or whatever do not have a high power supply rejection ratio at high fre quencies You can resolve that problem with decoupling so you have a chance to make the scheme work I hasten to point out however that I haven t actually built such a system myself very often Don t Let Ground Loops Knock You for a Loop The need for multiple separate power supplies or at least multiple regulators comes of course from the many paths taken by ground currents flowing to and from the power supplies If you don t keep these paths scrupulously separate the ground loops can cause bad crosstalk between various parts of the system low level analog high power analog and digital So be very careful to av
202. eves an excessive trimming potentiometer adjustment range a The circuit in b suits TTL much better The need for good communications is critical good oral and written communica tions to prevent confusion or false assumptions After all it s not realistic to expect the system design and every one of its definitions to start out perfect from the first day The chief troubleshooter for the system should be the Program Manager or whatever he is called He ll have his computers Pert charts GANTT charts and so forth but most valuable he has his people who must be alert for the signs of trouble Systems and Circuits 153 These people have to be able to communicate the early signs of trouble so the leader can get things fixed Well OK he or she How to Trim without Trimming Potentiometers Speaking of keeping circuits well trimmed some people like to use trimming poten tiometers to get a circuit trimmed just right Other people hate to because the po tentiometers are expensive or unreliable or drifty Worst of all if a circuit can be trimmed it can also be mis trimmed some person may absentmindedly or misguid edly turn the potentiometer to one end of its range or to the wrong setting How long will it take before that error is corrected For just this reason 0 people prefer fixed voltage regulators I because they always h have aivalidim me not always easy to engineer the correct values ib
203. ew Vou Can shift by 2 3 mV the amount and direction depend only on the value of the previous signal And that s for an expensive Teflon hold capacitor most other ca pacitors have soakages three to five times worse If the timing frequency and rep rate don t change you may be able to add a circuit to provide some compensation or ihe soakage Ref 7 but the problem isn t trivial and neither is the solution Cascading two S H circuits a fast one and a slow one with a bhig hold Capacitor won t help the soakage but will tend to minimize the problem of leakages Some people wish that a S H circuit would go from sample to hold with a neghgible 132 10 The Analog Digital Boundary jump or glitch Although you can build such a circuit it s a lot more difficult than building a more conventional S H circuit You usually find glitch free S H circuits only in deglitchers which are more expensive than most S H circuits Several module and hybrid manufacturers provide this kind of precision device Even though it doesn t settle out instantly a deglitcher is fast and consistent in its settling gt However it still does take some time to settle within 5 mV Aperture Time Still Causes Confusion There s one area of specsmanship where the S H circuit is clouded in confusion That area is the aperture delay specification Maybe someday lIl write a data sheet and drive away the cloud Ask me for a data sheet on the LF6197
204. ew data sheets Noise When an analog signal moves slowly from one level to an other it would be nice if the ADC put out only the code for the first voltage and then at the appropriate threshold began to produce only the code for the other voltage In o practice there is a gray area where noise causes codes to come up when they shouldn t On a good ADC the noise can often be as low as 0 1 or 0 05 LSB p p But when you come to a worst case condition which with successive approximation converters often occurs at or near a major carry for example where the output changes from 1000 0000 to 0111 111 1 the noise often gets worse sometimes climbing to 0 5 LSB p p or more I wouldn t want to buy an ADC without knowing how quiet it was I d have to measure the noise myself as shown in Figure 10 7 because virtually nobody specifies it That s not to say all ADCs are bad just that ADCs Can Be Tough and Temperamental 129 manufacturers don t make much noise about noise Ron Knapp of Maxim wrote a nice explanation of this ADC noise measurement technique in EDN recently Ref 3 I recommend his article on this subject Most ADC data sheets spell out that the only correct way to test or use an ADC is with the analog signal s ground the digital supply s ground and the analog supply s ground tied together right at the ground pin of the ADC If you don t or can t inter connect the grounds at the specified point all bets are of
205. f CONVERT COMMAND O O MSB OUT e ADC e UNDER e TEST INPUT LSB 1 1k O LATCHES O 0 OPTIONAL LSB OUT 2k 100 ANALOG GROUND V STABLE ADJUSTABLE LOW NOISE REFERENCE VOLTAGE SOURCE VERTICAL HORIZONTAL INPUT INPUT TRIANGULAR WAVE GENERATOR 1 TO 100 Hz AS APPROPRIATE Kianna WARN Rabie b c d Figure 10 7 A reference source a triangular wave generator and a scope are the major building blocks of an ADC crossplot tester a that can reveal how much noise a converter adds to the signal ir is digitizing In b the noise performance is ideal whereas in c it is merely acceptable In d the noise performance is unacceptable 30 IU he Analog Digital Boundary With ADCs Paper Designs Aren t Adequate On one 10 bit ADC I designed when the customer found some problems that I couldn t duplicate in my lab I bought one plane ticket for me and one for my best scope After a few hours we arrived at the scene and in less than an hour I had the problem defined The customer expected our converter to meet all specs with as much as 0 2 V DC plus 0 2 V AC at frequencies as high as 5 MHz between the analog ground and the digital ground Outrageous Amazingly our architecture was such that by deleting one resistor and adding one capacitor I could comply with the customer s wishes Most ADCs couldn t have been adapted to work the cus tomer was fantastically lucky that I had used a weird d
206. f the two transistors came from different manufacturers or from the same manufacturer at different times Similarly transistors from different manufacturers will have different characteristics when going into and coming out of saturation especially when you re driving the transistors at high speeds In my experience a components engineer is a very valuable person to have around and Can save you a lot of grief by preventing unqualified components from confusing the performance of your circuits Another assumption engineers make has to do with a transistor s failure mode In many Cases people say that a transistor like a diode fails as a short circuit or in a low impedance mode But unlike a diode the transistor is normally connected to its leads with relatively small lead bond wires so if there s a lot of energy in the power oa supply the short circuit will cause large currents to flow vaporizing the lead bonds As the lead bonds fail the transistor will ultimately fail as an open circuit More Beta More Better It s nice to design with high beta transistors and if some is good more s better But as with most things in life too much can be disastrous The h parameter hyp is equal to AVge AVop with the base grounded Many engineers have learned that as More Beta More Better 79 beta rises so does h As beta rises and h rises the transistor s output impedance decreases its Early voltage falls its v
207. f there s an oscillation on the power supply bus or an excessive amount of phase shift in the feedback circuit If the step response looks lousy you check your scope or your probes or your signal generator because they re as likely to have gone flaky as the op amp is These failures are the reason we studied sO many passive components The overall performance of your circuit is often deter mined by those passive components And yet there are exceptions There are still a few ways an op amp itself can goof up Don t Sweat the Small Stuff Before we discuss serious problems however you should be aware of the kinds of op amp errors that aren t significant First of all it generally isn t reasonable to ex pect an op amp s gain to be linear nor is its nonlinearity all that significant For ex ample what if an op amp s gain is 600 000 for positive signals but 900 000 for nega tive signals That sounds pretty bad Yet this mismatch of gain slope causes a nonlinearity of about 10 uV in a 20 V p p unity gain inverter Heck the voltage coefficients and temperature coefficient errors of the feedback resistors will cause a lot more error than that Even the best film resistors have a voltage coefficient of 0 ppm V which will cause more nonlinearity than this gain error Recently I heard a foolish fellow argue that an op amp with a high DC gain such as 2 000 000 or 5 000 000 has no useful advantage over an amplifier with a DC gain of
208. f you are having stability problems with followers just go ahead and try these techniques it s as easy as adding a resistor box or a pot to your existing circuit I should also mention that some of these concepts were used by 10k 10k re R c 4 d 4 Figure 8 9 You can easily modify the basic inverter a and integrator c to decouple capacitive loads b and d PRs sey 2 2 pae REC O R nR mR a TF NOISE GAINen 1 b T NOISE GaiNe 2s i Figure 8 10 By adding just a single resistor you can tailor the noise gain of a standard integrator a c 10k Figure 8 11 102 8 Operational Amplifiers The Supreme Activators 10k NOISE GAIN 1 b NOISE GAIN 6 SIGNAL GAIN 1 d OC NOISE GAIN 1 NOISE GAIN 6 AC NOISE GAIN 6 SIGNAL GAIN 1 SIGNAL GAIN 1 By manipulating the noise gain of an amplifier you can stabilize unity gain followers while maintaining the desired closed loop gain Glenn DeMichele in his Design Idea for which he won EDN s 1988 Design Idea award Ref 3 gt 0o o p My third recommendation to prevent oscillation in general purpose op amps is to add a feedback capacitor across Ry unless you can show that this capacitor isn t nec essary or is doing more harm than good This Capacitor s function is to prevent phase lag in the feedback path Of course there are exceptions such as the LF357 or LM349 which are stable at gains or noise gains greater than 10 Adding a
209. face from seeming like a visit to an unreal world Many classes of circuits are neither entirely analog nor entirely digital Of course as an analog engineer I don t have a lot of trouble thinking of all circuits as analog Indeed when problems develop in circuits containing both analog and digital ele ments finding a solution is more likely to require that you summon your analog expertise than your digital knowledge Timers D A and A D converters V F and F V converters and S H circuits all fall right on the boundary line between the analog and digital worlds Digital ICs have more than a few analog subtleties And even multi plexers which you may have thought of as purely analog have some quirks that result from their close association with the digital world Time for Timers A timer is basically a special connection of a comparator and some logic which is usually built with analog circuit techniques The familiar 555 timer can do a lot of useful things but it sure does get involved in a great deal of trouble PI treat the most classical fiascoes For one thing people try to make timers with the crummiest leakiest usually electrolytic capacitors Then they complain because the timers are not accurate or their timing isn t repeatable Some people insist on building timers to run for many seconds and then have trouble tweaking the time to be exactly right Sigh These days I tell people Yes you could make a 2 minute ti
210. few inches of wire for the circuit s inductance to make even a good fast rectifier look bad with bad overshoot One diode that does turn ON and OFF quickly is a diode connected transistor A typical 2N3904 emitter diode can turn ON or OFF in 0 1 nsec with negligible over shoot and less than pA of leakage at 1 V or less than 10 pA at 4 V This diode does of course have the base tied to the collector However this diode can only withstand 5 or 6 V of reverse voltage and most emitter base junctions start to break down at 6 or 8 V Still if you can arrange your circuits for just a few volts these diode connected transistors make nice fast low leakage diodes Their capacitance is somewhat more than the 1N914 s IpF Other Strange Things That Diodes Can Do to You If you keep LEDs in the dark they make an impressive low leakage diode because of the high band gap voltage of their materials Such LEDs can exhibit less than 0 1 pA of leakage when forward biassed by 100 mV or reverse biassed by V Of course you don t have to reverse bias a diode a lot to get a leakage problem One time I was designing a hybrid op amp and I specified that the diodes be con nected in the normal parallel opposing connection across the input of the second stage to avoid severe overdrive Figure 6 3 thought nothing more of these diodes until we had the circuit running the op amp s voltage gain was falling badly at 125 C Why Because the
211. fficient are required Sometimes it was just a bad choice and a conversion to a stable metal film resistor such as an RNS5D or RN60C with a TC of 50 or 100 ppm C max considerably improves accuracy and stability In other cases the engi neer says No I tried a metal film resistor there but when I put in the carbon re sistor the overall TC was improved In this case the engineer was relying on the carbon composition resistor to have a consistent TC which must compensate for some other TC problem I have found that you can t rely on consistent TC with the carbon composition type and I do not recommend them in applications where preci sion and stability are required even if you do see some TC improvement in your circuit However carbon composition resistors do have their place I was recently reviewing a military specification that spelled out the necessary equipment for the ESD electrostatic discharge testing of circuits An accurate 1500 Q resistor was required for use as the series resistor during discharge of the high voltage capacitor In this case you would assume that a metal film resistor would be suitable however a metal film resistor is made by cutting a spiral into the film on the resistor s ceramic core Figure 3 1a Under severe overvoltage conditions the spiral gaps can break down and cause the resistor to pass a lot more current than Ohm s Law predicts the resistor will start to destroy itself Therefore t
212. finally decide which way to go Or if the data comes just a little earlier or later you might get an abnormally narrow output pulse a runt pulse When you feed a runt pulse to another flip flop or counter the counter can easily respond falsely and count to a new state that might be illegal Thus you should avoid runt pulses and make sure that you don t clock flip flops at random times Fig 10 1a L7 contains an example of a D flip flop application that can exhibit this problem When the comparator state changes at random times it will occasionally change at precisely the wrong time on the clock s rising edge making the output pulse narrower or wider than normal In certain types of A D converters this effect can cause nonlin earity or distortion A good solution is to use a delayed clock to transfer the data into a second flip flop as in Fig 10 1b A glitch is an alternate name for a runt pulse A classic example of a glitch occurs when a ripple counter such as a 7493 feeds into a decoder such as a 7442 When the Perfect Waveforms Don t Exist 123 PRE CLR SET Q 74C74 ra CLK PULSE OUTPUT WITH DUTY CYCLE PROPORTIONAL IN a COMPARATOR PRE CLR SET 4D zca o COMPARATOR _ DUTY CYCLE i CLOCK O Figure 10 1 Runt pulses cause problems in this simple ADC a The comparator state changes at random times Occasionally the state will change at pr
213. find out the problem What a completely useless notion the CRT and all other functions were dead S0 icouldn t possibly use its diagnostic software Fortunately Tihave acouple technicians who cannot imagine the meaning of cannot be repaired Ka Paul climbed in and found a shorted rectifier replaced it and Iiwas back on the nja couple days Tft d had to take it to a repair shop I hate to think of the and days ust to get 2 00 rectifier replaced Tim sure the power supply card would cost 90 not to mention the Jabor When Iwas wandering through Kathmandu last year Isaw workers repairing things that would notibe worthithe effortin ithe USA But Nepalis do not have enough money to join in the thi 00 sety s so they make the effort to repair things Cars tires stoves t i could ipossibly be repaired usually is And if itcan t be paired i l ets re ycledi T Support that approach and T myself am usually willing to put i ina lot more hours of effort to troubleshoot some thing than the cost of replacing i it would justify Why Because sometimes I learn something Once I had an oldi 1970 VW witioh I retired ae it ad 249 850 miles on it and because it was leaking oil badiy ii i ally started to dismantle the engine I found nota cracked block but that the bolts that fasten down the oilicooler had loose Because there that anybody vi ork of nuts c
214. for these wanderings is observing vistas of America that few people have seen The curiosity that motivates Bob s exploration of old railroad routes is reflected in many of his other activities both at and away from work For example another of Bob s hobbies is designing voltage to frequency con verters VFCs Most people who design VFCs do it as part of a job Although Bob sometimes designs VFCs for use in National products he often does it just for fun and because he finds the activity educational and challenging A couple years ago on such a lark he put together a VFC that used only vacuum tubes This circuit proved that the company where he spent the first 14 years of his career George A Philbrick Researches more recently Teledyne Philbrick now Teledyne Components of Dedham MA could have gone into the VFC business in 1953 eight years before Pease received his BSEE from MIT Twenty years after he designed it one of Bob s first solid state VFCs the 4701 continues to sell well for Teledyne Philbrick The story of how Pease pioneered the voltage to frequency business is recounted in a chapter of Analog Circuit Design Art Science and Personalities Butterworth Heinemann 1991 edited by Jim Williams See the ad at the end of this book Bob also loves to write he clearly enjoys communicating to others the wisdom he has acquired through his work He has published about 60 magazine articles not counting the series in EDN tha
215. frequencies And even more insidious than 1 f noise is Popcorn noise a type of electrical noise in which bursts of square steps are added to the normal thermal noise at random times Popcorn noise occurs rarely these days but unfortunately it s not at 0 not even with the cleanest processing and the best manufacturers I ve been chastised and told that some of my amplifiers are noisy compared with those of certain competitors But when I look at the competitor s data and plots I see 1 f and popcorn noise lurking unnoticed in a corner On high performance parts we try to screen out the noisy ones But when a few parts have a spacing of 2 to 10 seconds between bursts of pop corn it s not cost effective to look for those parts Only a small percentage of our customers would want to reject that one noisy part and pay for the testing of all the good parts too Remember 10 seconds of testing time equals 30 cents time equals money Although oscillation and noise problems may be the most common ones you ll encounter when you use op amps there s a host of other characteristics that are wise to look out for These characteristics include overload or short circuit recovery set tling time and thermal response Many op amps have a fairly prompt recovery from overdrive when you make the output go into the stops that is when you force the output into the power supply rails For most op amps this recovery characteristic is not defined or s
216. g dimmer and dimmer that s a clue Ask Questions Take Notes Record Amount of Funny l When you ask these four questions make sure to record the answers on paper preferably in a notebook As an old test manager I used to work with Tom Milligan used to tell his technicians When you are taking data if you see something funny Record Amount of Funny That was such a significant piece of advice we called it Milligan s Law A few significant notes can save you hours of work Clues are where you find them they should be saved and savored Ask not only these questions but also any other questions suggested by the an swers For example a neophyte product engineer will sometimes come to see me with a batch of ICs that have a terrible yield at some particular test I ll ask if the parts failed any other tests and I ll hear that nobody knows because the tester doesn t continue to test a part after it detects a failure A more experienced engineer would have already retested the devices in the RUN ALL TESTS mode and that is exactly what I instruct the neophyte to do Likewise if you are asking another person for advice you should have all the facts laid out straight at least in your head so that you can be clear and not add to the confusion I ve worked with a few people who tell me one thing and a minute later start telling me the opposite Nothing makes me lose my temper faster Nobody can help you troubleshoot effec
217. ge Check Output op amp ee aa Comments on Table for Instrumentation Amplifiers As above this circuit should be set up with sockets for ease of evaluation This cir cuit does offer a little more interaction but it is not really too difficult when you figure out what has to be going on Back to Electronic Circuits 7 32V FEEDBACK UNREGULATED DC INPUT o Table 14 8 Troubleshooting LM2575 Switching Regulators Indication of Troubl ossible Cause 183 45V 1A REGULATED OUTPUT o Solution Output V much too Ow iV top low i we Booed Op ae Output V muchitooihigh FeedBack Inductor overtieats Shorted rectifier IC overheats Frequency too high gt Output shorted Rectifier overheats Frequency too high Bad ripple Check V with scope Remove load Check ohms output to ground Check voltage at pin 5 Check volts ohms on D1 Check replace swap Check ambient cool it Check Viatpin 4 Cheek swap Check frequency at pin 2 Cheok L check its dossiness vs aknown good inductor Check rectifier Check frequency at pin 2 Check Vout lioad n Luppi Check frequency at pin2 Compare to good diode oS rs Cheok frequency at Check capacitor for Rs Check waveforms etc etc Comments on Table for LM2575 Switching Regulator Even though this is a very simple looking circuit with only a few parts you shoul
218. gn Letters to Bob 171 development and debugging Very few people can appreciate the artistic or seat of the pants techniques we apply I recently supervised somebody in the design of a circuit pack It took him a few days to do the actual logic design It took him several months on and off to prepare that design so that somebody could actually build it a printed circuit board He was amazed He never learned any of that in school He still has to get the circuit pack get parts a nontrivial task and debug it Needless to say it s quite an educational experience for a novice Even for an experienced de signer like me it s no trivial task This fact is probably responsible for the attitude I have developed over the years I expect the worst and am surprised when anything works at all I am rarely disap pointed I design with this philosophy in mind I detect this philosophy in Bob s articles too I run into many people with the opposite frame of mind They expect the best and are surprised when it doesn t work I have little patience with these people They are either geniuses and I have met two or three of those over the years in this trade or they ve never gone through the design and debugging process It is truly a humbling process One more comment touched on in Bob s article People are continually amazed when I tell them that I solve most of my problems at my desk with an ohmmeter That fact simply points
219. good results If you are working on static sensitive compo nents an antistatic solder sucker is less likely to generate high voltages due to in ternal friction than is an ordinary solder sucker I have been cautioned that a large solder sucker may cause problems when working on narrow PC traces in that case solder wick may be the better choice Hand tools Among the tools you need are sharp diagonal nippers suitable pliers screwdrivers large cutters wrenches wire strippers and a jack knife or Exacto knife Signal leads connectors cables BNC adapters wires clip leads ball hooks and alligator clips as needed Scrimping and chintzing in this area can waste lots of time shaky leads can fall off or short out Freeze mist and a hair dryer The freeze mist available in aerosol cans lets you quickly cool individual components A hair dryer lets you warm up a whole circuit You ll want to know the dryer s output air temperature because that s the tempera ture to which you ll be heating the components NOTE Ideally we should not use cooling sprays based on chlorofluorocarbons CFCs which are detrimental to the environment have a few cans that some people would say I shouldn t use But what else should Ido send the can to the HOT JUNCTION CHROMEL ALUMEL THERMOCOUPLE TYPE K 408 pire NOTE ALL RESISTORS 1 YELLOW 24 2 Choosing the Right Equipment 15V POWER SWITCH SIA OI pF TEMP
220. h a blown fuse After shopping unsuc cessfully at several electrical supply houses I finally went in to a Radio Shack They had them and I realized that was the first place I should have gone I replaced the fuse and turned on the power would the fuse blow for a good reason or had the old fuse just fatigued out The new fuse has held for several months so it was justa fatigue failure Most fuses are fully rated for 115 or 230 VAC but not more than 32 V of DC That s because the alternating current flow gives time for an arc to be extinguished which would not happen with DC So for high voltage DC the answers aren t so simple Some circuit breakers are rated for as much as 65 VDC but often that s not enough There is a CD Series that is good up to 125 VDC anda larger GJ Series that is rated up to 150 V available from Heinemann Another approach in circuits with rectified power is to put the sensing coil in the DC circuit but connect the breaker into the AC circuit That s no help if you just have a 120 V battery supply These days high powered MOSFETs can be used to make such a good high voltage high current switch that you can build your own fast turn off switch acti vated by over current an electronic equivalent of a fuse I built one of these and it didn t work very well the first time It blew out the FET Twice I haven t really given up on it and when I get some time after I put this book to bed I ll go bac
221. h that signals leak and crosstalk to each other or controlled impedance lines are interfering thus causing reflections and ringing Avoid PC Board Problems at the Outset The fixes for these problems and ways to avoid them in the first place are fairly straightforward These days the G10 and G11 fiberglass epoxy materials for PC boards are quite good and reasonably priced Trying to use cheaper phenolic or fishplate is not economical in most cases Conversely a special high temperature material or an exotic material or flexible substrate may be justifiable If you don t have an expert on these materials the PC board maker or the manufacturer of the substrate material can 50 Manufacturer Avoid PC Board Problems at the Outset 5l usually provide some useful advice See Table 5 1 for a comparison of PC board materials In some RF applications phenolic material has advantages over glass epoxy it has a lower dielectric constant and superior dimensional stability And for ultra broadband oscilloscope probes some types of glass epoxy have a definite disad an tage due to mediocre dielectric absorption especially if the epoxy has not been properly cured gt As for quality there is almost never an excuse for buying your boards from a vendor whose products are of unknown quality Low cost would be one poor alibi Can t get acceptable delivery time from our normal vendor would be another One time
222. hadn t ever been washed after soldering In this case the impedance of the scorched flux was as low as 500 Q when measured across a 0 1 X l in area of the PC board The leakage was from the V to the output and it pulled the output voltage up out of regulation So even if you are not trying to achieve 10 2 Q of leakage resistance you should still observe rudimentary standards of cleanliness or even your simplest circuits won t work right Similarly one of our PC boards designed for a S H circuit was yielding 10 Q of leakage resistance which was unacceptable We tried cleaning the board with every organic solvent but had no luck Finally I took a few boards home and set them in the dishwasher along with the normal charge of Calgonite After a full wash and rinse cycle I pulled out the boards banged them to shed most of the water beads and set them in my oven to dry at 160 F The next day they checked out at the more accept able value of 10 3 Q I have used this technique several times on leaky PC boards and sockets and it works surprisingly well It can work when alcohol TCE and organic solvents are not helping at all After you get your board clean and dry you I want to keep it that way For this purpose you may want to use a coating such as the urethane acrylic or epoxy types sprays or dips Humiseal is the pioneering name and they have a broad catalog of different types for various production needs In a similar ve
223. hat critical it s just a starting point You really must build and trim and test the circuit for overshoot ringing and freedom from Oscillation If the equation said 1 pF and you get a clean response only with 10 pF you d be suspicious of the formula Note that when you go from a breadboard to a PC board the stray capacitances can change so you must recheck the value of Cy Tin some Cases you may not need a separate capacitor if you build 0 5 pF into the board Ih any case you certainly don t have to just trust my equations build up your breadboard and fool around with different values of Cp and check it out for yourself S e if you don t agree My last recommendation is that when you think the circuit is okay that is free from oscillation test it anyway per Pease s Principle to make sure it s as fast or stable as you expect Be Sure that your circuit isn t ringing or oscillating at any ex pected operating conditjoni or toad or bias f Noises Theoretical and Otherwise In addition to oscillatory behavior another problem you might have when using op amps is noise Most op amps have fairly predictable noise It s often right down at theoretical levels especially at audio frequencies There s a pretty good treatment of noise and its effects in various applications in Thomas Frederiksen s book Ref 4 Also if you want to optimize the noise for any given source resistance or impedance National Sem
224. he LOW level of 0 2 V or the HIGH level of 18 4 V How could the outputs have a me dian level of 9 V How do you get an R S flip flop to hang up at an output level halfway between the rails Unlikely Then he pointed out some other statistics the 3 o values of the output were 30 V and 8 V Now that is pretty bizarre for a circuit that has only a 20 V supply and ground and it is not running as a switching regulator it s just sitting there at DC The meeting broke up before I could find the facts and protest so that product was not released on schedule It turned out of course that the tester was running falsely so while the outputs were all supposed to be SET to 18 4 V it was actually in a random state so that half the time the outputs were at 18 4 V and half the time at 0 2 V If you feed this data into a statistical program it might indeed tell you that some of the outputs might be at 9 V assuming that the data came from a Gaussian distribution But if you look at the data and think it is obvious that the data came from a ridiculous situation Rather than just try to ram the data into a statistical format the engineer should start checking his tester Lies Damned Lies and Statistics 147 Unfortunately this engineer had so much confidence in his statistical program that he spent a whole week preparing the Beautiful Report Did he go report to the design engineer that there were some problems No Did he check his data check
225. he op amp in Figure 5 4 forces the DUT s output current through the Darlington transistor and then through a 0 1 Q precision resistor The only way to use a 0 1 Q resistor with any reasonable accuracy and repeatability is to use the 4 wire Kelvin connections as shown The op amp can force the upper Sense lead to be precisely 300 mV above the lower Sense level even if the lower end of the resistor does rise above ground due to various IR drops in wires or connectors There are several places in this circuit that we could call ground but the only ground we can connect to the 300 0 resistor and get good results is the Sense lead at the bottom of the 0 1 2 resistor If you connect the bottom of the 300 C resistor to any other ground shifts in the IR drops would cause relatively large and unpre dictable and unacceptable shifts in the value of the 3 A current in other words Inaccuracy and Trouble So when you re running large currents through circuits think about the effects of IR drops in various connectors and cables If you think IR drops will cause trouble maybe Kelvin connections can get you out of it Figure 5 4 s fourth Kelvin connection is hidden inside the LM323 5 V regulator which has separate Force and Sense connections to the Output terminal A fifth Kelvin connection is also concealed inside the current limit circuitry of the regulator Here the device senses the load current with a 4 wire Kelvin connected resistor and send
226. he spec should have called for the use of a carbon composition resistor whose resistive element is a large chunk of resistive material Figure 3 1b This resistor can handle large overloads for a short time without any such flash over Even when you are applying a 200 to 400 overload for just a short time the nonuniform heating of the spiraled section of a metal film resistor can cause the resistor to become unreliable You can also get around this problem by using a series connection of metal film resistors If you put fifteen 100 Q 1 4 W metal film resistors in series each individual resistor would not see overvoltage or excessive power Carbon film resistors are now quite inexpensive and have become the most common type of resistor around most labs Their main drawback is that they are very similar in appearance to metal film resistors and have some similar characteristics Carbon film resistors have 1 tolerances are normally manufactured with spiral cuts and have the same kind of voltage overload limitations as metal film types But carbon film resistors have much higher TCs 500 to 800 ppm C It s easy to erro neously insert a drifty carbon film resistor for the intended metal film type Don t confuse the two Precision film resistors on the other hand are available with greatly improved accuracy and TC Compared to ordinary RN5SD and RNSSC resistors with TCs of Typical Resistor Characteristics TC Resistor Type Range Q
227. he transistor and not 2 or 3 inches away can cure this tendency to oscil late Sometimes a small ferrite bead is more suitable than a resistor because it will degrade the transistor s frequency response less Oscillations Crop Up Not all problematic oscillations are high frequency ones An unstable switching regulator feedback loop can oscillate at low frequencies For troubleshooting switching regulator feedback loops I first recommend a network analyzer to save you troubleshooting time A network analyzer facilitates taking data and checking out variations of the circuit in case of trouble However I do tend to put more faith 2 2K O tp gr CERAMIC ds TAS Figure 9 i This was a popular circuit for the RESET function until the engineers discovered how badly it oscillated 110 9 Quashing Spurious Oscillations in real time step response If that is consistent with the frequency domain response fine if not I get suspicious Secondly if an earlier version of your circuit has worked OK what s the differ ence between the new one that does not work well and the old one that does Be sure to keep one or more examples of the old version around so that you can make com parisons when the new circuits have troubles Note that I said when not if Thirdly look for components such as capacitors whose high frequency characteristics can change if someone switched types or suppliers Optoisolators in switching regula
228. he wafer but cannot be tested after the die is packaged because that signal is not accessible any longer To avoid frustrating and confusing the customer some manufacturers are establish ing two classes of guaranteed specifications The tested limit represents a test that cannot be doubt ed one that is actually performed directly on 100 percent of the devices 100 percent of the time e The design limit covers other teats that may be indirect implicit or simply guaranteed by the inherent design of the device and is untikely to cause a failure rate on that test even as high as one part per thousand Why was this distinction made Not just because customers wanted to know which specifications were guaranteed by testing but because the qQuality assurance group insisted that it was essential to separate the tested guarantees from the design limits so that the AQL assurance quality level could be improved from 0 1 Percent to down below 100 ppm Some data sheets guarantee characteristics that are quite expensive and difficult to test even harder than noise such as long term drift 20 ppm or 50 ppm over 1 000 hours The data sheet may not tall the reader if it is measured tested or estimated One manufacturer may perform a 100 percent test while another states Guaranteed by sample testing This is not a very comforting assurance that a part is good especially in a critical case where only a long term test can prove
229. hematics for your test equipment you can more easily explain these incompatibilities And finally the data sheets and schematics of any ICs used in your circuit will also come in handy Choosing the Right Equipment 21 CUARENT LIMITED SUPPLY 50 mA SHORT CIRCUIT PATH T CIRCUIT IN TROUBLE NOTE ALL RESISTORS t0 oly voy Aris ane F nea Rites LEVER BR eA oe ai piv PAAA ESANS R ii ne ma ONE Ee aes ey OY I un f A short circuit detector This tool comes in repair a lot o BPC be he the exact location of the short you simply slide the positive input probe the bidses ln ahis aikanaple iF you slide it from A toward B or D the pitch won t change because there is no change ih voltage at these points no current flowing along those busses But if jo slide the probe along the path from A to C or from K to M the pitch will change because the voltage drop is changing along those paths It s an easy and natural technique to learn to follow the shifting frequency signals An AM radio What do you do when trouble is everywhere A typical scenario starts out like this You make a minor improvement on a linear circuit and when you fire it up you notice a terrible oscillation riding on the circuit s output You check everything about the circuit but the oscillation remains In fact the oscillation is riding on the output the inputs on several internal nodes and even on ground You 22 2 Choosi
230. hese days are well behaved and you only need to take four basic precautions to avoid oscillations First always use some power supply bypass capacitors on each supply and install them near the op amp For high frequency op amps the bypass capacitors should be very close to the device for best results In high frequency designs you often need ceramic and tantalum bypass capacitors Using bypass capacitors isn t just a rule of thumb but a matter of good engineering and optimization Second avoid unnecessary capacitive loads they can cause an op amp to develop additional phase shift which makes the op amp circuit ring or oscillate These effects are especially noticeable when you connect a X scope probe or add a coaxial cable or other shielded wire to an op amp to convey its output to another circuit Such connections can add a lot of capacitance to the output Unless you re able to prove that the op amp will be stable driving that load you d better add some stabilizing Circuits It doesn t take a lot of work to bang the op amp with a square wave or a pulse and see if its output rings badly or not You should check the op amp s response with both positive and negative output voltages because many op amps with pnp follower outputs are less stable when Vout is negative or the output is sinking current Refer to the box Pease s Principle I ve seen pages of analysis that claim to predict capacitive loading effects when the op a
231. his exalted station he views the activity as part and parcel of his job Indeed he revels in it and his writing effectively communicates his pas sion for exorcising the hobgoblins that bedevil electronic circuits That fact was not wasted on the readers Their reaction was overwhelmingly posi tive Never in the magazine s history almost 35 years has something that EDN published evoked such an enthusiastic outpouring Every few weeks a new deck of reader service cards circulates around our offices There are the cards on which readers have written comments in addition to making requests for more informa tion on products advertisediand mentioned in the magazine The decks for the issues that contained the series installments invariably contained scores of hand written notes asserting that Bob s articles i were marvelous the best that EDN had ever printed In fact EDN s readers voted all 12 articles the best read contributed manu scripts in their respective issues Once the series ended we started getting cards that asked for the articles in book form First the book requests were a trickle but they rapidly swelled to a torrent So for all of you who asked for it and for those who never saw the series in EDN but who have decried the lack of a compendium on troubleshooting from a designer s perspective here it is Lest it appear that I was the sole EDN staff member who edited Bob s work let me make it clear that I wasn t ev
232. i and 0 45 X V are almost the only choices Flyback regulators are the simplest and cheapest magnetically coupled regulators However at power levels aboye100 W their disadvantages become objectionable and forward or push pull sohemes are more appropriate At the highest power levels bridge type designs are best Ifyou Ary to use a configuration at an inappropriate power level you may have to struggle to get it working Likewise the use of current mode regulation may help you get faster loop response but the concept is difficult to understand let alone execute Current limiting is always a problem with switchers The choice of a sense resistor is not easy because the resistor must have low inductance As with most aspects of switch mode regulation to achieve good reliability and to avoid trouble you have to spend the time to design and test the current limiting circuit carefully Some newer Switch mode controller ICs have been engineered to make it reasonably easy Older ones like the LM3524 haven t been usually Similarly a soft start circuit is important for a large switcher especially when the switcher strains to put out a lot of current to quickly charge up the output filter capac itors and especially for a boost configuration where the inductor s current might saturate and refuse to pull the output high enough For a large supply this current 140 i i Dealing with References and Regulators could damage transistors wires
233. ic 35 3839 110 li f P flux solder AB 54 SONIO Frederiksen Tom M 103 107 freeze mist 118 frequency high 22 35 41 43 45 51 66 88 91 94 97 99 1084109111 116 117 127 130 150 156 frequency response 15 31 73 84 89 99 101 103 109 1 It 116 118 128 136 156 191 205 207 See also at step response or bandwidth frequency to voltage converters FVC 67 131 134 as analog isolation amplifiers with VFC 131 as tachometer 131 phase locked loop as FVC 131 ripple filtering 131 function generators 17 22 25 37 92 93 94 95 150 pulse output 150 set up problems 150 sine output 17 150 sine distortion and cure 150 Square wave output 17 150 triangle output 17 150 212 funny amount of 5 fuses See also circuit breakers 33 34 140 164 fatigue failures 38 ratings AC 34 ratings DC 34 FVCs See frequency to voltage con verters Giles Martin 44 gimmick See under capacitors glass epoxy beams low picofarad glitches See under digital ICs grid dip meter 22 109 ground 57 129 130 145 156 161 162 See also under printed circuit boards See also under analog to digital converters ground loops 25 63 110 129 130 156 hrb 78 79 HA2525 81 hacksaw 9 23 hair dryer 23 hairline opens 8 51 hairline shorts 24 51 hammer 9 38 hand tools 14 23 heat sinks 82 116 135 165 178 180 bolts just right 83 180 bolts too loose 82 180 bolts to
234. iconductor s Linear Applications Note AN222 Ref 5 has some good advice as does the article in Ref 6 You ll have difficulty with noise when it s unpredictable or when op amps of a particular type have varying noise characteristics This problem rarely happens at audio frequencies but is likely to happen sporadically at low frequencies such as 10 or 100 Hz or even lower frequencies Every manufacturer of transistors and ampli fiers tries to keep the noise low but occasionally some noisy parts are built Sometimes the manufacturer is able to add tests that screen out the noisy parts But these tests aren t cheap if they take even one second of tester time which may cost three cents or more We are trying to add some 0 3 second tests to some of our more popular amplifiers but it s not trivially easy Here s a tip we find that true RMS testing for noise is a big waste of time be cause amplifiers that are objectionably noisy have much worse p p noise than you would guess from their RMS noise data So the p p test is the best discriminator We get the best resolution with a bandwidth from around 30 Hz to 3 kHz after we roll off g SAperanona Amplhecs fhe Supreme Activators the broadband noise A sample time of 0 1 second works pretty well the ones that pass a 0 1 second test but fail a 0 5 second test are uncommon Popcorn Noise Can Rattle Sensitive Circuits Flicker noise also known as 1 f noise is AC noise that exists at low
235. idth op amp such as an LF356 and 50 or 70 mV is quite for an 1 M741 If you want an op amp to Move its output al there has to be a significant error volage reasonable any significant speed across the inputs for atleast a short hing 7 8 Operational Ampilitiers lhe Supreme Activators Also beware of op amp models and what they might mistakenly tell you For in stance the standard equation for a single pole op amp s gain is A A 1 j OT This equation implies that when the DC gain A changes the high frequency gain A changes likewise Wrong With the growing popularity of computer modelling I have to explain this to a would be analyst every month No there is almost no corre lation between the high frequency response and the spread of DC gain on any op amp you can buy these days There are several ways to get an op amp s DC gain to change Change the temperature add on or lift off a load resistor Or swap in an am plifier with higher or lower DC gain Although the DC gain can vary several octaves in any one of these cases the gain bandwidth product stays about the same If there ever were any op amps whose responses did vary with the DC gain they were aban doned many years ago as unacceptable So your op amp model is fine if it gives you a fixed constant gain bandwidth product But if the model s gain at 1 MHz doubles every time you double the DC voltage by reducing the load you re headed for trouble and c
236. if the device did meet the manufacturer s Specification If in doubt question the manufacturer TYPICALS Next to a guaranteed specification there is likely to be an other in a column labeled typical It might mean that the manufacturer once actually saw one Part as good as that It could indicate that half the parts are better than that specification and half will be worse But it is equally likety to mean that five years ago half the parts were better and half worse It could easily signify that a few better and a few parte a lot worse after all if the noise of an amplifier is extremety close to the theoretical limit one Cannot expect to find anything much better than that but there wif always be a few noisy ones If the specification of interest happens to be the bias current lp of an op amp a user can expect broad variations For example if the specification is 200 nA maximum there might be many parts where ly is 40 NA on ane batch where the beta is high and a month later many parts where the Ip is 140 nA when the beta is tow 199 200 F How to Get the Right Information From a Data Sheet Absolute Maximum Ratings note 11 if Miiitary Aerospace specified devices are required Lead Temp Soldering 4 seconds please contact the National Semiconductor Sales TO 46 Package 300 C Office Distributors for availability and specifications TO 92 Package 260 C Supply Voltage 38V to 0 2V
237. ifthe gain from the reference to the output changes by more than 1 LSB when you go from a code of 1000 0000 to a code of 0111 1111 In fact if the fre quency is above 5 kHz you may find a 0 2 or larger error because the multiplying DAC s ladders whose attenuation is a linear function of the input code at DC be come slightly nonlinear at high frequencies due to stray capacitance The non linearity can be 0 2 and the phase change as you vary the input code can exceed 2 even with a 5 kHz reference So don t let these AC errors in multiplying DACs surprise you a yo Another problem with DACS is the output glitch they can produce when going from one code to an adjacent one For example if a DAC s input code goes from 1000 0000 to 0111 1111 and the delay for the rising bits is much different from that for the falling bits the DAC output will momentarily try to go to positive or negative full scale before it goes to a value corresponding to the correct code Though well known this problem is aispecializedione One possible solution calls for precisely synchronous timing Multiple fast storage registers can also help to save the day But if the best synchronous timing is not good enough a deglitcher may be the solution ADCs Can Be Tough and Temperamental roe Like DACs many A D converters ADCs do exactly what they are supposed to so what can go wrong Most problems involve a characteristic that is mentioned on too f
238. illations as high as 250 MHz The literature that comes with the HD 1250 dip meter kit also lists several troubleshooting tips When grid dip meters first became popular the fastest oscilloscope you could buy had a bandwidth of only a few dozen megahertz These days it is possible to buy a scope with a bandwidth of many hundreds of megahertz so there are fewer occasions when you might need a grid dip meter Still there are times when it is exactly the right tool For example you can use its oscillator to activate passive tuned circuits and detect their modes of resonance Also in a small company where you can t af ford to shell out the many thousands of dollars for a fast scope the dip meter is an inexpensive alternative 16 A few working circuits if available By comparing a bad unit to a good one you can 1 Heath Company Benton Harbor Michigan 49022 1 800 253 0570 20 21 Choosing the Right Equipment 23 often identify problems You can also use the good circuits to make sure that your specialized test equipment is working properly A sturdy broad workbench It should be equipped with a ground plane of metal that you can easily connect to the power ground The purpose of this ground plane isto keep RF 60 Hz and all other noise from coupling into the circuit Place insulating cardboard between the bench and the circuit under test so that nothing tends to short to ground Another way to prevent noise from interfering
239. illion which is a lot more educational than a cold 119 2 dB statement Besides you learn rather quickly that the slope and the curvature of the display are important Not all ampli fiers with the same 119 2 dB of CMRR are actually the same not at all Some have a positive slope some may have a negative slope and some curve madly so that if you took a two point measurement the slope would change wildly depending on which two points you choose If you increase the amplitude of the input signal you can also see plainly where severe distortion sets in that s the extent of the common mode range Limitations If you set the noise gain as high as 100 then this circuit will be 3 dB down at Fgpw divided by 100 so you would only use this up to about 1 kHz on an ordinary MHz op amp and only up to 100 Hz at a gain of 1000 That s not too bad really To look at CMRR above kHz you might use R100c 2 k to give good results up to 10 kHz In other words you have to engineer this circuit a little to know where it gives valid data Thinking is required Sorry about that For really fast work I go toa high speed low gain version wae RI R Sk R2 R1I2 5k and R100 2k or k or 0 5k This works pretty well up to SO KH or more depending on what gain bandwidth product your amplifier has For best results at AC it s important to avoid stray Capacitance of wires or of a real switch at the points where you connect to R 100a o
240. in the guys up at Essex Junction VT told me about some varnish made by John Armitage Co which is a rather thick heavy high impedance coating It takes a while to dry but it s pretty durable and I like it When I was building some little 1 3 ounce modules that some scientists were going to carry up to the top of Mt Everest I chose a couple well baked coats of the Armitage to keep the modules clean and dry it s much lighter than potting in epoxy which is important when a guy has to carry a scientific package on his back up to 29 000 feet Of course with any of these coatings it is not trivial to cut in and repair the circuit or change components So your choice of a durable coating should be tempered by the awareness of how much fun it is to go in and remove the coating and do your repairs When I was at Philbrick we potted most of our products in epoxy and it gave good reliability and security If you get a good circuit in there well potted in epoxy it has an excellent chance to survive forever with no moisture getting in and with everything held isothermal protected from shock and physical abuse Of course if somebody abuses the circuit electrically and damages it it s substantially impossible to get inside to repair it You may have to drill down to the PC board just to do some troubleshooting It s fun and a challenge to have to delve in and troubleshoot a potted circuit Sometimes the potting material adds extra stre
241. in stacked up in cascade But the feedback capacitor to the negative input makes this safe and easy Nasty Latches A circuit that inadvertently latches up presents a problem exactly opposite that of an oscillating circuit Or you could correctly say that a latched up circuit is an oscillator Rin Re e a z GAIN 095 Rin RF 20x Rin GAIN 20 b T Rw Re Z gt 4 GAIN 10 OPTIONAL C C 10 100 OR 1000 pF R Rin AS SUITABLF c _ Figure 9 5 Depending on the gain of the buffer you can use these three schemes to stabilize a buffered anyplifier 118 9 Quashing Spurious Oscillations with zero frequency Although latched up circuits demand troubleshooting the good thing about them is that they sit right there and let you walk up to them and touch them And you can measure every thing with a voltmeter to find out how they are latched This state of affairs doesn t mean that troubleshooting them is easy because sometimes you can t tell how the latched up circuit got into its present state And in an integrated circuit there can be paths of carriers through the substrate that you can t put your finger on The worst aspect of latched up circuits is that some are destructive so you can t just sit there and let them remain latched up forever Two approaches for attacking destructive latches are Turn off the power quickly so the latched up circuit cannot destroy anything Try tur
242. ines and toggle in spite of a low high condition at clock time This anomaly happens if the clock is left high and is not pulsed J Koontz Chief Engineer gt Computer Automation Irvine CA Dear Bob If an engineer wants to see how to properly control EMI at its source he or she should look at the chassis of any TV and tuner Chances are any radiation from inter nally generated 15 kHz to 950 MHz signals meets FCC Part 15 rules Obviously most TV receivers do not depend on their plastic cabinets to contain spurious radiation For some time now I have been working with labs that attempt to certify radiating equipment that was not designed using basic radiation containment methods These methods date back to the time of tube circuits and include using specialized compo nents such as feedthrough capacitors ferrite beads and toroidal coils These methods also include using very light gauge low cost tin plated steel cans to enclose radi ating components Soldering the tabs of these 5 sided cans to the PC board forms a complete 6 sided enclosure around the radiating component Note that TV receivers PC boards have considerable ground plane area If you do not design out EMI someone will have to design in some Band Aids to fix your bad design Here are some points to consider Incorporate as much ground plane as possible on one side of each PC board that contains digital or analog signals above audio frequencies Be sure this gro
243. ing Tell me if this list is incorrect or incomplete but I have not heard any complaints Example The DM7486 and DM74S86 and DM74LS86 and MM74C3386 all have the same pinout But the DM74L86 and MM74C86 have a different pinout When the L86 came out it was not pin compatible with the 7486 and of course the C86 came out to be compatible with the L86 Several of these parts are rare and obsolescent and sole sourced and you will probably never see one but they are listed here for completeness Here is the complete list 74H01 74L51 LS51 74H53 74L54 LS54 H54 74H55 74L71 74L78 LS78A 74L85 C85 74L86 C86 74L95 C95 Readers might also want to be aware that the DM74107 for example is pulse triggered but the 74LS107 is edge triggered so in some cases they might not be exactly interchangeable There are other similar cases which can be gleaned from the fine print of the data sheets of digital ICs Refer also to Chapter 10 pages 123 124 for a discussion of other incompatibilities of digital ICs Appendix B Operational Amplifiers with Nonstandard Pinouts The other day A was writing to an old timer and vale one that the new IC op amps and so is the I po cae Yes each one is easy to trim but they have dif ferent trim schemes the L607 left after the two tri iS 6 aksigned is almost never standardized and Dam not even going to mention it here In some cases it s an over comp pin and oth
244. ing into my ex periment causing rectification If data arise from a well behaved phenomenon and conform to a nice Gaussian distribution then I don t care if people use their statistical analyses it may not do a Jot of harm Personally I think it does harm because Photo Courtesy Fran Hoffart Figure 12 2 SPICE printouts are almost always good for something 146 12 Roundup of Floobydust when you use the computer and rely on it like a crutch you get used to believing it and trusting it without thinking However when the data get screwy classical statistical analysis is worse than useless For example one time a test engineer came to me with a big formal report Of course it didn t help things any that it arrived at 1 05 for a Production Release Meeting that was supposed to start at 1 00 But this was not just any hand scrawled report It was handsome neat and computerized it looked professional and com pelling The test engineer quoted many statistical items to show that his test system and statistical software were great even if the ICs weren t Finally he turned to the last page and explained that according to the Statistics the ICs outputs were com pletely incompetent way out of spec and thus the part could not be released In fact he observed the median level of the output was 9 V which was pretty absurd for the logical output of an LM1525 type switching regulator which could only go to t
245. ing your time again When you WID LARIZE something that is not going to happen And the late Bob Widlar is the guy who showed me how to do it Troubleshooting by Phone A Tough Challenge These days I do quite a bit of troubleshooting by telephone When my phone rings never know if a customer will be asking for simple information or submitting a rou tine application problem a tough problem or an insoluble problem Often I can give advice just off the top of my head because I know how to fix what is wrong At other times I have to study for a while before I call back Sometimes the circuit isso com plicated that I tell the customer to mail or transmit the schematic to me On rare occa sions the situation ts so hard to analyze that J tell the customer to put the circuit in a box with the schematic and a list of the symptoms and ship it to me Or if the guy Is working just a few miles up the road will sometimes dropin on my way home to look at the actual problem Sometimes the problem is just a misapplication Sometimes parts have been blown outand have to guess what situation caused the averstress Here s an example In 10 l First Things First June a manufacturer of dental equipment complained of an unacceptable failure rate on LM317 regulators After a good deal of discussion I asked Where did these failures occur Answer North Dakota When did they start to occur Answer In February I put two and two together
246. inition of 116 buffers problems with 116 117 150 distortion 116 loop stability 116 117 117 local oscillations at high frequen cies 116 over heating 116 117 termination of cables 116 buffers digital 124 127 128 130 Bulleid H A V 13 cables See wires and cables Calibration Lab 149 155 158 196 calibrator 15 18 21 53 88 capacitance of resistors 29 30 capacitance of transformer 38 capacitance of operational amplifier 97 capacitance meters 17 25 46 97 capacitor substitution box 48 106 See also R C substitution box capacitors 5 6 9 10 18 20 37 40 48 54 56 70 75 89 99 109 111 113 116 117 120 121 127 131 135 140 145 147 149 153 155 160 162 164 165 168 170 176 179 180 183 185 191 193 204 206 types film 42 48 extended foil 42 43 43 121 Mylar 42 121 180 See also under polyester oil filled 47 paper 42 polycarbonate 42 polyester 42 47 120 See abo under Mylar polyphenylene 42 polypropylene 40 42 47 141 192 polystyrene 42 131 polysulfone 42 stacked foil 42 209 210 capacitors Continued tab type 43 Teflon 42 45 48 131 ceramic 44 48 162 111 130 179 COG low tempco 45 46 131 glass 42 NPO See COG porcelain 42 X7R stable K 45 ZSU high K 44 45 I 11 silver mica 44 46 electrolytic 40 46 48 49 120 121 135 136 147 160 164 179 185 193 aluminum 40 45 nonpolar 41 161 tantalum
247. ions that pretty much correspond to data sheet limits If you use them wisely they may be helpful subject to the conditions I listed in th previous paragraph These models are not free They are not even heap But we think they are worth what you pay for them Still they are not guaranteed an at Now seriously where can you get a model of a transistor that is guaranteed To run under all conditions I don t think you can beg or steal or borrow or buy a model of a transistor that is guaranteed 2 Or of a capacitor Or even of a resistor But I can guarantee that every op amp you can buy or make has some characteris 1 Analogy Inc P O Box 1669 Beaverton Oregon 97075 503 626 9700 2 For information on guaranteed models for CMOS transistors inquire with James Smith Semiconductor Physics Inc 639 Meadow Grove Place Escondido California 92027 4236 619 741 3360 G More on Spice 207 tics that cannot be absolutely modeled by any computer model If you happen to depend on that feature or the absence of that feature it is only a matter of time be fore you get in trouble J will also guarantee that just because you made one breadboard and it works well you cannot put that circuit into production and get 1000 units in a row to work well unless you are a smart engineer and design the circuit properly and do your worst case design studies and plan for well behaved frequency response etc And I think that
248. ircuit to test your board for leakage current The transistors are connected across the op amp s feedback path so that they act like a current to voltage detector with a wide range logarithmic characteristic Figure 5 2 You can calibrate the logarithmic current meter circuit of Figure 5 1 to sense currents be tween mA and I pA and between pA and 1 mA Leakage Can Be a Problem When a PC board comes from its manufacturer it is usually very clean and exhibits high impedance Sometimes a board starts out leaky but normally a board doesn t begin to leak until you solder it or wash it with a contaminated solvent the fifth problem Looking for leakages When you have a leaky board or a slightly less than infinite impedance connector or insulator how do you test for leakage You can t just slap a DVM on it because even on the highest range for example 20 MO the display will just read Over range That s no help if you want to read 2000 MO or 20 000 or 200 000 MQ or even higher Some DVMs or digital multimeters have a scale for microSiemens conductance which will let you resolve as high as 100 MQ But this scale does not normally have more resolution than that Leakage Can Be a Problem 53 DIGITAL VOLTMETER LOW NOISE 1 237 Vs VARIABLE NOTES S RESISTOR SENSE SELECTOR R DVM INPUT IMPEDANCE FOR SOME DVMs R iS 10M IN SOME RANGES CONSULT USER S MANUAL Figure 5 3 The DVM appr
249. is a favorite trick of mine Sometimes it drives my technicians wild but then they learn the trick and find it awfully useful Let s say I want to compare a Mylar capacitor with a ceramic ca pacitor in a small precision circuit The technician starts to remove the Mylar capac itor and install the ceramic one Wrong Instead remove one lead of the first capac itor and lift it up slightly Then tack solder one end of the second capacitor to the circuit At this point neither capacitor is actually in the circuit both Capacitors are Just waving in the breezes After the solder thoroughly cools down I can use the springiness of the leads to let me touch in one of the capacitors or the other or both as needed It only takes a sec ond to go from one mode to the other Of course I m assuming there s not enough voltage to bite my finger If there is I ll just push the end down with a popsicle stick or a bare piece of glass epoxy material If I actually desoldered and resol dered the capacitors and allowed enough time for these temperature sensitive compo nents to cool off I d probably forget what the difference between them looks like So this technique can save a lot of time and greatly facilitates A B comparisons it lets me use my eyeball to evaluate the nuances of small performance changes Of course if I have two or three of these spring loaded options at one time and they begin to get wobbly it may be
250. is true no matter where you buy your op amps What s new What color is the king s new underwear Dirty grey same as everybody else s I was at an evening session at the IEEE Bipolar Circuits and Technology Meeting in Minneapolis recently Several companies that sell CAD tools had done some se rious work to analyze the circuit for a 12 bit A D converter Even the ones that had only a little time to put in were able to show that macro models were feasible and effective as a way to do good analysis while saving computing time that was the primary objective of the study But even the ones that put in the most time at analysis did not recognize or did not comment about it that the noise of the reference and the comparator were rather large and you could not achieve 12 bit resolution without slowing down the response a lot more than you would have to do otherwise fora circuit where you did not have to consider noise If a good designer of A D converters had these tools and he knew where to look for noise or where to insert lead inductance or extra substrate capacitances he might use some of these CAD tools to help him design a better ADC But if he just believed what the computer told him he would probably be badly fooled Once a customer called me up and asked me how to get my LM108s to stop oscil lating in his circuit He explained it was a simulated LM108 with some simulated feedback resistors and simulated switches and filters Oh H
251. ise when say coupling op amps with tantalum capacitors Second I have replaced more tantalum capacitors than I can remember because they short circuited for no good reason The worst place for a bad tantalum capacitor is on a computer motherboard which is just where I found the last one I replaced The job started out as a short that was shutting down the power supply Ascertaining which rail the problem was on was easy enough I decided that because of the number of components on the bus I would have to try something less radical than temporarily isolating a section of the rail I tackled the problem by feeding a 10 kHz sine wave at about 1 V rms through a kQ resistor and monitoring the traces with a current probe I found the offending compo nent in less than a minute I may have been lucky but on the other hand my test signal wouldn t cause the ICs to draw any significant current and was of a low enough frequency not to cause enormous current in the good decoupling capacitors In the same article you said that floating TTL inputs is reasonably OK In some situations particularly noisy ones I would have to disagree I have seen nasty prob lems where this practice causes random glitches The circuit in Figure 13 2 is an absolute nightmare Yet this practice is common on IBM PC clones from Asian man ufacturers in of all things the glue logic of 80386 motherboards where high speed clocks are the norm Malcolm Watts Wellington Po
252. istors 57 58 examples in tests 58 Kelvin clips 57 Kelvin sockets 57 58 178 remote sense regulators 58 59 kilovolts 27 Koontz J 162 Lamoureux Richard 170 laser 6 latches 18 60 108 117 118 140 156 comparator latches 113 current limited power supplies 18 18 destructive 118 non destructive 118 power supply sequencing 118 storage scopes 15 leakage current 10 41 42 44 52 56 61 62 65 70 81 121 128 177 leakage currents measurement See current meter leakage inductance 35 LEDs See light emitting diodes Lembo Dr Nicholas 11 12 LF147 LF347 116 190 LFI55 LF355 188 189 LF156 LF356 18 93 96 97 188 189 LF157 LF357 102 189 LF198 LF398 132 159 LF351 116 188 LF400 LF401 91 189 LF411 188 LF6197 132 light emitting diodes LEDs 73 74 196 197 See also diodes DEADs Darkness Emitting Arsenide Diodes 74 polarity 73 reliability 73 uniformity 72 list of things that can t be causing the problem 3 7 Index LM3xx See LMIxx Linear Monolithic ICs made by National LM10 21 24 96 105 188 LM12 105 M35 temperature sensor IC 24 25 LM101 LM101 A LM301A 77 96 188 LM107 LM307 96 188 7 LM108 LM308 72 96 118 188 205 207 LM110 LM310 100 21 116 LM111 LM311 113 115 194 LM117 LM317 10 11 37 38 53 54 80 105 106 130 136 177 178 190 192 LM120 LM320 130 179 LM122 LM322 120 LM123 LM323 58 LM124 LM324 96 115 116 124 1
253. istors are often installed so close to a printed circuit board that you can t see if the leads are crossed erie the leads were often crosisil and a about every tenth transistor was the wrong gender pnp where an npn shiould have be n or Vice versa I ve thought about it a lot and I can t think of any circuits that work equally well whether you install a transistor of the opposite sex So mind your Ps and Qs your Ps and Ns your 2N1302s and 2N1303s and your 2N39048 antl 2N3906s In addition to installing a tr istor correctly you must design with it correctly First of all unless they are completely protected from the rest of the world transis tors require input protection Most transistors can withstand dozens of milliamperes of forward base current but will die if you apply only a few volts of forward bias One of my pet peeves has to do with adding protective components MIL HDBK 217 has always said that a circuit s reliability decreases when components are added Yet when you add resistors or transistors to protect an amplifier s input or output the circuit s reliability actually improves It just goes to show that you can t believe everything you read in a military specification For detailed criticism of the notion of computing reliability per MIL HDBK 217 see Ref 1 Similarly if you pump current out of the base of a transistor the base emitter junc tion will break down or zener Thi
254. ither a clipping amplifier a or diode limited bridge b will give you a clean crisp square wave whole megillah I prefer to build up modular chunks and to test each section as I get it built Then if it works that provides a pleasant positive kick several times along the length of a project But if it doesn t work it gives me a chance to get it on track before I go charging ahead and get the whole thing finished Sometimes it s just a missing capacitor Other times I ve got the whole concept wrong and the sooner I find it out the better So if you see one of my systems made up of 14 little 7 inch square sections all lashed together on a master framework don t be surprised I mean if you can make a big system work the first time more power to you often remind my technician This may not work the first time but it will be really close You may have to tweak an R here or a C there but it won t be disas trously bad Similarly when I have a circuit that does not work right do I just want to get it working right Rather not What I want is to learn what was wrong and learn what happens when I try changes So I don t give my technician a long list of changes to make all at once tell him make this change first and see if the gain gets better If that doesn t work make that change and then that one and keep an eye on the gain and the phase Then try this tweak on the output stage and trim it for lower distor
255. itle H Series 621 3815 ISBN 0 7506 9184 0 ISBN 0 7506 9499 8 softcover Butterworth Heinemann 313 Washington Street Newton MA 02158 1626 Linacre House Jordan Hill Oxford OX2 8DP United Kingdom 1098 Printed in the United States of America Editorial design and production services provided by HighText Publications Inc Solana Beach California IO nrmO Ow Contents Foreword vii Acknowledgments x About the Author xi First Things First The Philosophy of Troubleshooting 1 Choosing the Right Equipment 14 Getting Down to the Component Level Resistors and Inductors 26 Getting Down to the Component Level Capacitor Problems 40 Preventing Material and Assembly Problems PC Boards and Connectors Relays and Switches 50 Understanding Diodes and Their Problems 65 Identifying and Avoiding Transistor Problems 77 Operational Amplifiers The Supreme Activators 89 Quashing Spurious Oscillations 108 The Analog Digital Boundary A Never Never Land 120 Dealing with References and Regulators 135 Roundup of Floobydust Loose Ends That Don t Fit Elsewhere 143 Letters to Bob 155 Real Circuits and Real Problems 172 Appendixes Digital ICs with Nonstandard Pinouts 187 Operational Amplifiers with Nonstandard Pinouts 188 Understanding and Reducing Noise Voltage on Three Terminal Regulators by Erroll Dietz 191 Testing Fast Comparators for Voltage Offset 194 Vp vs Ipon
256. its It is a good circuit to practice and sharpen up your troubleshooting skills on A DVM is not a useless tool but a scope does a lot of things better Now let s move on to the Positive Adjustable Regulator Vin O Q GROUND Positive Adjustable Regulator Troubleshooting Positive Adjustable Regulators Indication of Trouble Possible Cause Vout 1 25V R 1 GROUND Solution Output V much too low Output V much too high Output oscillates Output t00 noisy Input much too low R2 or R3 shorted C3 shorted or reversed Load is short circuit or too heavy CRI is shorted or backwards C2 shorted or reversed Leakage at ADJ pin Output 0 8 V Pot wiper open Ri too low Input V much too high Leakage at ADJ pin General Check for C1 Load bounces too much Input too noisy Load oo noisy or jumpy Check input with scope Check resistors Check C3 or remove Unit gets hot Try disconnecting load Check diode Check C2 or remove Check for dirty board Output shorted to supply Check voltages on pot Check value Check with scope Check for dirty board Note the frequency C1 should be 0 1 uF or more see data sheet Add a lot of C2 and see what helps Study noise on Input Increase CI C2 or C3 Increase C2 and see what happens also Cl or C3 178 14 Real Circuits and Real Problems Output drifts falls Unit much too hot Increase the
257. ive Most VFCs have excellent linearity but the linearity depends on the timing capacitor having low dielectric absorption Teflon makes the best VFC timing capacitors but polystyrene polypropylene and ceramic capacitors with a COG characteristic are close behind Refer to the LM131 LM331 data sheet for examples and notes Trimming a VFC to get a low temperature coefficient is not easy because the overall temperature coefficient depends on several components including the refer ence as well as various timing delays See Ref 4 for VFC trimming procedures or at least to appreciate how much effort is involved when you buy a well trimmed unit The Other Way Back Frequency to voltage converters FVCs are often used as tachometers or in conjunc tion with a VFC and an optoisolator to provide voltage isolation in an analog system FVCs are about as linear as VFCs and about as drifty so the temperature trimming problem is the same as for a VFC One exception is if you re using cascaded VFC FVC pairs in which both circuits are in the same location and at the same tem perature In that case you can often get by with trimming only one of the pair or by just making sure the TCs match Another problem with FVCs is that you often want the response to be as fast as possible but need to keep the ripple low The design of a filter to accomplish both objectives will of course be a compromise My rule of thumb is that you can keep the ripple dow
258. k A piece o f equipment bea vare t that you are going into battle with inad K equate tools certain clues m mayi take you j longer than necessary to spot In many lt Cases when you spent too much simply because you were foolish ox were u unaware of a particular troubleshooting technique but f i finding one small problem the time was wasted it in other cases the time was wasted because of the lack of a particular piece of equipment It s important foryoute recognize this last mentioned situation Learning when you re wasting t time because you lack the proper equipment is part of your education asa troubleshooier ei In addition to the proper tools you also need to have a full understanding of how sed to work Pm sure you ve seen engineers or technicians work fi 1 i they finally find the solution say Oh I didn t know it was supposed to work that way You can avoid this scetiati y sing equipment that you are comfortable and familiar with re EA A L The following equipment is essential for most analog circuit troubleshooting tasks This list can serve as a guide to both those setting up a lab and those who just want to make sure that they have everything they need that they aren t missing any tricks A dual trace oscilloscope It s best to have one with a sensitivity of 1 or 2mV cm and a bandwidth of at least 100 MHz Even when you are working
259. k and get it running When I get it running I ll publish it somewhere where you can all see it The equivalent of a solid state fuse that can handle as much as 200 V of DC Meanwhile when you need some fuse protection on a DC power supply just put a fuse in the secondary of the transformer so it sees AC current flow and AC voltage rather than DC Inductors and Transformers Aren t So Simple Inductors and transformers are more complicated than resistors nonlinearity is rife Their cores come in many different shapes and sizes from toroids to pot cores and from rods to stacks of laminations Core materials range from air to iron to any of the 3 Heinemann Electric P O Box 6800 Lawrenceville NJ 08648 609 882 4800 Equivalent Circuits Demystify Transformers 35 ferrites I am not going to presume to tell you how to design an inductor or trans former or how to design circuits that use them but I will discuss the kinds of trouble you can have with these components For example you can have a good core mate rial but if there is an air gap in the core and you don t carefully control the gap s width the energy storage and the inductance of the component can vary wildly If someone has substituted a core of the wrong material you may have trouble spotting the change an inductance meter or an impedance bridge can help But even with one of those tools you re not home free For most inductors and transformers with co
260. k hard to make goodjones Refer to How to Read a Data Sheet Appendix F and Ref 10 because bad data sheets can get the user into trouble Different Methods Uncover Different Errors Now that you know some op amp problems to look out for how do you actually troubleshoot an op amp circuit usually split my plan along two lines AC and DC problems Examples of AC problems include oscillations and ringing DC problems include bad DC output entors and pegged outputs which are outputs stuck at either the positive or negative supply rail Obviously you heed a scope to be sure the circuit isn t oscillating Ita ies nervous iwhen I find out that the customer I m trying to Help dbesy t scope 1 can und rstand if an engineer only has a crummy scope but there are certai in problems you Cannot expect to solve nor can you even verify a design if you don t have any scope at all If the problem is an AC problem I first make sure that the input signals are well behaved and at the values expect them to be Then I put my scope probes on all the pins and nodes of the circuit times it s appropriate to usea 10 probe and metimes I AC couple the scope sometimes I DC other times I shift to 1x mode couple it I check all the pins especially the power supply pins Then depending on around and gather symptoms by adding capacitors or RC se two Probes to see if the input nd output Py and I sim
261. king too many iterations Sometimes problems happen that only a computer expert can address But when you ask the computer guru for advice you might get no advice or what s worse bad advice After all many computer wizards know nothing whatsoever about linear circuits If the wizard tells me Hey don t worry about that or Just change the voltage resolution from 0 1 mV to 10 mV then I must explain to the wizard that although that advice might make some computers happy it gives me results that are completely useless Talking to computer wizards is sometimes difficult Even if you do everything right the computer can lie to you Then you have to make a test to prove that you can get the right answer and the computer can t For example one time we had a circuit with 60 transistors and a diff amp appeared to be oscillating even though it was clearly switched OFF The computer experts told us that we had obviously made a mistake So we disconnected and re moved 58 of the transistors there was nothing left but 2 transistors and one of them was biassed off by a full volt And its collector current was oscillating at 100 kHz between plus and minus 10 HA even though nothing in the circuit was moving When we confronted the computer experts they belatedly admitted there was an internal timing error which they proceeded to fix But it certainly took us a diffi cult week to get them to admit that Sugar a
262. ks like and how much trouble one can cause in a critical circuit I feel sort of sad that today s young people aren t building kits for electronic equipment In the old days you could learn all about cold soldered joints before you got into industry by building a Heathkit or a Knightkit I built several of each I made a few cold soldered joints and I had to fix them With modern wave soldering equipment it s fairly easy to avoid cold soldered joints in your production line But on hand soldered circuits it s always a possibility to have cold soldered joints so if you have a nasty problem don t forget the old solution Re solder every joint Once in a while you ll find a joint that never got any solder at all If for some reason you have acid core solder around it s mainly used for plumbing and is not found in most electronics labs for good reason Kkeep it strictly labeled and segregated from ordinary rosin core solder Acid will badly corrode conductors Also keep specialty solders such as high temperature solder low tem perature solder silver solder and aluminum solder in a separate place to avoid con fusion There is also solder for stainless steel which requires special flux Recently I have heard people promoting silver solder as a kind of superior solder for splicing speaker cables The Golden Ear set claim that this solder makes the audio sound better However I must caution you that silver solder
263. l by itself and I was just sitting there watching it not doing anything But when you look inside one of the input s lead bond wires has blown out evaporated and in the usage circuit there are only a couple 10 kQ resistors connected to it Well you can t blow a lead bond with less than 300 mA Something must have bumped against that input lead and shorted it to a source that could supply half an ampere There are many cases where looking inside the part is very educational When a capacitor fails or a trim pot get my hammer and pliers and cutters and hack saw and look inside just to see how nicely it was or wasn t built To see if I can spot a failure mechanism or a bad design I m just curious But sometimes I learn a lot Now when I have finished my inspection and I am still mad as hell because I have wasted a lot of time being fooled by a bad component what do I do I usually WID LARIZE it and it makes me feel a lot better How do you WIDLARIZE something You take it over to the anvil part of the vice and you beat on it with a hammer until it is all crunched down to tiny little pieces so small that you don t even have to sweep it off the floor It sure makes you feel better And you know that that compo nent will never vex you again That s not a joke because sometimes if you have a bad pot or a bad capacitor and you just set it aside a few months later you find it slipped back into your new circuit and is wast
264. le And eee going to go back and tweak the ause T TTL paris would be damaged if someone igure 12 Sb is better Here sa chunk of late breaking floobydust the topic i is those solderless breadboards which consistiof anumber of metal strips and solderless connectors hidden underneath ha plastic panel with lots of holes in it Schools often use them to itance The breadboan usual y have 2 3 ors pF between 1 adjacent strips On a a good day only a wise engineer could plana out that all the capacitors sprinkled inroashout the cirouit ts acitors elbse toa chip difficult pect phi alt oughithey are not inexpensive use cheap plastics such as nylon Otiaiwann humid day cheap plastics do not offer high insulation resistance Nobody wants to talk about what kind of plastic the breadboards are made of Finally Mr Scott Bowman of Dublin CA points out that after you insert enough wires into any given hole the solderless connector will scrape sufficient solder off the wire so thatthe scraps of solder will pile up and start to intermittently short out to an adjacent strip Further the adhesive that holds on the back panel tends to hold the solder scraps in place so you can t clean the scraps out with a solvent or a blast of air I didn t even think about these solderless breadboards when I wrote my series because see them so rarely at work They just have too many disadvantages to b
265. like the proverbial alligator and chomp on your ankle and cause you a great deal of misery Instead when you think you have designed and installed a good fix for the oscillation BANG on the output with square waves of various amplitudes frequencies and amounts of load current One of the easiest ways to perform this test is to connect a square wave generator to your circuit through a couple hundred ohms in series with a 0 2 uF ceramic disc capacitor Connect the generator to the scope so you can trigger on the square wave signal Also apply an adjustable DC load that s capable of exercising the device s output over its full rated output current range or in the case of an op amp over its entire rated range for output voltage and current To test an op amp try various capacitive loads to make sure it can drive the worst case you expect it to encounter For some emitter follower output stages the worst case may be around 0 to 50 pF The Pease s Principle oscillation may disappear with heavier capaci tive loads The adjacent figure does not show a rec ommended value for some of the parts If you are testing a 5 A regulator you may want a load resistor as low as an ohm or two If you are evaluating a low power amplifier or a micropower reference a value of a megohm may be reasonable both for the load resistor and for the resistor from the square wave generator Thinking is recommended Heck thinking is
266. ll behaved way The as witha zener is to starve it If you pass too little ly them In con srformers ren from ergy or you Could shop for a controlled avalanche to survive safely and reliably repetitive excursions ceed their rated breakdown voltage The manufacturers of these devices can give you a good explanation of how to keep out of trouble If you do need a zener to conduct a surge of current check out the specially de signed surge rated zener devices also called transient voltage suppressors from General Semiconductor Industries Inc Tempe AZ You ll find that their 1 W de vices such as the IN5629 through IN5665A can handle a surge of current better than fa 6 Understanding Diodes and Their Problems most 10 or 50 W zeners If you need a really high current zener a power transistor can help out Figure 6 4 As mentioned earlier a diode tends to fail by becoming a short circuit when overpowered and zeners cannot absorb as much power as you would expect from short pulses How dreadful but can IC designers serendipitously take advantage of this situation Yes The Vos of an op amp usually depends on the ratio of its first stage load r sistors IC designers can connect several zeners across various small fractions of the load resistor When they measure the Vos they can decide which zener to short out or zap with a 5 ms 0 3 to 1 8 A pulse The zener quickly turns into a low impedance 1 Q short
267. lled back wards Electrolytic capacitors installed back wards Broken wires Links installed in the wrong place or MISSING Ftakey connector MP P O Box o0 Harasbure PAST TEs E OSOL ODLOG Hnterconmectian Products Ine 2604 SOUND Garnsev Sanpa Wipe Ye ee Fy dO sg Drakes PO Bast Piet Rivel Pah tuinesobr oo dp ey bl pass 186 14 Real Circuits and Real Problems Solder shorts Unsoldered joints cold soldered joints IC plugged in with the wrong pins e Transistors backwards e Faulty meter or other tester If you can check for these problems you have a good chance of catching 50 of the problems and then the really tough ones will be left over for you to solve Best of luck Aptitude and Attitude If you have to treat your troubleshooting work as a chore as a necessary but boring task well I can t tell you how to do your work but I have always found that if you can treat any such work as a game then you have a chance to bring more innovative approaches to the job and chances of doing it better and having fun I mean in my job if I get the job done but I am not having any fun then something is wrong Hell sitting here typing this at 12 05 AM is fun If it weren t I would stop and go to bed But hey fun or not this is the end so good night Appendix A Digital ICs with Nonstandard Pinouts _ I printed this little list of ICs that have nonstandard pinouts in EDN Magazine back in the fall of 1990 say
268. lly I just need a couple simple Rs and Cs with an alligator clip to select the right ones 28 Line adapters those 3 wire to 2 wire adapters for your 3 prong power cords You References need several of them You only need them because too many scopes and function generators have their ground tied to the line cord s neutral You need some of these to avoid ground loops You also need a few spares because your buddies will steal yours For that matter keep a few spare cube taps When they rewired our benches 2 few years ago the electricians tried to give us five outlets per bench I stamped my feet and insisted on ten per bench and that s just barely enough most of the time You ve come to the end of my list of essential equipment for ordinary analog circuit troubleshooting Depending on your circuit you may not need all these items and of course the list did not include a multitude of other equipment that you may find useful Logic analyzers impedance analyzers spectrum analyzers program mable current pumps capacitance meters and testers and pulse generators can all ease various troubleshooting tasks Each of you will have your own idea of what is essential and what is unnecessary for your special case and I would be delighted to get feedback on this subject You can write to me at the address in the Acknowledgments section of this book Collins Jack and David White Time domain analysis of ahasinp helps to alleviate DSO
269. long term stability are not quite as good as those of Teflon capacitors but are comparable to those of other good precision film capacitors And the TC is better than almost any thing you can buy So if you want to make a sample and hold circuit usable over the military temperature range you ll find that COG capacitors are more compact and less expensive then Teflon parts Many but not all ceramic capacitors smaller than 100 pF are made with the COG characteristic You can get a 22 000 pF COG capacitor in a Q 3 in square package if you re willing to pay a steep price About every year or so a customer calls me about a drift problem His V F con verter has a poor TC even though he said that he had put in a COG 0 01 pF capacitor 46 4 Getting Down to the Component Level Capacitor Problems as the main timer Troubleshooting by phone it s always a wonderful challenge I ask him This COG ceramic 0 01 F capacitor is it as big as your little finger nail He says Oh no it s a lot smaller than that I reply Well that s too small it can t be a COG Problem solved Actually there are some small COG 0 01 uF capac itors but they are pretty uncommon unless you order them specially o One observed failure mode for ceramic capacitors can arise when the capacitor S leads are attached to the dielectric with ordinary low temperature solder When the capacitor goes through a wave solder machine the lead may bec
270. look at them but not when you are not looking at them You can eliminate this effect by making your own 1M very high impedance probe that presents only a pF capacitive load 100k 11 pF 10x PROBE 1 pF SILVERED MICA HIGH SPEED OSCILLOSCOPE 11 pF 10x PROBE rkers wh n he carries one of his large chor Figure 10 6 The author irritates co wo i ties to the photocopier and tries to figure out how S to figure out how Fto duplicate such a large drawing In fact some say that he irritates co workers rhdst ofthe time supply voltages In some cases the DC rejection can be 80 or 100 dB but high fre quency noise on a supply oan come through to the output virtually unattenuated So you must plan your system carefully It might be a good idea in a critical application to use a completely separate power supply regulator for your precision DAC At least you should add plenty of good power supply bypass capacitors right at the power supply pins ceramic and tantalum capacitors Sometimes when you feed signals to a DAC without passing them through buffers the noise ringing and slow settling of the digital signals can get through to the analog side and show up on the DAC output Nobody has a spec for rejection of the noise on DAC bit lines in either the HIGH or LOW state Maybe vendors should 128 10 The Analog Digital Boundary specify this parameter because some DACs are good and some
271. lowers which you d expect to be very docile because their voltage gain is less than 1 have a tendency to scream or oscillate at high frequencies So whether you buy a buffer or roll your own watch out for this problem Also a buffer can have a high frequency roll off whose slope increases suddenly at 40 or 60 MHz and thus can contribute phase shift to your loop back down at 6 or 10 MHz You can beat this problem but you have to plan A buffer can also add a little distortion which the op amp cannot easily cancel out at moderate or high frequencies Since buffers don t usually have a spec on this distortion beware Also if you re run ning the output s quiescent bias current as Class AB you must be sure that the DC operating current is stable and not excessive You must set it high enough so that you don t get distortion but not so high that power consumption becomes excessive One of my standard procedures for stabilizing a unity gain follower Stage is to put feedback capacitance around just part of the loop Figure 9 5 This circuit tolerates capacitive loads because the buffer decouples the load while the feedback capacitor around the op amp provides local stability Most unity gain buffers whether mono lithic hybrid or discrete are unstable with inductive sources so keep the input leads short A series resistor may help stability as it does for the M310 but it will slow down the device s response Many high
272. lpi thinimize arcing or burning You gotta read the data sheet But these days not all relays are mechanical First of all there are mercury wetted relays which are credited with giving no contact bounce Most of these must be held upright or they do not work There are mercury wetted reed relays that do not bounce 3 Coto Type 1240 12 2104 Coto Corporation 55 Dupont Drive Providence Rhode Island 02907 401 943 2686 62 5 Preventing Material and Assembly Problems and can be used in any position But even they will not work at temperatures colder than 38 C where mercury freezes Then there are the solid state relays Some of them can switch many amperes but they often use SCRs and thus have a loss of more than a volt You can t use that for a little signal Others have low ohm MOSFETs and can handle a few amperes with low losses and low offset voltage But the big ones have a lot of leakage and capaci tance which is not always mentioned The little ones are nice and delicate for preci sion switching but cannot carry many milliamperes So for high reliability you have to be pretty knowledgeable and thoughtful and selective when you choose a relay or you ll pick one that s inappropriate for your application and you ll be embarrassed when some of them provide poor performance or fail sooner than expected When Is a Relay Not a Relay When it is just a switch mechanically operated by ha
273. lunch the engineer told him I found the problem it s a mismatch between Q17 and R18 Write up the ECO and when I get back from lunch Ill sign it Unfortunately the good rapport between the engineer and the technician broke down there was some miscommunication The technician got confused and wrote up the ECO with an incorrect version of what should be changed When the engineer came back from lunch he initialed the ECO without really reading it and left for a two week vacation When he came back the modules had all been fixed potted and shipped and were starting to fail out in the field A check of the ECO revealed the mistake too late The company went bankrupt It s a true story and a painful one Don t get sloppy with your paperwork don t let it happen to you Failure Analysis One of the reasons you do troubleshooting is because you may be required to do a Failure Analysis on the failure That s just another kind of paperwork Writing a report is not always fun but sometimes it helps clarify and crystallize your under standing of the problem Maybe if a customer had forced my engineer friend to write exactly what happened and what he proposed to do about it that disaster would not have occurred When I have nailed down my little problem I usually write down a scribbled quick report One copy often goes to my boss because he is curious why it s been taking me so long I usually give a copy to friends who ar
274. ly exhibit lowleakage However if you need a guarantee of very low leakage you will have to test and select the devices yourself as many people do Digital Computers To avoid making unpleasant comments I will simply say that I hope somebody else writes a good book on troubleshooting these Software NO comment References 134 10 The Analog Digital Boundary So we take leave of the analog digital world sort of In the next chapter we ll visit another area of great importance to analog digital electronics but it is a purely linear region perhaps the most purely linear References Armed with knowledge about references we ll move on to the troubleshooting of power electronics including switching regulators I Jung Walter 555 Timer Cookbook Howard Sams and Co Indianapolis IN 1977 Pease Robert A Wideband phase locked loops take on F V conversion chores EDN May 20 1979 p 145 Also available as AN 210 in NSC s Linear Applications Book 1986 1989 etc New Phase locked loops Have Advantages as Frequency to Voltage Converters and more Knapp Ron Evaluate your ADC by using the crossplot technique EDN November 10 1988 p 251 Pease R A Versatile monolithic V Fs can compute as well as convert with high accuracy Electronic Design December 6 1978 p 70 Also available as Appendix D in National Semiconductor Corp Linear Applications Handbook Santa Clara CA 1
275. lytechnic Wellington New Zealand Dear Mr Watts Thank you for your comments You question the practice of pulling current out of a transistor s base circuit If you were to actually pull current out of the base and Figure 13 3 2 Letters to Bob 161 This scheme will make your tantalum capaci tors immune to damage from reversal forward bias the transistor s intrinsic base emitter zener you could cause damage If instead the circuit prevents zener breakdown and its clamps prevent excessive re verse Vpp then everything is fine I probably wasn t clear enough about those points Note that many discrete transistors are not as fragile are not going to be damaged or degraded by zener breakdown as monolithic transistors are I m not sure why Also you recommend against plugging CMOS components into live powered up circuits and working without a ground strap You say you have seen such procedures directly cause unreliability and failures OK you have and other people have but I haven t Perhaps RAM chips are more fragile than 74Cxx chips So I must retract my cavalier and flippant n remarks In general you should use ground straps and not plug ICs into live circuits unless you a are sure gt you know what you re doing and are pre _ pared to accept Ic failures tee However when troubleshooting sometimes you may have to resort to these mea sures You should then be aware that they may not n
276. mance For example if a buffer can drive heavy loads and can han die fast signals cleanly at no load the maker isn t doing anybody any favors if there is no mention that the distortion goes sky high if the rated toad is applied 201 Another example is the application hint for the LF156 family Exceeding the negative common mode limit on either input will cause a reversal of the phase to output and force the amplifier output to the corresponding high or low state Ex ceeding the negative common mode limit on both inputs will force the amplifier output to a high state in neither case does a latch occur since raising the input back within the common mode range again puts the input stage ahd thus the amplifier in a normal operating mode That s the kind of information a manufacturer should really give to a data sheet reader because no one could ever guess it Sometimes a writer slips a quirk into a characteristic curve but it s wiser to draw attention to it with a line of text This is because it s better to make the user sad before one gets Started rather than when one goes into production Con versely if a user is going to spend more than 10 minutes using a new product one ought to spend a full five minutes reading the entire data sheet FINE PRINT What other fine print can be found on a data sheet Some times the front page may be marked advance or pretimi nary Then on the back page the fine pri
277. me thank heaven most of them have been driven out One of the stori s that keeps rattling around the industry is about a group of engi neers who decide to band together and start a new computer company The smartest one is assigned to do the main processor board Another smart engineer does all the interfaces And the smart but green kid engineer is assigned to do the switch mode power supply because of course that s the easiest part to do Anybody who has worked on a big switcher will probably speak up right away The switcher is not as easy as it seems In the end the power supply d sign takes a lot longer than every ict Ly E AA aA h ge o One day the young engineer opens up the compartment where the balky power _ supply resides and it blows up in his face After his co workers take the poor fellow to the hospital they ask around and find a consulting engineer who makes a living out of fixing exactly this kind of switcher problem The switcher design was slightly ed th fian expert before it would work correctly So re no Simple task Don t hesitate to call in an expert bsbanti ily true the consulting engineer would have ieniaie S OOenfing engiheer woul Regulators Suit Different Power Levels There are several different configurations of switch mode regulators At low power levels capacitiyely coupl d switcher d signs are simple but don t provide much choice of Vou 1 9 X Vin 0 9 X V
278. ments tend to lose notes Well where I work drafting departments doa task and if that includes adding notes then they add what ever the engineer requires Actually I usually do my own drafting Some people gripe but the information is all there I don t usually get too enthusiastic about using incandescent lamps for amplitude control It s true that most oscillators don t have distortion as low as 80 dB but you can put the output of a mediocre oscillator through a filter and get distortion lower than 80 dB Yes I believe that We have found that our lowest input current amplifiers such as 18 19 22 23 24 25 26 27 Letters to Bob 159 LMC662 have the lowest leakages in plastic DIPs which are made by an automated process and are untouched by human hands The TO 99 and CERDIP packages are not nearly as good as the plastic minidips which do 10 5 Q repeatably Protection diodes can rectify high frequency noise I have never seen this one Boy Roy you must live in a nasty neighborhood for ambient RF noises You can probably run a transistor radio without any batteries I forgot to even mention thermal probes We use such probes more for characterizing than for troubleshooting Often a soldering iron does the troubleshooting job faster but more crudely The LF198 data sheet mentions that you should not let the Sample input move too slowly Are there other S H circuits that ar
279. mer with an LM555 ora 10 minute timer with an LM322 but that would be wrong Instead you could make a simple 4 Hz oscillator using one quarter of an LM324 or LM339 and cheap small components This oscillator can drive a CD4020 or CD4040 the last output of that counter Q12 or Q14 can time very accurately and conveniently Such an arrangement is cheaper and much more accurate and compact than what you get if you blow a lot of money on a 47 uF polyester capacitor for a long interval timer or put up with the leakages of a tantalum capacitor which no manufacturer wants to guarantee In addition in just a few seconds you can trim the moderate frequency oscillator by looking at an early stage of the divider trimming a long interval timer can take hours The CMOS counters are inexpensive enough and these days for 2 to 20 minute timer applications I can usually convince customers not to buy the linear part The LM555 data sheets tell you to avoid timing resistors with values higher than 20 MQ Nowadays though you can get a CMOS version LMCS5S5 or equivalent or use a CMOS comparator or a CMOS op amp to work at 120 Time for Timers 121 100 MQ or more Just be careful about board leakage and socket leakage as you would with a high impedance op amp circuit Then you can use a smaller higher quality capacitor Furthermore it is a nontrivial statement that not all 555s work similarly some manufacturers 555s have different intern
280. milar to the Schottky s Just the other day read about some new Germanium Schottky diodes that have even lower Vis High speed and ultra high speed sometimes also called high efficiency silicon 65 Log if 66 6 Understanding Diodes and Their Problems 100 mA 10 mA 100 pA SER tt tt Ey 100 pA 22 Figure 6 1 ve Eii s low Wea asi Schottky yare ni ii Hab eeveee volta rat rely ut the popular nthe 1N4148 actually do have about 10or 15 nA of Ise of their gold ame So alti these diodes gt them in ous noway ne nde use transistor sicollector base j junction 1 popular 2N930 0r n have low seakage these hg j thar lpA leakage eveniat 7V or r10 pA at 50 V Although this low leakage is not guaranteed it s usually quite consistent However this c b diode generally doesn t turn ON or OFF very quickly Another source of ultra low leakage diodes are the 2N41 17A and the PN4117A 18 A and 19 A These devices are JFETS with very small junctions so leakages well below 0 1 pA ar standard with 1 0 pA max guaranteed not bad for a 0 40 part The capacitances are small too Turn Em Off Turn Em On 67 Speed Demons When a diode is carrying current how long does it take to turn the current off There s another wide range phenomenon Slow diodes can take dozens and hundreds of microseconds to turn off For example the collector base ju
281. modern op amp Avoid the absurdity of trying to measure a common mode gain of zero to compute that your CMRR slope is infinite You ll get more meaningful An Uncommon Mode 9i 100pF Figure 8 1 If you run an op amp at such high impedances that lg X Ris more than 20 mV you ll be generating big errors and a Vogser trim pot can t help you cancel them out Please don t even try an OFFSET wn WW ADJUST Vow Figure 8 2 The CMRR of an op amp can t be represented by a single number It makes more sense to look at the CMIRR curve AVGs versiis AV ki rand note its nonlinearities compared to a straight line with a constant slope of part in 100 000 results if you just measure the change in offset voltage Vos as a function of common mode voltage Vem and observe the linear and nonlinear parts ofithe curve What s a good way to measure the change in Vos versus Voy the CMRR of an op amp I know of a really good test circuit that works very well First How Not to Test for CMRR ee The first thing always tell people is how notte measure CMRR In Figure 8 3 if you drive a sine wave or triangle wave into point A it seems like the output error as seen by a floating scope will be N 1 times Vey divided by the CMRR But that s not quite true you will see N 1 times the CM Error plus dhe Gain Error So at moderate frequencies where the gain is rolling off and the CMRR is still high you will see mos
282. mp s output is resistive but as far as I am concerned they re a complete waste of time The output impedance of an op amp is usually not purely resistive And if the impedance is low at audio frequencies it often starts to rise inductively at high frequencies just when you need it low Conversely some op amps such as the NSC LM6361 have a high output impedance at low frequency which falls at high frequencies a capacitive output characteristic so when you add more capacitance on the output the op amp just slows down a little and doesn t change its phase very much But if an op amp is driving a remote low resistive load that has the same impedance as the cable the terminated cable will look resistive at all frequencies and capacitive loading may not be a problem But you still have to be able to drive that low impedance 75 Q load You can decouple an inverter s and integrator s capacitive load as shown in Figure 8 9 If you choose the components well any op amp can drive any capacitive load from 100 pF to 100 uF The DC and low frequency gain is perfectly controlled but when the load capacitor gets big the op amp will slow down and will eventually just have trouble slewing the heavy load Good starting point component values are R1 47 to 470 Q and CF 100 pF These values usually work well for capacitive loads from 100 pF to 20 000 pF If you have to make an integrator or a follower you ll need an additional 4 7 kQ resisto
283. mproving by a factor of 2 every time you cool them off another 11 degrees more I m not sure who led him to believe that but in general modern electronic compo nents are not greatly improved by cooling or the absence of heating In fact those of us who remember the old vacuum tube days remember that a good scope or volt meter had an advantage if you kept it running nice and warm all the time because all the resistors and components stayed dry and never got moist under humid conditions I won t say that the electrolytic capacitors might not have liked being a little cooler But the mindless effort to improve the reliability by keeping components as cool as possible has been overdone I m sure you can blame a lot of that foolishness on MIL HBDK 217 and all its versions In some businesses you have to conform to 217 no matter how silly it is but in the industrial and instrument business we don t really have to follow its every silly quirk and whim One guy who is arguing strenu ously about 217 is Charles Leonard of Boeing and you may well enjoy his writing Ref 4 So if something is drifting a little and you think you can make a big improvement by adding a fan and knocking its temperature down from 75 to 55 C I m cautioning you you ll probably be disappointed because there s not usually a lot of improvement to be had It is conceivable that if you have a bad thermal pattern causing lots of gradients and convection you can cu
284. n to nearest authorized Varoom dealer Well it s nice to know that one out of the five problems can be solved by the car s owner But personally I prefer driving a car that can be fixed and troubleshot by ordinary human beings Go ahead ask me what do I drive that fits that description A 1968 VW Beetle My wife has a newer car she has a 1969 Beetle If it doesn t run which is a fairly rare occasion I know how to troubleshoot it Do I look at a table Yes but not in a book I look at a table inside my head What if I suspect the carburetor or fuel pump I pour a tablespoon of gas down the carburetor s throat If it fires and runs and then dies I know that I can provide gas but the carburetor can t Then there are a number of things I can do such as holding a gallon of gas on top of the car to provide fuel No pump needed I haven t had to do that but I ve seen a friend do it If I suspect the spark I always have enough spare parts that I can check the spark check the timing swap in spare points rotate the distributor whatever is needed keep spare parts in my car to help a friend when I spot a VW broken down along the highway I even keep a spare envelope SASE in the car so if I try to help a guy and E we can t fix it and we can t figure out why I give him the envelope and he can mail it back to me to tell me what the problem turned out to be Last year I got a letter back from a guy who broke down
285. n meet the eye Heck I just had a couple yards of Teflon insulated wire sitting outside my kitchen window running over to a sensor for an electronic thermometer The wire would only get on the average 1 hour per day of direct sunlight After 10 years the yellow wire was still in good shape but the white insulation had just about died utterly Who wants to explain that one Recently an engineer showed me the results of a study of wire for loudspeakers He showed that the inductance of ordinary two conductor wire per 20 feet can cause a small but noticeable phase shift perhaps 10 degrees at 20 kHz even with the large and ultra expensive speaker cables 10 per foot and up But when he took flat ribbon cable with 40 conductors which is typically used to bus digital signals 64 5 Preventing Material and Assembly Problems around and tied every other wire in parallel as the hot side and every other wire in parallel as the return side the inductance was lower by a factor of 10 When you think of it if you take some cables that are supposed to look like 75 Q and you par allel 10 of these pairs it should have pretty flat response quite suitable for driving an 8 2 load Much better than some of the monstrous huge cables and less bulky and much cheaper too References 1 Pease Robert A Programmable Pulser Takes 5 nsec Steps EDN May 10 1990 p 150 2 Kidder Tracy The Soul of a New Machine Avon Books New
286. n to about 0 01 of the Vjullscale but with the simplest filters you must keep the carrier at least 100 times the Finin With more sophisticated filtering such as two Sallen Key filters cascaded the 3 dB point can be 1 10 of the slowest carrier For example with a carrier frequency in the range 5 10 kHz the signal can go from DC to 500 Hz Ref 5 If you need still faster response see Ref 2 which shows in a cook book circuit how to use a simple phase locked loop to make a surprisingly quick FVC S H Circuits Electronic Stroboscopes A VFC produces an output proportional to the average value of its analog input during the conversion If you need to digitize rapidly changing signals for example to reconstruct waveforms in the digital domain you need a different type of ADC and you almost always have to precede it with a sample and hold circuit Designing S H circuits is a complicated challenging endeavor Meeting exacting specs often requires an expensive module or hybrid circuit A major problem of S H circuits is dielectric absorption or soakage in the hold capacitor Ref 6 If you need to runa relatively short sample time with a long hold time and if the new output voltage can vary con siderably from the previous sample the soakage may be your biggest problem For example if an S H circuit acquires a new voltage for 5 us and then holds it for 500 us you can tell approximately what the previously held signal was because the n
287. n turn causes as indicated in Figure C 3 the output inductance to decrease until the current limit resistance bond wire resistance and lead resistance becomes the dominant resistance at the output This is true for positive and negative regulators for adjustable and fixed regulators and for large and small regulators In the past it has been assumed that Zou VS Frequency was a fixed curve but really there is a family of curves at different current levels see Figure C 4 In conclusion the typical values of output bypass capacitance that users of 3 terminal regulators have traditionally used may provide the expected noise reduction at some frequencies but not at every frequency In most cases a few microvolts of power supply noise peaking at 5 or 10 kHz will not cause any problems However if 193 i C Understanding and Reducing Noise on 3 Terminal Voltage Regulators Output Inductance vs Load pie Current LM317 Vout i uld be avoided in R The most effective noise willbe operated 20 pF sho 8 over which the regulator 10k tk Frequency Hz 1 Ta TD ubinducte currents a it aK 3 engihees utp if ue lication EEN et have 10 applic 1 change the range of load the cular hie o and ae Figure 4 Output impedance vs frequency at different current levels Appendix D Testing Fast Comparato
288. nal regulators is inductively reactive over a certain frequency range one can surmise that adding output capacitance to improve noise performance and transient response can also have other effects The examples given in this appendix will use the LM317 adjustable voltage regulator but this information can be scaled and then applied to all other kinds of 3 terminal voltage regulators As shown in Figure C 1 the output impedance of the LM317 over the kHz to 1 MHz frequency range is inductive This has nothing to do with long wires but is simply another way of looking at the fact that the gain of any operational amplifier or Output Impedance vs Frequency N T e T eT TER LL L g 1 000 1 tH pene i a a fate f A FHH a 0 100 Hte F i ttt gt SLE U A TTI EB 0 010 font il tf tert e we os A O H fo ttt A HEEE 100 1K 10K 100K tM Frequency Hz LM317 s output impedance vs frequency at 500 mA Reprinted with permission of Electronic Design Figure C 2 192 C Understanding and Reducing Noise on 3 Terminal Voltage Regulators 25 22 5 Noise Output Vs C zo adanan 10 Frequency kHz Typical noise peaks produced by an LM317 and various capacitive loads regulator is designed to roll off at 6 dB per octave This condition is usually not of much concern to the av
289. nalog or a digital designer can encounter with digital ICs First unless proven otherwise you should have one ceramic power supply bypass capacitor in the range 0 02 to 0 2 wF or even 1 pF if the IC manufacturer requires it for each digital IC plus a tantalum capacitor in the range 2 to 10 pF for every two three or four ICs The ceramic capacitors provide good local high frequency bypassing the tantalum parts damp out the ringing on the power supply bus If you can t use a tantalum capacitor you can use 10 or 20 wF of aluminum electrolytic or if you are desperate you can try a l or 2 F extended foil Mylar unit in series with a 1 Q carbon resistor to provide the needed lossiness If your linear circuit really de pends on clean crisp digital outputs CMOS outputs make dandy square wave gener ators as long as the power supply isn t ringing and bouncing you may even want more bypassing possibly hundreds of microfarads Floating Inputs Can Leave You at Sea On TTL parts you can leave an unused input floating and it will normally go HIGH on CMOS you must tie unused inputs such as the preset and clear inputs of a flip flop to the positive supply or ground as appropriate Otherwise these inputs will float around and give you the screwiest intermittent problems Also when these inputs float for example on unused gates they can cause considerable unwanted power drain and self heating With CMOS people used to tell you tha
290. nation resistors at one or both ends of the bus to provide damping and to cut down on reflections and ringing When you have to drive long lines in an analog system you must do the same Note that for really fast signals digital de signers don t even lay out their PC traces with square corners they bend the foil around the corner in a couple of 45 turns Many digital engineers are not just bit pushers they ve been learning how to handle real signals in the real world They are actually pretty expert in some analog techniques and analog engineers can learn from them Perfect Waveforms Don t Exist Even though many digital engineers are familiar with real problems they often sketch the waveforms from gates and flip flops showing nice crisp vertical rises and showing the output of a gate changing at the same time as the input But smart engi neers are aware that when it comes down to the fine print they must be prepared to admit that these waveforms have finite rise times and delays These nit picking de tails are very important when your signals are in a hurry For example if the data input of a D flip flop rises just before you apply the clock pulse the output goes high If the data input rises just after you apply the clock pulse the output goes low But if the D input moves at just the wrong time the output can show metastability it can hang momentarily halfway between HIGH and LOW and take several dozen nanoseconds to
291. ncies than bipolar transistors For example the first high fidelity all MOSFET audio amplifier ever saw blew up It worked okay in the lab but some misguided engineer decided that if a band References l 88 7 Identifying and Avoiding Transistor Problems width of 5 Hz to 50 kHz was good then 0 5 Hz to 500 kHz was better Consequently when the speaker cables were extended from 10 feet to 20 feet for a demonstration the amplifier broke into a megahertz region scream and promptly went up in smoke because of the lack of damping at the sources I was told that after a minor redesign the amplifier was perfectly reliable The redesign involved cutting the bandwidth down to a reasonable level adding some ballasting in the sources and tying anti snivet resistors directly to the gate pins Note A snivet is a nasty high frequency oscillation originally found in vacuum tube TV sets an oscillation similar to the oscillation of a MOSFET with no resistance in series with its gate As with bipolar transistors MOSFETs are very reliable if you don t exceed their voltage current or temperature ratings Dissatisfaction with a device s reliability or performance usually stems from the drivers or the related circuitry Most MOSFETs have a maximum Vgs rating of just 20 or 25 V A MOSFET may temporarily survive operation with 30 or 50 V on the gate but it s not safe to run it up there forever If you apply excessive gate voltage gradual g
292. nction of a 2N930 can take 30 us to recover from 10 mA to less than mA and even longer to the nanoam pere level This is largely due to the recombination time of th carriers stored in the collector region of the transistor Other diodes especially gold doped ones turn off much faster down into the nanosecond region Schottky diodes are even faster much faster than 1 ns However one of my friends pointed out that when you have a Schottky diode that turns off pretty fast it is still in parallel with a p n junction that may still turn off slowly at a light current level If a Schottky turns off from 4 mA in less than 1 ns there may still be a few microamperes that do not turn off for a mi crosecond So if you want to use a Schottky as a precision clamp that will turn off very quickly as in a settling detector Ref 2 don t be surprised if there is a small long tail Switching regulators all have a need for diodes and high current rectifiers and transistors to turn off quickly If the rep rate is high and the current large and the diode turns off slowly it can fail due to overheating You don t want to try a IN4002 at 20 or 40 kHz as it will work very badly if at all Sometimes if you need a mod erate amount of current at high speed you can use several 1N914s in parallel I ve done that in an emergency and it seemed to work well but I can t be sure I can rec ommend it as the right thing to do for long term reliability Th
293. nd But when you choose a switch the contacts have almost exactly the same limitations as the relay s contacts There are high current ones there are delicate ones there are hermetically sealed ones So in the same way be thoughtful when you make your choice You do have the advantage here that if you try to wear out a relay a few million operations can cause failures in just a few weeks but most people couldn t wear out a switch by hand fast enough to get in trouble As with relays if you aren t sure what the data sheet is trying to tell you talk to the manufacturer s good people for advice and inter pretation They may have a switch in their back room that is just what you need Weird Wired World Now I ll add a few pithy comments about wire and cable Not all wire is the same For example when I first got a job in electronics I was having a lot of trouble with Teflon insulated wire The wires would often break right at the point where the solder stopped After several engineers assured me that all wire was the same and suggested that I was just imagining things I was ready to scream Finally I found an engineer who admitted that cable manufacturers couldn t put individually tinned wires into a Teflon insulator as they do with plastic insulated wire At the temperatures at which the Teflon is extruded the solder would ail flow together thus making the stranded wire a solid wire Instead cable manufacturers use silver
294. nd Spice and Nothing Nice 145 My boss tells me that I should not be so negative that computers are a big part of our future When he says that I am tempted to go out and buy stock in companies that make Excedrin and antacids be cause that also will be a big part of our future I have given a couple lectures at major conferences with com ments about SPICE and some of its problems Ref 1 and 2 After the lecture engineers from other companies have come up and told me Yeah we have those kinds of problems too For additional comments on SPICE refer to Appendix G One guy gave me a tip Don t put a 50 2 resistor in your circuit put in 50 1 Q and it may converge better In other cases we discov ered that a parallel resistor capacitor combination that was connected only to ground was helping to give us convergence when we commented out the R and C we could not get convergence any more Other people comment that if you change the name of a re sistor or its number or its position on the list of components conver gence may be improved or ruined So this convergence bird is a Photo Courtesy Al Neves Figure 12 1 hurled this computer to its doom from atop National s 3 story parking very fragile and flighty thing garage As the dust settled My boss reminds me that some versions of SPICE are better than knew that computer would others at converging and I shouldn t Just be a com
295. nent and its reference designation It s also a good idea to label numbered test nodes and the correct polarity of diodes and electrolytic capacitors Arrange the signal paths so that if you are desperate you can easily break a link and open a loop be it analog or digital Many modern PC boards have multiple layers and sophisticated patterns of ground planes power supply busses and signal flows Troubleshooting such a board re quires specialized techniques and skills and all sorts of maps so you don t get lost or confused Make sure that all the board s nodes are accessible not hidden or buried on an inside layer or under a large component When possible leave adequate space around components especially ones that are more likely to fail and need to be replaced Such components might be part of circuits that lead off the board and into the realm of ESD transients and lightning bolts and thus might occasionally fail Locate delicate components away from the edge of boards where they might be damaged by rough handling Beware of using eyelets to connect different layers of foil on your PC board Years ago plate d through holes were considered risky so we used eyelets to con nect the top and bo om foils When these eyelets went through temperature cycling the thermal stresses would cause the eyelets to lift the foil right off the boards Even in the last year have seen advertisements that sing oul the praises of evelets
296. ng point to ground witha grounded foil surrounding the critical nodes on both the top and bottom of the boards With the addition of these grounds the circuit can perform well even under worst case humidity conditions After all the internal volume of the glass epoxy insulator is always dry whereas the surface is the place where you can easily get a leakage problem due to dirt or moisture That s where you have to prevent the leakage Of course crosstalk and high frequency capacitive coupling problems are caused by adjacent foil locations and are cured by the same guarding and shielding just dis cussed to prevent foil leakages To help you plan a good layout think about what dv dt and di dt will do in a poor layout Location Location and Location The field of PC board layout is a subject unto itself But there are some things you can do or add to a layout that will make testing the circuit much easier Thoughtful designers have a store of these tricks but I bet very few people write them down In my world the unwritten rules are the ones that are broken so we are trying to write them down I recommend that all designers write down a list of their good ideas Some layout tips from my list are Make sure that the signals you need access to for troubleshooting or analysis are easy to find and probe Make a small hole in the solder mask for accessibility Include a silk screen layer of labels in your layout artwork showing each compo
297. ng the Right Equipment turn off the DVM the function generator the soldering iron and finally even the power supplies but the oscillation is still there Now you start looking around the lab to see who has started a new oscillator or switching regulator that is doubling as a medium power transmitter Aside from yelling Who has a new circuit oscillating at 87 kHz what can you do to solve the problem One useful tool is an ordinary AM transistor radio As we have all learned FM radios reject many kinds of noise but AM radios scoop up noise at repetition rates and frequencies that would surprise you How can a crummy little receiver with an audio bandwidth of perhaps 5 kHz detect noise in the kilohertz and megahertz regions Of course the answer is that many repetitive noise pulse trains whose repetition rates are higher than the audible spec trum but below the AM frequency band have harmonics that extend into the vicinity of 600 kHz where the AM receiver is quite sensitive This sensitivity extends to signals with amplitudes of just a few microvolts per meter If you are skeptical about an AM radio s ability to detect these signals tune its dial down to the low end between stations Then hold it near a DVM or a computer or computer keyboard and listen for the hash Notice too that the ferrite stick antenna has definite directional sensitivity so you can estimate where the noise is coming from by using either the null mode or p
298. nging really badly Recently one of our senior technicians was helping a customer with some applica tions advice He found that the AC output impedance of an LM317 was changing considerably as a function of the load current coming out of the output transistor We had always assumed that the curve in the LM117 data sheet was invariant versus current load that was a mistake Then we found that every other monolithic regu lator has the same sliding scale of output inductance For additional notes on this phenomenon I recommend the Erroll Dietz article Ref 1 which I have included as Appendix C because this tendency of the output inductance to be modulated by the output current may help to explain why regulators are happy in some cases but grouchy in other similar situations Another regulator problem can occur when you add an external transistor to in crease the output current Since this transistor adds gain at DC it s not surprising that you have to add a big filter capacitor on the regulator s output to prevent oscillation Some of the applications in old National Semiconductor data sheets recommend specific values for the filter capacitor and specific types for the boost transistors but some of these circuits are quite old When customers find out that 2N3234s are no longer available they re likely to substitute a more modern transistor that has a faster response and is likely to oscillate In this case a customer mi ght complain abo
299. ning on power for short pulses and watching the circuit as it approaches the de structive latch condition See Chapter 2 Use an adjustable current limited supply with zero or small output capacitance such as the example in Chapter 2 so when the circuit starts to latch the fault condition can easily pull the current limited power supply s voltage down Another way to inadvertently generate a latched up condition is to turn on the outputs of your multiple output power supply in the wrong sequence Some ampli fiers and circuits get quite unhappy when one supply usually the positive one turns on first Automatic power supply sequencers can help you avoid this problem An antireversal rectifier across each supply can help too and is always a good idea for preventing damage from inadvertently crossed up power supply leads or supply short circuits I used to get calls every few months from people who asked me if it was okay to ship or launch products that contained LM108s that may have had 15 V on their 15 V pins and vice versa It was always painful for me to tell them Don t ship it junk it And next time put antireversal diodes on each supply Specifically you should add these antireversal rectifiers across each bus in your system to protect the loads and circuits Also add an antireversal rectifier across each power supply s output to protect the supplies Figure 9 6 Some people think that leaving parts out is
300. noise measurements and the heat from the transformer and regulators will degrade the instrument s accuracy Instead I build a separate power supply box on the end of a 3 ft cable the heat and magnetic flux are properly banished far away from my precision circuits Passive Components A User s Guide ian Sinclair Heinemann Newnes Halley Court London England 1990 p 225 Order from Butterworth Heinemann 80 Montvale Avenue Stoneham Mass 02180 4 Getting Down to the Component Level Capacitor Problems Earlier chapters have described a good analog troubleshooter s mind set armamen tarium of test equipment and requisite knowledge of resistors inductors and trans formers Next we reveal some of the secrets of an often underestimated class of components capacitors And much of what you need to know to troubleshoot ca pacitor related problems is not in any book it s not even in data sheets Capacitors are rather remarkable We consider capacitors like resistors to be passive But if you charge up a really good capacitor such as a 47 pF polypropy lene capacitor to 10 V and then take a 2 week vacation when you come back the voltage may not have decreased by as much as 20 or even 10 The Capacitor may have stored and retained enough energy to run a nano power circuit for hours or to light an LED for a shorter interval Calling components with such exceptional prop erties passive is more than a little
301. nsformer is not normally an isolation transformer You may need to cascade one of cach to get safe adjustable power L Registered PM of GENRAB Corp Concord MA Varntes can be purchased from JLM dectromes SG Somerset SE VO Box 10317 West Hartford CT 06110 203 23 0600 20 7 2 Choosing the Right equipment ISOLATED LINE OUTPUT Figure 2 8 You can use this back to back transformer configuration to achieve line isolation similar to that of an isolation transformer oy i 11 A curve tracer A curve tracer can show you that two transistors may have the same saturation voltage under a given set of conditions even though the slope of one may be quite different from the slope of the other If one of these transistors works well and the other badly 2 a curve tracer can helpiyou understand why A curve tracer can _ also be useful for spotting nonlinear resistances and conductances in diodes capaci tors light bulbs and resistors AC curve tracer can test a battery by loading it down or i recharging it It can check semiconductors for breakdown And when you buy the right adapters or cobble them up yourself you can evaluate the shape of the gain the CMRR and the PSRR of op amps ivs 13 1A complete supply of resistor an rae Yoi should have resistors in the range from 0 1 Ato 100 MQ and cap ac fro 10 pF to 1 pF Also 10 100 and 1000 pF capacitors come in handy Just becausey uin circiit design doe
302. nt may say some thing such as This data sheet contains Preliminary limits and design Specifications Supplemental information will be published at a later date The manufacturer reserves the right to make changes in the products contained in this document in order to improve design or performance and to supply the best Possible products We also assume no responsibility for the use of any circuits described herein convey no license un der any patent or other right and make no representation that the circuits are free from patent infringement in fact after a device is released to the marketplace in a preliminary status the engineers love to make small im Provements and upgrades in specifications and characteris tics and hate to degrade a Specification from its first pub lished value but occasionally that is necessary Another item in the fine print is the manufacturer s tele phone number Usually it is best to refer questions to the local sales representative or field applications engineer be cause they may know the answer or they may be best able to put a questioner in touch with the right person at the factory Occasionally the factory s applications engineers have all the information Other times they have to bring in product engineers test engineers or marketing people And some times the answer can t be generated quickly data have to be gathered opinions solidified or policies formulated be fore tha manufacturer
303. nversely if you study these precau tions and think about what can happen you may be able to avoid getting into trouble in the first place That s even better than being able to get out References References 49 In fact maybe some of the warnings I have presented here will explain why you once had a problem whose cause you could never figure out Every once in a while I learn something that stops me in my tracks That explains why the oscillator I made two years ago never worked right If you stand on my shoulders you may be able to get to places that neither of us could get to alone I certainly didn t figure out every one of these ideas by myself J am passing along many ideas that I gleaned from other people s experience Furthermore I doubt if they invented all those ideas themselves Surely they benefited from other peoples ideas that they picked up along the way I am just trying to pass along insights that are not book learning but that I learned i in the College of Hard Knocks It may or may not be troubleshooting but ie s close enough for me us wy an ye po 5 Preventing Material and Assembly Problems PC Boards and Connectors Relays and Switches In addition to your choice of components the materials you use to assemble your circuit will have an impact on how well it performs This chapter covers what you need to know to solve the occasional problems caused by PC boards
304. ny do you plan to build If the answer is justia few dozen and the design is a full featured high power job I encourage the engineer to buy an existing design But if large numbers are involved an engineer usually has the time to do the design right and spread that effort over a few thousand circuits An alternative to designing your own switcher is using one of the new Simple Switchers the simple but complete switching regulator ICs Some of these 1 It was a bastard file gan a a cache TS tee T ogi Switch Mode Regulators 139 chips LM2575 LM2576 LM2577 LM2578 are about as foolproof for a switcher as you can get The data sheets of these parts explain that You may need a couple resistors a few capacitors an inductor and a fast rectifier and then it s done You ll have a cookbook circuit that really does work And if you want to get the component selection information from a program on a floppy disk I am told that works quite well too and is considered pretty user friendly If you only need to supply a few hundred milliamps to your circuit you may not even need a power transistor or a heat sink Even in the last couple years there have been advances in designs that really do work as opposed to paper designs that have no chance of surviving a short on the output or of working under worst case conditions Although a few of these useless paper designs still pop up from time to ti
305. o say that part never became popular hor did anything else from that company l So let me caution YOu Whether yout circuit is a loop of sequential logic or an analog loop with i pos tivoan negative feedback be sure to design the start up circuit carefully Adda test to huke abad p rtwilt be ejected AND build up a few bad poreen 4 YE Dy t Figure 11 2 The addition of R and D to this standard data book design of a i5 V 0 5 A switching regu lator provides the circuit with better soft start and restart capabilities References 142 I i Dealing with References and Regulators parts units with the start up circuit broken or disconnected AND make sure they fail the test Then leave the test in the flow Don t drop the test just because nobody has ever seen a part fail Dropping that test would be courting disaster Here at National Semiconductor we ve appointed a Czar of Start up Circuits He is the repository of all knowledge about circuits that do and don t start properly Since this shy fellow I shan t give his name began to reign the goof up rate has been cut by many decibels Dietz Erroll H Reduce Noise in Voltage Regulators Electronic Design Dec 14 1989 p 92 12 Roundup of Floobydust Loose Ends That Don t Fit Elsewhere In any serious troubleshooting situation it is usually wise to plan what tests are most likely to give you
306. o tight 83 180 burrs between heat sink and tran sistor 83 just right 83 84 too big 84 too small 82 86 106 135 Heath Company 22 59 107 109 Hoffman John Paul 167 humidity 10 55 61 147 hypothesis 6 107 hysteresis See under comparators problems with impedance analyzer or bridge 25 30 35 identical 168 inductance 110 116 136 139 144 148 162 183 185 191 193 207 inductors 34 39 138 See also trans formers and relays air 34 air gaps 35 beads 8 37 84 87 109 162 core material 34 35 core losses 35 38 ferrite 35 39 incoming irispection of 36 kick from 36 37 materials 34 39 measurements of ohmmeter and problems of 35 36 38 inductance meter 35 36 Q meter 35 R C damper 37 saturation 35 36 38 shorted turn 35 38 snubber 37 38 steel 34 38 test conditions 35 37 turns See windings used in switch mode regulator 138 139 183 184 windings 35 38 wire losses 35 38 instrumentation amplifier 182 insulation leakage measurement See under current detector integrator operational amplifier circuit 89 99 104 153 intelligence artificial AD 11 See also under stupidity artificial intelligence genuine 1 See also stupidity genuine Interconnection Products Inc 185 intermittent problems techniques for solving 15 32 50 60 63 121 122 143 144 153 170 175 judgment 12 56 81 87 Kelvin connections 57 58 59 80 178 4 terminal res
307. oach is an alternative to the approach illustrated in Figure 5 1 for testing leakage You can calculate the leakage current from Ohm s Law V X Rs or IL Vo Rs There are basically two ways to measure leakage current The approach I have used for many years is to connect a couple of transistors as a wide range logarithmic current to voltage detector across the feedback path of a low bias current op amp These days I m not using vacuum tubes I have graduated to an LMC660 as shown in the circuit of Figure 5 1 I calibrated the meter with a hand drawn scale to sense currents ranging from 1 pA to 1 mA and 1 pA to 1 mA Figure 5 2 As long as the air conditioning doesn t break down I know my calibration won t drift much more than 10 or 20 which is adequate to tell me which decade of current I am working in The Vgg does have some temperature sensitivity but not enough to bother this circuit very much Because these transistors are of course quite non linear as current to voltage sensors you do have to shield the summingpoint away from AC noises 60 Hz 120 Hz 1 MHz etc to prevent rectification and false read ings So the whole test circuit and the unknown impedance are best located in a shallow metal box grounded with an optional metal cover It s true that the DVM approach shown in Figure 5 3 has a little more accuracy and perhaps more resolution but it too is easily fooled by noise and the digital readout doesn
308. oblem occurred when the LM317 was used as a battery charger When the charger output was shorted to ground the LM317 started drawing a lot of current But the transformer s inductance kept supplying more and more current until the LM317 went into current limit and could not draw any more current At this point the transformer s secondary voltage popped up to a very high voltage and destroyed the LM317 The addition of the 1000 uF snubber also solved this problem inductors Like Resistors Can Overheat How do you spot a bad inductor or transformer I have already discussed several mechanisms that can cause the inductance or Q of an inductor to be inferior to that of a normal part And as with a resistor you can smell an inductor that is severely over heating Overheating can be caused by a faulty core a shorted turn incorrect wire gauge or anything else that causes losses to increase An open winding is easy to spot with an ohmmeter as is a short from a primary to a secondary If the pattern of winding has been changed from one transformer to another you may not see it unless you test the components in a circuit that approximates the actual application However you may also be able to see such a discrepancy if you apply a fast pulse to the two transformers Changes in winding pattern even clockwise vs counter clockwise have been observed to cause significant changes in transformer perfor mance and reliability Tightly coupled win
309. oesn t say whether Iican Sigh We cheerfully and dutifully explain that you can run any operational ampli fier on a single supply An LM108 does not have any ground pin It can t tell if your power supplies are labelled 15 V and 15V or 30 V and ground or ground and 30 V That is merely a matter of nomenclature a matter of your standpoint a matter of which bus you might choose to call ground Now it is true that you have to bias your signals and amplifier inputs in a reason able way The LM108 1M308 can amplify signals that are not too close to either supply the inputs must be at east a couple volts from either supply rail So if you need a circuit that can Handle inputs near the minus rail the LM108 is not suitable but the LM324 LM358 LMC660 LMC662 and LM10 are suitable If you need an amplifier that works near the rail the LM101A and LM107 are guaranteed to work there The LF355 and LF356 typically work well up there but are not guaranteed But if you keep the inputs biassed labout halfway between the rails just about any op amp will work on a single supply It s just a matter of labels Someday we plan to write an applications note to get these silly questions off our back and to answer the customers reasonable questions but right now everybody s a little too busy to write it Tr eee nee Measure Bias Current Rather Than Impedance Another op amp spec you don
310. of 2 7 to 4 7 Q in series with some of the capacitors Adding resistance in series with bypass capacitors might seem a bit silly but it s a very useful trick High K ceramic capacitors also can exhibit piezoelectric effects When you put a good amount of AC voltage across them they can hum audibly and if you rattle or vibrate them they can kick out charge or voltage Other types can do the same thing but high K types are worse Be careful when using these capacitors in a high vibra tion environment The capacitance of stable K capacitors such as X7R typically decreases by less than 15 from the room temperature value over the 55 to 125 C range These capacitors are general purpose devices and are usually available in the 100 to 10 000 pF range in the larger packages you can get as much as 300 000 pF However you can buy a 10 000 pF capacitor in either a high K or a stable K type and you can t be sure of the kind you re getting unless you check the catalog and the part number Or measure the capacitance as you heat or cool it The last type of ceramic capacitor was originally called NPO for Negative Positive Zero and is now usually called COG Everybody calls them COG C oh G but it really is C zero G I ve seen the EIA document Ref 2 The COG NPO capacitors have a really high grade low K dielectric with a guaranteed TC of less than 30 ppm C Their dissipation factor dielectric absorption and
311. of course be converted to sterling at the cur rent rate of exchange presently about 2 to the pound It should however be noted that prices in the U K are eusually somewhat in excess of the straightforward con verted sum The symbol is used to denote number Hence Tesistor 20 is equivalent to R20 and so on Various items oftesteat ipmentof US origin e Bs Hewlett Packard Fluke etc are Farnell Electronic Component Canal Road E Leeds hoe West Yorkshire pE LS12 2T es Telephone 0532 63631 9 P O Box 99 Corby Northants NN17 9RS Telephone 0536 201234 Maplin Professional Supplies P O Box 777 Rayleigh Essex SS6 8LU Telephone 0702 552961 Also from the EDN Series for Design Engineers published by Butterworth Heinemann Analog Circuit Design 2 Art Science and Personalities edited by Jim Williams Coping with analog circuit design is a lot easier if you have an experienced analog wizard to guide you through the design process Jim Williams is such a wizard and in this book he s assembled 23 other masters of the analog art to share their experience knowledge insights and often wit Analog Circuit Design Art Science and Personalities is far more than just another tutorial or reference guide it s a tour through the world of analog design combining theory and applications with the philosophies behind the design process You ll read h
312. of humor and his passion for puns something of a legend As a form of humor plays on words are denigrated by all too many people However at least a few openly admit to enjoying puns and that group includes Bob and myself Many years ago when J first read material Bob had written I suspected that if I ever met him Fd probably like him When started to read what he had just submitted to EDN the experience was a bit like a chance encounter with an old friend after not meeting up with him for a long time The material was somewhat out of the ordinary for LON It was technical yes but it was lighter than most of what we publish There were few equations and no Vi vil Foreword complex schematics Would the readers like it My guess was they would the manuscript was filled with pithy and trenchant observations The style was different too In the past rigid constraints forced articles into a ho mogenized mold albeit one that despite the magazine s highly technical content was almost always eminently clear and readable Now we re a bit more relaxed We still rewrite extensively for clarity but we try to let a writer s style and personality show through I think part of the reason for our change in attitude was the success of Bob s series Bob s style not only displays his sense of humor it showcases his idiosyncratic and sometimes quirky nature Most of all however his perfectionism and consum mate craftsmanship are cle
313. often a symptom o that a transistor OF circuit has fi eu l oiijust replace the resistor the new one will S algo burn out or exhibit the same s ige characteristics Around i our lab if anybody smelis an overheated resistor he makes sure that we oe understand what it is Usually when itt holler Who has a resistor overheated an engineer or technic ian will sheepishly say Jj just cooked my circuit But some times it is a failure i in a piece of unattended equipment and the sooner we can turn soffi its power or fetch a fire extinguisher the better a How do you check for resistor error Ify you re desperate you can disconnect one o end of the resistor and actually measure aula is ts value It s s often easier to just measure the TX R drops i in the network and ded ce hy Which resistor if any seems to be of the we wrong value If one resistop is sus 6 ped of being temperature sensitive you can heat E iit with a soldering iron orc bol J M ifrbeze mist ai as You monitor its effect In some solid state circuits the signals Is are curte urrents so it s riot easy to probe the circuit with a ms voltmeter In this case you may iets make e implicit 1 Measurements to decide if a ma resistor is the problem Also remember that a sneak k path of current can often cause the same effect as a bad i resistor n DOEN
314. oid ground loops when you can Although the electrical engineering faculty at your local university might not agree a general solution to the ground loop problem would be an excellent subject for a PhD thesis If you write such a dissertation please don t forget to mail me a copy Some successive approximation ADCs have separate buffers feeding their output pins but other designs try to save money parts power or space by using the internal registers to drive both the internal DAC and the output pins In this case external loads on the outputs can cause poor settling and noise and can thus degrade the per formance of the converter If you re using ADCs you should find out if the outputs are connected directly to the DAC Sometimes as previously mentioned a preload on each bit output can help to accelerate settling of an ADC s internal DAC After all TTL outputs must be able to drive more current than their DC specs state they have to meet their AC specs VFCs and FVCs Frequently Find Favor The voltage to frequency converter VFC is a popular form of ADC especially when you need isolation between the analog input and digital outputs You can easily feed a VFC s output pulse train through an optocoupler to achieve isolation between VFCs and FVCs Frequently Find Favor 131 different ground systems The VFC can cover a wide range with 14 to 18 bits of dynamic range The less expensive VFCs are slower the faster ones can be expens
315. ointing the antenna to get the strongest signal So the humble AM radio may be able to help you as you hike around the lab and smile pleasantly at your comrades until you find the culprit whose new switching regulator isn t working quite right but which he neglected to turn off when he went out to get a cup of coffee The grid dip meter On other occasions the frequency and repetition rate of the noise are so high that an AM receiver won t be helpful in detecting the problem What s the tool to use then Back in the early days of radio engineers found that if you ran a vacuum tube oscillator and immersed it in a field of high power oscilla tions at a comparable frequency the tube s grid current would shift or dip when the frequencies matched This tool became known as the grid dip meter I can t say that I am an expert in the theory or usage of the grid dip meter but I do recall being impressed in the early days of monolithic ICs A particular linear circuit was oscil lating at 98 MHz and the grid dip meter could tickle the apparent rectified output error as I tuned the frequency dial back and forth That was 25 years ago and of course Heathkit has discontinued their old Grid Dip and Tunnel Dip meters in favor of a more modern design The new one simply dubbed HD 1250 Dip Meter uses transistors and tetrode FETs At the bargain price of 89 every lab should have one They ll help you ferret out the source of nasty osc
316. ok he and his colleagues at National Semiconductor were planning to write on power supply design Tarlton said he thought Bob had already put together some material on troubleshooting We needed to find out whether National would grant EDN the rights to publish a portion of the book Tarlton would open the discussions Shortly afterward a good sized package arrived at EDN s offices In it was the text of what would eventually become the first three installments of Bob s series By then Tarlton had left the East Coast to seek fame and fortune in Silicon Valley so the task of reviewing Bob s material fell to me I skimmed through it quickly and became quite intrigued I am a contemporary of Bob s actually I am a few years older Though we did not know each other at the time I was a graduate student in EE at MIT while Bob was an undergraduate there I first became aware of Bob when he was working for his previous employer George A Philbrick Researches now a part of Teledyne Components in Dedham Massachusetts Even in those days the sixties and early seventies Bob was a prolific writer He shared his musings and technical insights with Philbrick customers and other analog engineers who read the firm s house organ The Lightning Empiricist and with readers of trade magazines such as EDN Those earlier writings did a lot to burnish Bob s image as a technical expert but they had a secondary effect as well They made his sense
317. oltage gain decreases and its common emitter breakdown voltage BV cgo may also decrease The Early voltage of a transistor ts the amount of Vcg that causes the collector current to increase to approximately two times its low voltage value assuming a constant base drive Vearly is approximately equal to 26 mV X 1 h So in many circuits there is a point where higher beta simply makes the gain lower not higher Another way to effectively increase beta is to use the Darlington connection but the voltage gain and noise may degrade the response may get flaky and the base current may decrease only slightly When I was a kid engineer I studied the ways that Tektronix made good use of the tubes and transistors in their mainframes and plug ins Those engineers didn t use many Darlingtons To this day I keep learning more and more reasons not to use Darlingtons or cascaded followers For many years it s been more important in most circuits to have matched betas than to have sky high betas You can match betas yourself or you can buy monolithic dual matched transistors like the LM394 or you can buy four or five matched transistors on one monolithic substrate such as an LM3045 or LM3086 monolithic transistor array One of the nice things about bipolar transistors is that their transconductance gm is quite predictable At room temperature gm 38 6 X Ic This is much more consis tent than the forward conductance of diodes as men
318. ome disconnected from the capacitor If this problem occurs you ll have to switch to capacitors from a manufacturer that uses high temperature solder Don t Forget Silvered Mice A Vi many features similar to COG capacitors They have ppm C They can also work at temperatures above i temperature solder Unfortunately they have poor dly bad dielectric natn al ey ths marking on ithe capacitor hasn t 1000 means 10 100 or 1000 pF You A i meter Similarly inthe oldidays some vay I remember twolittle capaci itha K characteristic and both ithe same size and had the ou could buy a pretty good capacitor that had apao tance A bour 99 of the time they were excellent reliable capacitors Bu nce ina while some of the capacitors came through with a capacitance value completely different from the marked value One time I saw a Figure 4 3 If you saw a capacitor that looked like this you d know the manufacturer hadn t tested it before shipping right Photo by Steve Allen First Try Adding A Second 47 whole box of capacitors in which the two leads were still made of one loop of wire that had not been snipped apart Obviously the manufacturer wasn t interested in testing and measuring these capacitors before sending them out the door So if you are buying capacitors to a 1 AQL Acceptance Quality Level and not 0 1 or 0 01 you should be aware th
319. oming loon welll ithe effort So let s presume thi iy si i KAA and repair rather than just chuck the circuit ima wastebasket i was talking witha guy the other day and he said Bob be sure to spell out the difference between Lab Troubleshooting and Production Troubleshooting I don t think I know what that difference is In either case itcan be very important and a small amount of time and money can have great returns Of course on other occasions you can put in a lot of hours and get virtually nowhere As with any other system troubleshooting is an art which can be developed with practice You have to learn the failure modes the patterns of abuse the procedures for replacing bad parts the documentation and all the other things we have discussed But how about a Modus Operandi Let s look at the following table for a simple op amp inverter 174 14 Real Circuits and Real Problems Cr r x Rin R ViN gt Vin OV O Vout Figure 14 1 Basic Op Amp Inverter Table 14 1 Troubleshooting inverting Operational Amplifiers Indication of Trouble Possible Cause Solution Output voltage offset is excessive when input is zero Output pegged near one power supply rail Output is oscillating Feedback R too high Oscillation causes offset Sneak path for leakage into input R is wrong value Op amp s Vog too big Amplifier out of spec
320. on TM 51 beams made with copper clad material 16 multi layer boards 55 plated through holes 50 56 probe points 55 screened map of components 55 solder mask 50 51 problems with PC Boards bad layout 55 58 bypass capacitors 44 44 45 See also under capacitors for bypassing compensation capacitors 56 contamination 50 52 54 55 crosstalk 50 55 56 dirty grounds 57 eyelets 55 56 170 toil peeling or lifting 50 51 hairline opens 51 hairline shorts 51 lack of guarding 54 55 leakage 50 52 55 128 156 177 long tails dielectric absorption 51 need for high quality power and digital grounds 57 129 130 need for Kelvin connections 57 58 59 mislocation of components 8 55 moisture 50 55 plated through holes 50 55 56 probe points 55 ringing 44 45 44 56 100 solder shorts 50 51 176 180 Spaghetti syndrome 56 stray Capacitances too small smaller than breadboard 103 stray capacitances too big 103 144 strip line layout 122 too much neatness 56 unsuitable board material 50 51 washing dish washer 54 170 probes for ICs 15 16 21 probes oscilloscope 14 15 15 16 51 67 89 106 Ix 15 100 155 1x 10x 15 106 10x 15 51 89 106 150 156 158 active 15 current probes 15 35 160 161 frequency compensation of 10x 15 15 89 150 156 158 ground wires of probes 15 16 150 155 low capacitance 15 16 126 probing 6 7 40 60 126 162 168 206 problem
321. on is me It can take me five minutes to find what the problems are I love all those powerful versatile functions when I need them but they drive me nuts when the wrong button gets pushed Similarly a scope s trace can get lost and hide in the corner and sulk for many minutes on end if you don t realize that somebody maybe your very own errant fat finger pushed a treacherous button When the digital scopes with their multiple layers of menus and submenus start playing that game I find I need a buddy system some body to come and bail me out when I get hopelessly stuck What menu is that dratted beamfinder on anyway But scopes work awfully well these days Just don t expect precision results after you drive the trace many centimeters off scale by turning up the gain to look at the bottom of a tall square wave Most scopes aren t obligated to do that very well E Similarly be sure to keep the trimmers on your 10 X probes well adjusted and run a short ground path to your probes when you want to look at fast signals as discussed in Chapter 2 Troubleshoot As You Go Some people like to build up a big system and turn on the power and Voila it doesn t work Then they have to figure out what kind of things are wrong in the 151 Signal Sources 91V 15V IN914 OR 1N4148 47 ALL TRANSISTORS 2N3906 Va SETTLES QUICKLY TO 7V Vour 1x500 x500 Re 15V 07 a J a i b sv Figure 12 3 E
322. onfusion I once read an op amp book that stated that when the DC gain changed the first pole remained at the same frequency In other words the author claimed that the gain bandwidth product increased with the DC gain Wrong I wrote to the author to object and to correct but I never heard back from him I often see op amp spec sheets in which the open loop output impedance is listed as 50 Q But by inspecting the gain specs at two different load resistor values you can see that the DC gain falls by a factor of two when a load of 1 KQ is applied Well if you have an op amp with an output impedance of KQ its gain will fall by a factor of two when you apply a 1 kQ load But if its output impedance were 50 Q as the spec sheet claimed the gain would only fall 5 So whether it s a computer model or a real amplifier be suspicious of output impedances that are claimed to be unreal istically low Watch Out for Real Trouble What real trouble can an op amp get you into without much help trom the compo nents Well you could have a part with a bad Vos Or if the temperature is changing the thermocouples of the op amp s Kovar leads may cause small voltage differences between the op amp leads and the copper of the PC board Such differences can amount to 1 10 or 1 20 of a Celsius degree times 35 LV per degree which equals 2 to 3 uV A good way to avoid this problem is to put a little box over the amplifier to keep breezes and sunshine off the
323. ons clear the user was unlikely to be con fused or misted However the reader ought to be on the alert in a few cas es the data sheet writer is playing a specsmanship game and is trying to show an inferior to some users aspect of a Product in a light that makes it look Superior which it may be to a couple of users GUARANTEES When a data sheet specifies a guaranteed minimum value what does it mean An assumption might be made that the manufacturer has actually tested that Specification and has great confidence that no part could fail that test and still be shipped Yet that is not always the case For instance in the early days of op amps 20 years ago the differential input impedance might have been guaran teed at 1 MN but the manufacturer obviously did not mea sure the impedance When a customer insisted I have to know how you measure this impedance it had to be ex plained that the impedance was not measured but that the base current was The correlation between Ip and Zin por mitted the substitution of this simple dc test for a rather messy noisy hard to interpret test Heprrited by permission trom Electrome Engineering Times Every year for the last 20 years manufacturers have been trying to explain with varying success why they do not mea sure the Zin per se even though they do guarantee it in other cases the manufacturer may specify a test that can be made only on the die as it is probed on t
324. open or shorted to ground Such branching along would take a lot fewer than the 64 tests you would need to walk all the way down the string Further if an op amp circuit s output were pegged you would normally check the circuit s op amp resistors or conductors You wouldnt normally check the CAPAC tors unless you guessed that a shorted capacitor could cause the output to pep 6 First Things First Conversely if the op amp s Voyr was a few dozen millivolts in error you might start checking the resistors for their tolerances You might not check for an open circuited or wrong value capacitor unless you checked the circuit s output with a scope and discovered it oscillating So in any circuit you must study the data your clues until they lead you to the final test that reveals the true cause of your problem Thus you should always first formulate a hypothesis and then invent a reasonable test or series of tests the answers to which will help narrow down the possibilities of what is bad and may in fact support your hypothesis These tests should be perform able But you may define a test and then discover it is not performable or would be much too difficult to perform Then I often think Well if I could do that test the answer would either come up good or bad OK so I can t easily run the test But if I assume that I d get one or the other of the answers what would I do next to nail down the
325. ophy of Troubleshooting In this first chapter I will make the point that a significant part of effective trouble shooting lies in the way that you think about the problem The next chapter will cover the equipment you should buy and build to help you diagnose problems Other chap ters will illuminate some of the more subtle and elusive characteristics of passive and active components and the PC boards and cables that interconnect them Troubleshooting Is More Effective with the Right Philosophy If you recall that the most boring class in school was a philosophy class and you think this book will be boring that way well WRONG We are going to talk about the real world and examples of mistakes goofs and how we can recover from these mistakes We are going to talk about all the nasty problems the world tries to inflict on us We are talking about Trouble with a capital T and how to overcome it Here at National Semiconductor we decided a couple of years ago that we ought to write a book about switch mode power supplies Within the applications and de sign groups nearly all of the engineers volunteered to write a chapter and I volun teered to doa chapter on troubleshooting At present the status of that book is pretty dubious But the troubleshooting chapter is going strong and you readers are among the first to benefit because that one chapter has expanded to become this entire book Although I am probably not the world s bes
326. or 24 105 1E 156 170 thermstors 37 thermocouple 24 28 33 61 9 166 thermocouple meters 24 24 thermocouple amplifier for 24 24 thermometer See temperature meter three wire to two wire adapters dine cord 28 216 timers 46 120 121 134 P41 CMOS timers 120 leaky capacitors 120 LM555 LMC555 H20 logic diagram flow chart 12 slow oscillators 120 trimming oscillators 120 tools 3 12 14 23 58 70 126 138 169 173 177 203 206 207 See also equipment tools inadequate 14 touch in technique See under trouble shooting techniques transconductance 79 192 transformers 18 19 26 35 39 110 111 137 138 140 155 back to back transformers See under isolation transformers bifilar windings 38 comparison tests 35 37 cup core 35 E I cores 35 ferrite 35 insulation 37 isolation 18 19 20 inter winding capacitance 36 38 leakage inductance 35 36 magnetic flux 35 38 39 power transformers 34 37 39 10 111 primary inductance 35 36 rod core 34 39 saturation and overdrive 35 110 111 secondary inductance 35 36 similarity to inductors common features 37 test conditions 35 36 toroidal core 37 39 111 turns ratios 35 37 36 twisted pairs 38 variable 40 See also under Variac winding conventions transistors bipolar 77 79 81 88 104 108 109 115 118 176 177 180 184 204 206 2N918 78 2N930 66 2N2222 84 2N2369 78 2N3055 84 2N3055H 84 2N377
327. or ATE Automatic Test R nR OUTPUT ERROR Vernon n 1 VeRROR f FLOATING SCOPE FUNCTION GENERATOR OPTIONAL LINK TO GROUND Figure 8 3 Is this a CMRR test No because Verror Vem CMRR Vou Ay R nR OUTPUT ERROR Vernon n 1 FUNCTION GENERATOR V LINK TO GROUND X OPTIONAL LINK TO GROUND G Figure 8 4 Is this any better than the previous CMRR Test No it s exactly the same Still V Vem CMRR V Ay error Figure 8 5 Figure 8 6 First How Not to Test for CMRR 93 Equipment for production test and for stepped DC levels And it will give the same answer as my circuit at all low frequencies up to where it doesn t give the same an swer Now what frequency would that be Nobody knows Because if you have an op amp with low CMRR the servo scheme will work accurately up to one frequency and if you have an op amp with high CMRR the servo scheme will work accurately only up to a different frequency Also the servo amplifier adds so much gain into the loop that ringing or overshoot or marginal stability at some mid frequencies is in evitable That is much too horrible for me to worry about I will just avoid that by using a circuit which gives very consistent and predictable results Specifically I ran an LF356 in the circuit of Fig 8 3 and I got an error of 4 mV p p at 1 kHz a big fat quadrature error 90 degrees out of phase with the output
328. or is wiggled and jiggled and plugged in and out while the power is on But don t all the instruction books say that you shouldn t plug in the board with the power on Sure a lot of them do But I ve never gotten into more trouble plugging a board into a hot connector than the trouble I ve gotten out of There may be some boards that are destroyed or damaged by this method but they are in the distinct mi nority and should be studied One way to help avoid problems is to make the ground fingers on a printed circuit edge connector stick out longer than the other fingers Thus ground will be established before any other connection Still if you have a board that tends to latch up because the power supply sequencing may be improper you have to be prepared to stop plugging the board into that hot socket fast Learn by Fiddling and Tweaking There are many situations that can foul things up but we all tend to learn more from fiddling around with things tweaking and unplugging than by purely cerebral pro cesses Once I had a technician who thought that DIP sockets should not be secured in place by tack soldering but by glue This technique worked fine for a while but occasionally the sockets would act like an open circuit on one pin or another To solve the problem we used an old technique We traced the circuit coming into the IC and it was fine We traced the signal coming out of the IC nothing Then we traced the signals on the pin
329. ors Indication of Trouble Possible Cause Solution Bad DC errors Refer to table for positive regulators above a Oscillation Refer to table for positive regulators nt v8 ALSO gt Improper C2 capacitor Refer to manufacturer s oe data sheet Avoid ceramic or film caps Use tantalums or big C electrolytics Too many ceramic discs Swamp them with over connected on output bus compensation with tens a ee lt Of pF tantalum or a ene few hundred pF k aluminum electrolytic Comments on Table for Negative Adjustable Regulators These regulators have just about all the virtues and the freedom from problems as the _ Positive regulators but they are quite critical about having a good capacitor from the output to ground as noted above ey o a ee The fixed negative regulators LM320 family LM7900 family are likewise quite demanding that a decent capacitot be used for the output s damping This is inherent _ because the negative regulators all have collector loaded outputs and you need a good capacitor to roll off the extra gain a PINS SHOWN ARE FOR 14 PIN DIP PACKAGE tine EEE GROUND Figure 14 5 LM723 Type Regulator Table 14 5 180 14 Real Circuits and Real Problems Troubleshooting LM723 type Regulators Indication of Trouble _ Output voltage has small error Output voltage much too low Output voltage much too high Output cannot d
330. osis 5 11 106 173 diagonal nippers 23 Digikey 185 digital computers See under computers digital digital ICs 1 7 44 56 72 81 121 171 analog switches 79 81 121 133 155 select for very low leakage 133 buffered vs un buffered gates 124 choreography timing diagram 126 127 CMOS circuits 81 120 121 123 124 133 155 159 161 as linear amplifier 121 Index dirty waveforms 121 122 126 discrepancies TTL vs CMOS 74074 194 driving long busses as strip line 56 S ECL See high speed logic floating inputs 121 155 160 161 glitches 14 115 122 123 126 128 132 155 160 high speed logic 56 156 194 Si po LS TTL 161 metastability 122 156 nonstandard pin outs 123 124 187 overdrive of digital i inputs 124 125 161 ahs _ overshoot of TTL output 126 izi phase detector 121 oe bypassing runt pulses 122 123 See also glitches TTL circuits 123 124 130 187 wired or outputs 122 Py dee digital voltmeters DVMs 15 1 7 147 150 uu accuracy 149 false data 148 149 how to rebut false data i g poor linearity 148 149 poor gain accuracy 1492150 i a filters for input noises 17 17 150 Fluke 8810A 8842A 15i f integrating type 131 148 soe linearity See non linearity non linearity 148 149 Recirculating Remainder 148 Successive Approximation 148 3 digit 17 4 digit 16 5 digit 15 6 digit 7
331. ou drop them any of these imperfections may greatly increase until the meter is nearly useless or dead This is position sensitivity carried to an extreme Ideally you would use digital meters for every purpose But analo Meters have advantages for example when you have to look at a trend or watch for a derivative oran amplitude peak especially in the presence of noise which may clutter up the readings of a digital voltmeter So analog meters will be y iths for a long time especially in view fof their need for no extra power supply theif isolation and their low cost pedance of meter movements They look like a stalled motor ai few tundred millihenries at high frequencies However ifthe needle gs youll get ae ino yon of shied Rearies So if you putan analog meters mienon t when they i AA manufacturer of Power supplies decided to modernize its bench type supplies by replacing the old analog meters with digital meters Unfortunately these meters came with an accuracy of 5 Having a 2 172 digit digital panel metor RM with a resolution of 1 part in 200 but iN ii supplies from she tune 5 The steadiness and inrefutability of those e glowing unwavering diisi is psycholog ically h hard to i Jassify thi readings of the DVM or DPM sere any o other ce com For exampl jost ikindi dab shop ot iitobroting con version so they re inherently quite lin
332. our power transformer is not only running hot but spraying flux badly it s not usually easy to relocate it if you are nearly done with your project would love to recommend that you assemble your power supply in a little box 3 feet away from your main instrument but that is not always feasible You Comparators Can Misbehave 11 should at least start out by installing only a power transformer of known quality with good known freedom from excessive external flux and saturation Sometimes you can retrofit your circuit with a toroidal power transformer but most people don t keep these lying around They are more expensive but often worth it as they are more efficient and have less self heating i In concept you might try adding some shielding Go ahead put in 1 16 inch of aluminum It will have no effect at all for magnetic shielding you need iron Go ahead put in 1 16 inch of iron That s not much help because at 60 Hz it takes about 1 4 inch thickness of steel to do much good You can try but it s not an easy way Sometimes you can arrange your critical circuits to have smaller loops so they will pick up less flux Make neat compact paths use twisted pairs and use layout tricks like that those can sometimes help If you haven t tried these before ask an old timer 4 A mechanical vibration can be coupled in through a floppy wire or a high K ceramic capacitor If nobody tells you about this one this is a very tricky t
333. over a wide military temperature range exactly where people wish they had perfect reliability The commercial surface mount ICs with gull wing leads or J leads are more flexible and more forgiving and may cause less problems but you have to do your homework Too much solder and the leads get stiffened excessively too little and there s not enough to hold ion Kelvin Connections Improve Measurement Accuracy The usefulness of Kelvin contacts and connections is not widely appreciated In fact when Julie Schofield an Associate Editor at EDN asked me some questions about them recently I was surprised to find very little printed reference material on the subject I looked in a few dozen reference books and text books and didn t finda decent definition or explanation anywhere I did find some Kelvin clips which facilitate Kelvin connected measurements in a Keithley catalog I also found some Textool socket data sheets which mentioned in a matter of fact way the advantages of Kelvin contacts PI try to explain their usefulness in more detail here since nei ther Julie nor I could find a good description in any technical encyclopedia Perhaps the most common use of Kelvin connections is the remote sense tech inque Kelvin connections and sockets let you bring a precise voltage right to the 58 5 Preventing Material and Assembly Problems terminal or pin of your circuit under test If you don t control the supply voltage pre
334. ow leading analog circuit designers approach problems and you ll gain insight into their thought processes as they develop solutions to those problems If you work with analog circuits or want to gain new understanding of what can be a complex subject this is one book you must have Look for this book at your technical bookstore or order directly from Butterworth Heinemann 1991 389 pages ISBN 0 7506 9166 2 All you NEED is ACCESS The Only EOEM Publication Web Site With Full Editorial Content Includes new 1994 Includes company name address products Design ideas and all feature articles price and description Updated monthly complete with schematic diagrams tables and vendor lists gt Software Center replaces bulletin board system Users download valuable supplemental Fuil Editorial Archives dates back to software programs Much less expensive more January 1994 Search by keyword issue or user friendly than original BBS preselected topic Industry Forums provide outlet for gt Extensive New Product Database controversial stimulating dialog with peers in contains more than 4 000 products dating from the industry N ACCESS FOR DESIGN BY DESIGN http www ednmag com Don t Understand Analog Troubleshooting Relax Bob Pease Does Pools you are to say you learn by your experience prefer to profit by others mistakes and avoid the price of my ow ne Otto von Bismarck
335. pF capacitor your eye can resolve if the capacitor s value is Q 1 times or 10 times as large as it should be You can t perform this kind of test with a DVM Another advantage of analog meters is that they are passive devices They don t inject noise into your circuit as digital meters can even battery powered ones And they can have a lot less capacitance to ground A general purpose function generator While sine and square waves are popular test signals I have often found triangular waveforms to be invaluable when searching for nonlinearities Sometimes you need wo function generators one to sweep the Opel ating point of the DUT slowly back and forth over its operating range while vou watch the response of the output to a small quick square wave watching tor oscilla ton or ringing or trouble Power supplies with stable outputs Phey should have coarse and tine adjustment controls and adjustable current limits Digital controls arc usually noe suitable they Figure 2 5 9 18 2 Choosing the Right Equipment You can vary the output voltage of this DC power supply from 3 to 30 V by adjusting R R lt should be between 3 and 100 the short circuit current is equal to about 20 mA 600 mV R don t let you continuously sweep the voltage up and down while you monitor the scope and watch for trends In cases when the power supply s output capacitor causes problems you may want a power supply whose output circuit
336. pecified One recently advertised op amp claims to require only 12 ns to come out of the stops Just about all other Op amps are slower to one degree or another The recovery time for chopper stabilized amplifiers can be seconds Even if you have a fast op amp that doesn t have a delay coming out of limit there may be circuits such as integrators that take a long time to recover if you overdrive the output and inputs To avoid these cases a feedback bound made of zeners and other diodes may be helpful Ref 7 However if you have a differential amplifier you may not be able to use any zener diode feedback limiters I recall the time I de signed a detector circuit using a fast dielectrically isolated op amp When I went to put it into production nothing worked right It turned out that the manufacturer had just recently redesigned the chip to cut the die size by 50 The new and improved layout just happened to slow down the op amp s overdrive recovery time I wound up redesigning the circuit to use an LM709 I saved a lot of pennies in the long run but the need to change parts didn t make me very happy at the time Rely Only on Guaranteed Specs Don t rely on characteristics that aren t specified or guaranteed by the manufacturer It s perfectly possible for you to test a set of samples and find that they feature some desired performance characteristic that is not specified by the manufacturer But if t fulfill your requiremen
337. peed low saturation small die size and low cost This structure involves mesa etching which accounts for the slopes at the die edges Planar power transis tors b can achieve very small geometries small base widths and high frequency responses but they re less rugged than epitaxial base types in terms of Forward Biassed SOA wrote all about how these devices had even more Safe Operating Area than the epi base device so you might want to order these if you wanted a really gutsy tran sistor for driving inductive loads Unfortunately these transistors were obsolescent and obsolete they were slow perhaps 0 5 MHz of fa had a large die area and were expensive For example although these transistors required only one diffusion in some cases this diffusion had to run 20 hours Because of all thes technical reasons sales shrank until in the last 2 years all the single diffused power transistors have been discontinued So it s kind of academic to talk about the old single diffused parts see Figure 7 6 but I included a mention here just for historical interest Also I included it because if you looked in my old EDN write up and then tried to buy the devices I recommended you would meet with incredulity You might begin to question the sanity of yourself or the salesman or of Pease When I inquired into the availability of these parts I talked to many sales people who had no idea what I was talking about Finally when was a
338. pensive and come only in larger packages Wirewound resistors also have one major disadvantage the in ductance of an ordinary wirewound resistor makes achieving fast sub microsecond settling almost impossible However you can specify a special winding pattern that can greatly cut down the inductance of the windings This type is listed in several manufacturers catalogs as Type HS But Fve found that there are two different 1 For example Allen Bradley FO8B103A Beckman 694 3 R 10K A and Caddock T 914 10k 100 05 all consist of four 10 KQ resistors matched to 0 05 with 50 ppm C and 5 ppm C tracking in an 8 pin mini dip package Match the TC to the Application 29 Figure 3 2 Bob uses carefully selected precision resistors to model the resistance of the metal runs of a FET op amp Photo by Steve Allen types of HS One type has almost zero inductance and greatly increased inter winding capacitance the other type has low inductance and low capacitance and is well suited for fast settling amplifiers Be cautious of manufacturers oversimplified statements A tricky problem popped up a couple of years ago when we assembled a precision amplifier with wirewound resistors The output was drifting all over the place but the amplifier zeners and transistors were stable What was drifting It turned out that a wirewound resistor was drifting because we had mistakenly used a special temper ature compensating resistor
339. plainer But I am never fie to me again just reminding you that all kinds of computer simulation get criti cized and sometimes the criticism is valid the complainer is not just imagining things Ref 3 So if the computer persists in lying to you just tell your boss that the computer has proved itself incompetent Junk that digital piece of disaster What I really think you ought to do instead of using digital simulations is to make an analog computer model you ll have a lot less trouble Be sure to scale all the transistors capacitances at 100X or 1000x their normal values so the time scale is scaled down by 100 which makes the strays negligible That s what I do I have seen it work when SPICE cannot be beaten into cooperation You might call it an analog computer because that s exactly what it is I will listen to alternative points of view but be forewarned with frosty skepticism Lies Damned Lies and Statistics One thing that doesn t help me a darned bit is statistics at least in the sense that mathematicians use them I find most statistical analyses worse than useless What I do like to use is charts and graphs The data I took of the diodes Vp versus lp back in Chapter 6 were a little suspicious when I wrote down the numbers but after I plotted the data knew there was something wrong Then I just went back and took more data until I understood what the error was AC current noise crash
340. ply require ments Vishay Malvern PA produces some very accurate very stable 0 6 ppm C resis tors which I keep around to check ohmmeters Some companies think they are helping designers by taking instruments to the cali bration lab without letting the designers know They do not understand that small day to day drifts are less annoying than an unexpected step change produced by recalibration My computer brethren frequently lose scope probe ground clips the clips get in their way and sometimes short a power supply I gave up and bought a pile of the clips that I keep in my bench Sad but true sometimes adding a scope probe to a malfunctioning circuit makes the circuit work The probe adds enough capacitance to kill a glitch or stop a race On floating CMOS the DC impedance of the scope can be low enough to pull the signal down to a valid level At one place F worked I was called to the factory to make my no good circuit work Fhe complaint was that the DC offset of an Op amp was drifting When I got there I found the technician had a good DVM connected to the op amp through a piece of coaxial cable to keep out noise Of course the cable s capacitance was making the op amp oscillate You can t measure DC parameters when the op amp is oscillating Sometimes I find a scope connected this way because the tech wanted more gain or could not find a 10X probe 13 14 16 17 18 19 20 21
341. pride twisted pairs 20 62 Ii voltage to frequency converter VFC a au Problems with wires and cables xi 21 130 131 134 148 eqiiency degradation 63 as ADC 130 fhe k 47 RLU we capacitors for 131 ar isolated output 130 linearity 131 tempco 131 trim of tempco 131 hirre 63 shielded wire 62 63 wasting time 9 14 16 Watts Malcolm 160 5 i waveforms 8 15 17 20 35 60 70 122 131 150 194 waveforms weird 35 6263 113 a6 185 204 X acto knife 23 217 worst case analysis 55 72 74 90 112 128 137 140 207 quasi random short circuit test 138 wrist strap 23 81 160 Yamatake Mineo 24 88 Zener diodes 71 135 avalanche diodes 71 _ base emitter junctions 77 79 _ 159 161 composite with transistors 72 72 gt integrated circuit types 71 135 problems with abuse by surge of charge 71 instability when turned off on 71 long term stability 71 _ low voltage Tk Soft knee JLi a tempco imperfect 71 referenice grade Zener diodes T1 135 Symmetrical 72 _ tansient voltage suppressors 71 Vertical fuses or anti fuses 72 Zener to be gt e zapped for trimming 12 an r l Bigy n 2 1 Notes for the U K Reader The following notes are provided by the publisher specifically for readers in the U K The prices quoted in dollars should
342. r R100b Usually I get excellent results from just grabbing on to the resistor with a minigator chp You can avoid the Stray pf that way if you use a good selector switch with all the wires dressed neatly 96 8 Operational Amplifiers The Supreme Activators in the air which is an excellent insulator you may be able to get decent bandwidth but you should be aware that you are probably measuring the AC CMRR of your set up not of the op amp I was discussing this circuit with a colleague and I realized the right way to make this R100 is to solder for example 100 Q to the input and 100 Q to the input and then just clip their tips together to make 200 Q balanced strays and all that If you have an op amp with low gain or low g you may want to add in a buffer follower at a b so the amplifier does not generate a big error due to its low gain The National LM6361 would need a buffer as it only has a gain of 3000 with a load of 10 kQ and its CMRR is a lot higher than 3000 Altogether I find this circuit has better resolution and gives less trouble than any other circuit for measuring CM error And the price is right a few resistors and a minigator clip Single Supply Operation One of the biggest applications problems we have is that of customer confusion about single supply operation Every week or so we get a call from a customer Can I run your LM108 or LF356 or LM4250 on a single power supply Your data sheet d
343. r as indicated in Figure 8 9 d In some cases as with an LM110 voltage follower the feedback path from the output to the inverting input is internally connected and thus unavailable for tailoring In this case we can pull another trick out of our bag The tailoring of noise gain Noise gain is defined as 1 B where B is the attenuation of an op amp s feedback Oscillations Do Occasionally Accompany Op Amps 101 network as seen at the op amp s inputs referred to the output signal For instance the B of the standard inverter configuration Figure 8 10a equals Z Z Z2 so the noise gain equals N 1 You can raise the noise gain as shown in Figure 8 10b If you re using a low noise gain op amp configuration such as a unity gain fol lower that has a noise gain of 1 Figure 8 11a it s well known that for good sta bility the op amp and its feedback network can t have appreciable unwanted phase shift out near its unity gain frequency If you can increase the noise gain to4 or 5 the requirement for low phase shift eases considerably No you don t have to change the signal gain to 5 A noise gain of 5 or greater is easy to achieve Figure 8 11b while maintaining a gain of 1 for the signal Even the unity gain follower with a wire from the output to the inverting input can be saved as illustrated in Figures 8 11c and d You ll find a more complete description of these circuits in Ref 2 which I wrote in 1979 but meanwhile i
344. ransistor I ramped the V up and down ifrom 5 V to 15 V back and forth and ran several tests Then I added some compli cating factors so I wanted to look at th Circuit for the first 202 ps of a 10 kHz triangle wave Namely after the first two cycles of the triangle wave I decided to look at the collector current I Cx dV dt of the transistor at t 201 ys I got my answer Printed and plotted and it did not make any sense T studied the whole circuit and I used every troubleshooting technique I could think of and it did not make any sense _ The current through the 1 pF of Cpo was not 0 2 A but 5 pA How could that be After several hours I finally decided t look at the incoming waveform I had commanded it to go back and forth from 5 V to 15 V at the rate of 50 ps per each ramp so I knew what it had to be doing But when I looked at t 201 ws the dV dt had suddenly increased from 0 2 V us to 5 V s It turned out that because I had commanded the PLOT mode to stop at 202 ys the transient generator had decided to go from 15 V to 5 V not in the time from 200 to 250 us but in the span from 200 to 202 us The dV dt speeded up by a factor of 25 without being asked to for a com pletely unexpected reason Nothing in anything I had ever seen about SPICE nothing my friends had ever heard would lead you to expect this In fact SPICE sort of en courages you to look at the waveforms any time you want it offers a sort of
345. rcuit How would you search to detect a short or an open working with 2 layer metal I bring nodes up from the first metal through vias to the second metal Sometimes I leave holes in my Vapox passivation to facilitate probing dice The subject of testability has often been addressed for large digital circuits but the underlying ideas of Design For Testability are important regardless of the type of circuit you are designing You can avoid a lot of trouble by thinking about what can go wrong and how to keep it from going wrong before the ensuing problems lunge at you By planning for every possibility you can profit from your awareness of Murphy s Law Now clearly you won t think of every possibility Remember it was something that couldn t go wrong that caused the problems with Stanier s loco motives But a little forethought can certainly minimize the number of problems you have to deal with Consider Appointing a Czar for a Problem Area A few years ago we had so many nagging little troubles with band gap reference circuits at National that I decided unilaterally to declare myself Czar of Band Gaps The main rules were that all successful band gap circuits should be registered with the Czar so that we could keep a log book of successful circuits all unsuccessful circuits their reasons for failure and the fixes for the failures should likewise be logged in with the Czar so that we could avoid repeating old mistakes and
346. res composed of ferromagnetic mate rials you had better make sure that the test conditions the AC voltage and the fre quency that the measuring instrument applies to the device under test closely ap proximate those the component will see in your real life application If you fail to take such precautions your inductance measurements stand a good chance of seri ously misleading you and making your troubleshooting task much more frustrating The phenomena you are likely to run into as a result of incorrect test conditions in clude saturation which can make the inductance look too low and core loss which can lower the Q of an inductor For transformers make sure you understand which of the inductances in the device s equivalent circuit you are measuring Equivalent Circuits Demystify Transformers You can represent a transformer with a turns ratio of N as a T network Figure 3 5a N equals N N gt where N is the number of secondary turns and N is the number of primary turns However if you plan to make measurements on transformers it s helpful to keep the equivalent circuit shown in Figure 3 5b in mind For example the inductance you measure between terminals A and B is quite large if you leave termi nals C and D open but the measured inductance will be quite small if you short ter minals C and D together In the first case you are measuring the mutual inductance plus the leakage inductance of the primary But because the
347. reviews for all the guys This guy gave me a tip Don t design a circuit in SPICE with 50 Q resistors Use 50 1 Q It converges better H mmmm That sounds intriguing Right now I am struggling with a SPICE model of a circuit Not of a new circuit but of an old circuit The band gap reference of the old LM33 which I put into pro duction back in 77 It s a good thing I put it into production before we got SPICE because if I had first run this through SPICE I would have been pretty discouraged SPICE says this circuit has a rotten temperature coefficient and oscillates like a politician I went back and double checked the actual silicon circuits They soar like an angel have very low TC and are dead beat when you bang on them They have no tendency to oscillate they do not even ring So why does SPICE persist in lying to me Doesn t it realize I will break its back for the impertinence of lying to the Czar of Bandgaps The SPICE and CAD experts around here tell me Oh you must have bad models I ve been told that before when I was right and the experts were absolutely wrong I mean how can a single FET oscillate at 400 kHz With the help of two resistors More on this topic later I ve already gotten several letters from readers who have asked How about all these new models for op amps Won t they lead linear designers in a new direction My replies to them start out by covering a couple examples of old macro
348. rginally bad design and part of the circuit does need to be changed per haps an extra test or screening of some components before the circuit can run in production But you cannot just say that if there is ever trouble it is the design engi neer s fault What if the design engineer designed a switching regulator that never had any problems in production never ever but it only puts out 1 W per 8 cubic inches and all the parts are very expensive and then there is a lot of expensive testing on each component before assembly to prove that there is a good safety margin Is that a good design I doubt it Because if you tried to build a plane with too big a safety factor it might be bigger than a 747 but able to carry only 10 passen gers Every circuit should be built with an appropriate safety factor If you only use a transistor that is always SURE to work well that may be an uneconomic safety factor Judgment is required to get the right safety factor MOSFETS Avoid Secondary Breakdown When it comes to power transistors MOSFETs have certain advantages For many years MOSFETs have been available that switch faster than bipolar transistors with smaller drive requirements And MOSFETs are inherently stable against secondary breakdown and current hogging because the temperature coefficient of Ips vs Vag is inherently stable at high current densities If one area of the power device gets too hot it tends to carry less current and thus ha
349. rite toroids achieve their particular magnetic properties by means of oper ation at a particular point on the material s magnetization curve Saturating the core can move the operating point and drastically change the core s magnetic properties The likelihood of your being able to return the material to its original operating point is small to nonexistent In other cases as a result of applying excessive current the a b Figure 3 5 36 3 Getting Down to the Component Level core temperature increases to a point where the core s magnetic properties change irreversibly Regardless of the mechanism that caused the damage you may have to do as I once did package the inductors with a strongly worded tag to demand that nobody test them at Incoming Inspection Bob Widlar had a good solution to that He would instruct the Incoming Inspection Technician to count the number of leads Don t measure anything just count the number of leads If they follow that instruction they probably won t wreck the trans former If you choose too small a wire size for your windings the wire losses will be ex cessive You can measure the winding resistance with an ohmmeter or you can mea sure the wire s thickness But if the number of turns is wrong you can best spot the error with an L meter remember that L N Be careful when using an ohmmeter to make measurements o transformers and inductors some ohmmeters put out so many milliamp
350. rive rated load Short circuit current too big Poor load regulation Oscillation Output noisy Q dies at full load Possible Cause _ Resistors incorrect Oscillation Reference bad Amplifier bad Resistors incorrect Reference bad Comp cap shorted Q1 dead R4 broken Load too heavy Input much too low PC foil shorted PC foil open LM723 busted Same as above Q1 shorted out Input V too high R4 wrong value Q1 broken Something bad Current limiter busted Oscillation Bad gain of Q1 General C1 bad C2 needed C3 needed Q1 has bad response Input noisy Reference noisy Inadequate heat sink Heat sink bolts too loose or too tight Oscillation Power excessive Solution Check R1 R2 Check for oscillation Read V at pin 6 Z Check V at pins 4 5 Check R1 R2 Check V at pins 6 5 4 3 Check V at pin 6 Check V ohms at pin 13 Check V at pins 12 11 Pull out Q1 replace Measure R4 ohms Measure V across R4 Remove load Check V with scope Review all data above Review all data above Review all data above Test replace IC Same as above Check voltage from pin 11 to 12 If that s OFF transistor should be OFF Check Vin with scope Measure R4 Check replace Q1 Review all data above Check voltages at pins 2 3 Check R4 Replace 723 Check for oscillation Check and replace Q1 Note the frequency Add 001 across C1
351. rn out the delicate wirewound element Then he would write in his report that the potentiometer had failed What a dumb way to do incoming inspection Some trimming potentiometers are not rated to carry any significant DC current through the wiper This DC current even a microampere could cause electromi gration leading to an open circuit or noisy unreliable wiper action Other trim pots are alleged to be more reliable if a small amount of current at least a microampere DC is drawn through the wiper to prevent dry failures Carbon pots are not likely to be degraded by either of these failure modes If you have any questions about the suitability of your favorite trimming potentiometers for rheostat service you or your components engineer should ask the ROES Ananufacturer How do you spot resistor problems The most obvious way is to follow your nose When a resistor is dying i it usually gets quite hot and sometimes the strong smell of i Phenolic leads right to the amused component Just b be careful not to burn your fin nc ations icha a resistor hasn t truly failed but ut doesn t seem to be doing its job dither Sont thing seems to be wrong with the cir cuit anda resistor of the wrong value i is the easiest explanation So you measure the resistor in question and 90 of the time the resistor is just fine usually the trouble is elsewhere A resistor doesn t usually fail all by itself Its failure is
352. rrors or the occasional real killer mistake All technicians and engineers are invited Learn to Recognize Clues There are four basic questions that you or I should ask when we are brought in to do troubleshooting on someone else s project Did it ever work right What are the symptoms that tell you it s not working right When did it start working badly or stop working What other symptoms showed up just before just after or at the same time as the failure As you can plainly see the clues you get from the answers to these questions might easily solve the problem right away if not they may eventually get you out of the woods So even if a failure occurs on your own project you should ask these four al e WO PRR RI L ana saa i Eoee Agii T E A O pigo j I Figure I I Peer review is often effective for wringing problems out of designs Here the author gets his comeuppance from colleagues who have spotted a problem because they are not as overly familiar with his circuit layout as he is Photo by Steve Allen Methodical Logical Plans Ease Troubleshooting 5 questions as explicitly as possible of yourself or your technician or whoever was working on the project For example if your roommate called you to ask for a lift because the car had just quit in the middle of a freeway you would ask whether any thing else happened or if the car just died If you re told that the headlights seemed to be gettin
353. rs for Voltage Offset As mentioned in Chapter 9 it is not trivially easy to measure the V ottser Of fast comparators but it is possible if you think about all the aspects of the problem Fast comparators all have a tendency to oscillate when their input voltage is nearly zero And yet to measure offset you have to get the input voltage near zero The solution to this dilemma is to force the comparator to oscillate at the frequency you define The basic op amp oscillator of Figure D 1 is able to force the oscillation but it is not a precision circuit as the output amplitude is not well defined In fact fast comparators do not have big output swings or symmetrical swings the ones with ECL outputs have only a tiny output So we add in some gain from an LM311 as shown in Figure D 2 and use a few sections of MM74C04 to provide a symmetrical output This circuit puts 10 mV at the input of the DUT and the waveform at the input of the comparator is thus forced to ramp back and forth be tween 10 mV Vos Vaoise and 10 mV Vos Vnoise The average value of the input s voltage is thus equal to the Vos as required Now this circuit s offset would not be true if the LM311 had a bad delay in one direction and a worse delay in the other direction and that would be the case if we did not include some fast AC coupled hysteresis per the 4 7 kQ 100 pF network This forces the DUT to turn around and integrate back the other
354. rter suddenly developed a puzzling nonlinearity The trickiest part of the problem with the circuit s diodes was that diodes from an earlier batch had not exhibited any slow turn on behavior Further some diodes in a batch of 100 from one manufacturer were as bad as the diodes in Figures 6 2b and 6 2c Other parts tn that batch and other manufacturers parts had substantially no overshoot When I confronted the manufacturers of these nasty diodes they at first tried to deny any differences but at length they admitted that they had changed some diffu 68 6 Understanding Diodes and Their Problems Q 2N4275 OR 2N3904 Om liim RR a b Rik REALA AMANE I fF OOo guns d Figure 6 2 In this diode evaluation circuit a transistor Q simply resets V to ground periodically When the transistor turns OFF V rises to about 0 6 V at which point the diode starts con ducting In b when dV dt is 8 V s this IN4148 overshoots as much as 140 mV at input frequencies below 10 kHz before it turns ON At higher frequencies 120 240 480 960 and 1920 kHz as the repetition rate increases the overshoot shrinks and disappears Maximum overshoot occurs when fj lt 7 kHz In c when dV dt increases to 20 V s this same IN4148 overshoots as much as 450 mV at 7 kHz but only 90 mV at 480 kHz and negligible amounts at frequencies above 2 MHz in d various diode types have different turn on char acteristics Th
355. s Apply Vin 0 1 to 10 VDC and troubleshoot it per the procedure above for inverting op amps Check for reversed or shorted diodes Watch A output with scope Check out diodes in little sockets com pare to known good diodes Check amplifier with large and small AC signals fast and slow Compare Jeakage fo known good diodes a 182 14 Real Circuits and Real Problems Comments on Table for Full wave Rectifiers As with other complicated circuits if you have to keep a circuit even as complex as this in production you should have a breadboard all built up with sockets for any critical components to make it easy to evaluate them at a minute s notice Otherwise you may just try to duck the problem and that would be wrong gt Figure 14 7 Instrumentation Amplifier Table 14 7 Troubleshooting Instrumentation Amplifiers Indication of Trouble Possible Cause Solution Input stage works badly Anything Ground one input and put a signal in the other input troubleshoot as inverting amplifier Then swap inputs Bad output stage Ground one input of output stage and put a signal in the other troubleshoot as above Bad DC errors Anything Ground both inputs read all voltages with DVM remove suspected bad amplifier and test Bad CMRR Input stage Tie both inputs together and drive and Read all voltages Check input op amps CMRR Output stage Check resistors match and trim ran
356. s It finally turned out that I had an un used resistor and an un used capacitor each tied from one point to ground Nothing lse was connected to that point Originally th y were commented out by an as terisk But at one point I d leted the asterisk and the useless R and C were dropped into the circuit and they just happened to make the convergence a lot better When I removed the R and C things got worse again This lead me to appreciate two things That the convergence is a lot more fragile lt i thanwe suspect and that we may Be alle o randomly throw useless resistors into a a ov cit nity and sottietimes they couldn WOVE the Convergence In other words if you gence the computer might have a subroutine to o the circuit and see if that helps a kind of are still working on this but it may be a to surprising if you have ever heard that the byrne sistors randomly sptinkle a few tesisto auitoconverge scheme At pr s useful approach This concept is not too surprisir convergence of a circuit may be improved or degraded depending on the names and numbers you call the nodes of the cireuit Ifyou swap a couple nodes numbers and _ things get better or worse doesn t that mak you nervous Or at least suspicious 0 onn n ae oy ne Another serious problem I had with ICE was when I ran some simple transient tests triangle waves on the collector of a t
357. s that voltage to the current limit sense amplifier The use of Kelvin sockets is not confined to large power transistors or high current circuits Consider a voltage reference with 2 mA of quiescent current If you re trying to observe a ppm stable reference and the ground connection changes by 5 mQ which most socket manufacturers do not consider disastrous the 10 4V shift that results from this change in ground impedance could confound your measurements If you want to avoid trouble in precision measurements avoid sockets or at least avoid sockets that do not have Kelvin contacts Lord Kelvin William Thomson before he was appointed a baron did indeed leave us with a bag full of useful tools Avoid Cold Soldered Joints I have a few comments to add about solder most of the time we take it for granted You ll normally use ordinary rosin core tin lead solder If you avoid jiggling the Make Good Connections 59 0 TO 1 mA CURRENT SOURCE SV FORCE GROUND SENSE PROGRAMMABLE 300 POWER 0 1 SUPPLY NOTE IC LM607 OR SIMILAR OP AMP WITH 15V SUPPLIES Figure 5 4 By using Kelvin connections you can avoid measurement errors caused by IR drops in the circuit that you re trying to measure and in its connections In this circuit there are at least four pairs of Kelvin connections soldered joint as it is cooling you won t get a cold soldered joint But you should know what a cold soldered joint loo
358. s an inherent mechanism to avoid run ning away This self ballasting characteristic is a major reason for the popularity of MOSFETs over bipolar transistors However recent criticism points out that when you run a MOSFET at high enough voltages and low current the current density gets very small the temperature coefficient of Ips vs Vgs reverses and the device s inherent freedom from current hogging may be lost Ref 5 So at high voltages and low current densities watch out for this possibility When the Vpg gets high enough MOSFETS can exhibit current hogging and secondary breakdown similar to that of bipolars The newer power MOSFETs are considerably more reliable and less expensive than the older devices Even though you may need a lot of transient milliamps to turn the gate ON or OFF quickly you don t need a lot of amps to hold it ON like you do with a bipolar transistor You can turn the newer devices OFF quicker too if you have enough transient gate drive current available However MOSFETs are not without their problem areas If you persist in dissi pating too many watts into a MOSFET you can melt it just as you c melt a bipolar device If you don t overheat a MOSFET the easiest way to cause a problem isto forget to insert a few dozen or hundred ohms of resistance or a ferrite bead ri ght at the gate lead of the device Otherwise these devices have such high bandwidths that they can oscillate at much higher freque
359. s his DVM that had gone out of linearity not the LM331 Normally I hate to use a DVM s autoranging mode I have seen at least two other wise high performance DVMs that could not lock out the autorange feature The worst aspect of these meters was that I couldn t tell where they would autorange from one range to another so I couldn t tell where to look for their nonlinearity yet I knew there was some nonlinearity in there somewhere After an hour of searching I found a couple of missing codes at some such preposterous place as 10 18577 V And this on a 4000 DVM that the manufacturer claimed could not possibly have such an error could not have more than ppm of nonlinearity Another fancy DVM had the ability to display its own guaranteed maximum error saying that its own error could not be more than 0 0040 when measuring a 1 MQ resistor But then it started indicating that one of my better 1 000000 MQ resistors was really 0 99980 MQ How could I prove if it was lying to me Easy I used jiu jitsu I employed its own force against itself I got ten resistors each measuring exactly 100 000 kQ the fancy machine and all the other DVMs in the lab agreed quite well on these resistors values When I put all 10 resistors in series all the other meters in the lab agreed that they added up to 1 00000 MQ the fancy but erroneous machine said 0 99980 MQ Back to the manufacturer it went So if you get in an argument with a digital met
360. s of the DIP itself the signals were not the same as the signals on the socket not at all I finally realized that the glue was getting into the in ternal voids of the socket and preventing the IC s pin from making a true connection We banned the glue from that task and the problem went away mostly Still both before and after that time we have seen sockets that just failed to connect to an IC s pin You merely have to probe to the pin of the IC itself not just to the socket to nail down this possibility Sometimes the pin goes into the socket and actually fails to connect but more often than not the pin is simply bent under the package There is one other kind of problem you can have with a socket as a friend of mine recounted He was trying to troubleshoot a very basic op amp circuit but its wave forms did not make sense After several minutes he turned his circuit over and real ized he had forgotten to plug an op amp into the socket This example leads us to McKenna s Law named after an old friend Dan McKenna You can t see it if you don t look at it We invoke this law when we discover that we forgot to plug in a E line cord or connect something A vital part of troubleshooting is realizing that we are all at the mercy of McKenna s Law when we get absent minded Connectors and sockets usually do more good than harm They permit you to check options and perform experiments that may seem absurd and prepostero
361. s reverse current even if it s as low as nanoam peres or very brief in duration tends to degrade the low current beta of the tran sistor at least on a temporary basis So in cases where accuracy is important find a way to avoid reverse biassing the inputs Bob Widlar reminded me that the high 77 78 7 Identifying and Avoiding Transistor Problems current beta of a transistor is generally not degraded by this zenering so if you are hammering the Vp of a transistor in a switch mode regulator that will not neces sarily do it any harm nor degrade its high current beta Transistors are also susceptible to ESD electrostatic discharge If you walk across a rug on a dry day charge yourself up to a few thousand volts and th n touch your finger to an npn s base it will probably survive because a forward biassed junc tion can survive a pulse of a few amperes for a small part of a microsecond But if you pull up the emitter of a grounded base NPN Stage or the base of a PNP you risk reverse biassing the base emitter junction This reverse bias can cause significant damage to the base emitter junction and might even destroy a small transistor When designing an IC smart designers add clamp diodes so that any pin can survive a minimum of and 2000 V of ESD Many IC pins can typically survive two or three times this amount These ESD survival design goals are based on the human body model in which the impedance equals about
362. s that the are likely to saturate the component you are trying to mea sure andiat least tem pratily alter its characteristics Select an ohmmeter which puts out only a small arhounit of ourrent PRIMARY SECONDARY NCE ED TO PRIMARY oe yee sger D r PRIMARY lft ED TO PRIMARY is SESON AY DI Ep CAPAC CTED TO PRIMARY Cpg PRIMARY TO SECONDARY INTERWINDING G CAPACHANCE RIMARY a In most instances you can represent a transformer by its turns ratio b If you are measuring the characteristics of a transformer you should keep its equivalent circuit in mind Considering the effect of each component will help you understand the results of your measurements Protect Transistors From Voltage Kick 37 voltage high enough to damage or destroy almost any transistor You can avoid this problem by connecting a suitable snubber such as a diode an RC network a zener or a combination of these components across the inductor to soak up the energy The use of a snubber is an obvious precaution yet every year I see a relay driver with no clamp to protect the transistor The transistor may survive for a while but not for long The tiniest inductors are called beads They are about the same size and shape as beads worn as jewelry they are available in various types of ferrite materiat and they have room for only one or two or four turns of wire Beads are commonly used in the base or emitter of a fast transistor to help keep i
363. s with anything refer to specific topic of interest production test equipment 21 154 156 production testing 1 42 93 173 R C Substitution box 18 19 home made R C substitution boxes 18 19 high values of C 18 19 small values of R C 18 19 Twiddle box 18 VIZ Elec wnics 18 Radio Shack 3 record before trigger 15 recorder strip chart See under strip chart recorder rectifiers See under diodes rectifiers anti reversal See under power supplies rectifier circuit precision 181 regulators voltage See voltage regula tors relays 61 62 types of relays dry circuit See small signal relays gold plated contacts 61 high power 61 index mercury wetted 6 reed relays 61 small signal 62 solid state 61 62 problems with relays coil lacks clamp diode 3 37 contact bounce 61 contacts burning 61 dry failures 61 need for R C network 61 leakage 61 mercury freezes at 38 C 61 package leakage at warm temperatures 61 position sensitivity mercury wetted 61 small signals in high power type 61 62 thermocouples 61 rental equipment 14 83 repair technician 8 173 repairs 2 8 54 138 173 resistors adjustable 30 32 19 188 types of adjustable resistors carbon 30 32 cermet 30 conductive plastic 30 multi turn 30 31 potentiometers 30 32 153 177 188 rheostats 30 31 trim pots 30 128 153 174 188 wire wound 30 32 problems of adjustable resis
364. s you use Teflon or air when you need a superior insulator you have to be careful to get your best conductances A friend who is an amateur radio operator says that many kinds of problems in RF circuits arise because nuts and bolts are used to make ground connections If a lock washer or star washer is not included the mechanical connection can loosen the ground impedances will change with every little stress or strain and nasty intermittent electrical problems will result So a major factor in the reliability of these circuits is ensuring the integrity of all bolted joints by always including star washers And make sure that wires and connectors do not get so loose as to hurt the reliability of your circuit or system See also com ments on star washers i in Chapter 13 When you use shielded table should you ground the shield at one end or at both ends Many cases call fora a ground at the receiving end of the cable but there are cases in which the shield is the main ground return Neither way is necessarily bad but be consistent Likewise i in the design and the execution of the design avoid ground loops which can Cause weird noise problems In my systems I build my analog ground system completely separate from the digital ground and make sure that the case or package ground i is also Strictly divorced Then after I use an ohm meter to confirm that these prolinds a are really separate I add one link from the analog groun
365. sing cifcuitry is similar to Kelvin connections which are com monly used in test instruments A Kelvin connection uses four wires one pair of leads is meant to carry current and the other pair senses voltage across a device Keeping the idea of Kelvin connections i in mind when designing your PC board will label each set of grounds separately Ifi it has a lot of dirty currents flowing I keep that separate as Power Ground dates it t has to be especially clean that s a High Quality nected together at only on polit I don t have very much to thy about printed circuit boards for surface n mount de vices because I have not worked on them and I am not an expert about them I hear that they are of course more challenging and Tequire more meticulous work in every way In other words ifiyou are an expert at engineering ordinary PC boards you can probably stay really hard and do it well Ifyou are not an expert well it s not a good place to start After you get your board built one of the worst problems is that of thermal cycling and stresses Packages such as LCCs Leadless Chip Carriers have problems because of their zero lead length they have no mechanical compli ance If the PC board material does not have the same thermal coefficient of expan sion as the IC package the soldered joints can be fatigued by the stresses of temp cycling and may fail early This i is sp cially likely if you have to cycle it
366. sn t include a 0 1 Q or a 100 MO resistor doesn t meani that these values won t be helpful in troubleshooting it Similarly you may not havea big capacitor in your circuit but if the circuit sud denly stops misbehaving when you But a a3 R00 i F capacitor across the power supply you ve s en a quick and dramatic Konstrationi that power supply wobbles have a _ lot to do with the circuit s problentis Also several f et of plastic insulated solid wire telephone wire often come in handy J few inches of this type of twisted pair wire makes an excellent variable capacitor sometimes called a gimmick Gimmicks are cheap and easy to vary by simply winding or unwinding them Their capacitance is approximately one Picofarad p r i inch oe 14 Schematic diagrams It s a good idea to have several copies of the schematic of the _ circuit under test Mark up one copy with the normal voltages currents and wave _forms to serve as a reference point Use the others to record notes and waveform sketches that relate to the specific circuit under test You ll also need a schematic of any homemade test circuit you plan to use Sometimes measurements made with your homemade test equipment may not agree with measurements made by purchased test equipment The results from each tester may not really be wrong They might differ because of some design feature such as signal filtering If you have all the sc
367. solution Can I skip to the next test For example if I had to probe the first layer of metal on an IC with two layers of metal because I had neglected to bring an important node up from the first metal to the second metal I might do several other tests instead I would do the other tests hoping that maybe I wouldn t have to do that probing which is rather awkward even if I can borrow a laser to cut through all the layers of oxide If I m lucky I may never have to go back and do that very difficult or nearly impossible test Of course sometimes the actual result of a test is some completely unbelievable answer nothing like the answers expected Then I have to reconsider where were my assumptions wrong Where was my thinking erroneous Or did I take my measurements correctly Is my technician s data really valid That s why trouble shooting is such a challenging business almost never boring On the other hand it would be foolish for you to plan everything and test nothing Because if you did that you would surely plan some procedures that a quick test would show are unnecessary That s what they call paralysis by analysis All things being equal I would expect the planning and testing to require equal time If the tests are very complicated and expensive then the planning should be appropriately com prehensive If the tests are simple as in the case of the 128 resistors in series you could make them
368. speed buffers have the chore of driving loads in the range of 50 150 Q Driving these loads can require a lot of current which leads to overheating Plan your heat sinks carefully to keep the device from exceeding its rated maximum tempera Even Buffered Circuits Can Oscillate 117 ture Most buffers don t have any thermal shutdown feature but the new LM6321s and LM6325s show that a good op amp at least can have such features designed in When using buffers to drive remote loads be sure that the transmission lines or ca bles have suitable termination resistances on both ends to prevent reflections and ringing If you can afford the voltage drop it s best to put about 50 Q between the buffer and its cable lt When your buffer provides a lot of extra voltage gain you must make sure that the gain rolls off in a well engineered way at high frequencies or the loop will be unstable If the buffer amplifier has a positive gain as in Figure 9 5b you can use capacitive feedback around the main amplifier But when the buffer amplifier has a gain of 10 Figure 9 5c you may want to apply a feedback capacitor from the input of the buffer amplifier the output of the op amp to the noninverting input of the op amp In some cases you can achieve stability by putting a series RC damper from the non inverting input to ground to increase the noise gain but this trick doesn t always work Damping this loop is tricky because there is so much ga
369. sses to components Squeezing the resistors and capacitors can change their values and pouring epoxy around a cir cuit can add significant capacitance too If you pot a circuit that hasn t been baked and dried out thoroughly the moisture may get sealed inside the potted module Epoxy can cover up a multitude of sins but there is no substitute for good workmanship and good engineering Potting a piece of junk usually leads only to well potted junk Make sure the designers who lay out your PC board keep a list of rules to avoid troubles For example if your circuit has a high impedance point and you suspect leakage might be a problem don t run that high impedance trace beside a power 1 Humiseal Div of Chase Corp 26 60 Brooklyn Queens Expressway Woodside NY 11377 718 932 0800 2 Armitage MM 00941 Clear Brushing Alkyd Varnish John Armitage amp Company 1259 Route 46 Parsippany NJ 07054 201 402 9000 Location Location and Location 55 supply foil guard it with a stripe of ground foil or guard foil between the two A dozen times I have heard an engineer say The resistivity of this glass epoxy mate rial is 10 4 Q cm so you can t possibly expect to have a resistance of 10 Q from your summing point to the rest of the world Then I demonstrate that the mea sured impedance is typically a lot more than the specifications say but I agree that wouldn t dare count on that fact So I guard the summi
370. stance in which a S H circuit can have trouble is when its output is con nected to a multiplexer for example when multiple S H circuits drive a single ADC to achieve simultaneous sampling of many channels of dynamic analog data If the multiplexer which had been at a voltage of say 10 V suddenly connects to the output of a S H circuit whose output is at 10 V the circuit s output will twitch and then may jump to a false level because the multiplexer will couple a little charge through the S H circuit into the hold capacitor The industry standard LF398 is fairly good at driving multiplexers but if you get a big enough capacitance on the multi plexer output perhaps 75 pF and it s charged to a voltage more than 10 V away from the S H circuit s output voltage even the LF398 s output can jump I don t have a real solution for this problem but if you are aware that it can happen at least you won t tear out all your hair trying to guess the cause You will recognize the problem and then tear out your hair About all you can do is try to minimize the capacitance on the output of the multiplexer One way to do this is by using a hierarchical con nection of submultiplexers Not Much Agreement on Acquisition Time Definition Another area of S H circuit confusion is acquisition time I have seen at least one data sheet that defined acquisition time as the time required to go from HOLD to SAMPLE and for the output to then settle to a
371. t analog circuit trouble shooter I am fairly good and I just happened to be the guy who sat down and put all these stories in writing Furthermore the techniques you need to troubleshoot a switch mode power supply apply in general to a lot of other analog circuits and may even be useful for some basic digital hardware You don t have to build switchers to find this book useful if you design or build ath eR this book is for you are ie ts Who are knowledgeable about digital circuits computers microprocessots an ftware who may someday write about the trou bleshooting of those typesiof citeuits That sure would suit me fine because I am certainly not going to talk about those circuits Everybody has to be ignorant about something and that is exactly what I am ignorant about If Only Everything Would Always Go Right Why are we interested in troubleshooting Because even the best engineers take on projects whose requirements are so difficult and challenging that the circuits don t work as expected at least not the first time don t have data on switching regula tors but I read in an industry study that when disk drives are manufactured the frac tion that fails to function when power is first applied typically ranges from 20 to 70 Of course this fraction may occasionally fall as low as 1 and rise as high as 100 But on the average production engineers and technicians must be prepared to 2 First Things
372. t bias circuits ensure that a small but stable bias current starts the whole circuit going Then when the start up circuit is operating properly another circuit a hand reaches around and shuts off the start up circuit When the start up circuit works properly it saves power and doesn t waste much die area Unfortunately if the start up circuit is broken or inoperative the main circuit may still start if the supply voltage jumps up rapidly but may fail t to start if the supply ramps up slowly Once a customer returned a regulator complaining that the device would start if the supply ramped to 20 V in 30 seconds or less but would hot start if it ramped i in 36 seconds or more We checked it out and the customer was absolutely right We had to change one mask and add a start up test to prevent future troubles y 7 years a ICh mak e designed a micropower ic that didn t have a true DC start up circuit The IC Was st supposed to be started up by the transient rise rate of the supply voltage m temperature the circuit would always start no matter AMOEBA how slowly the supply ram ramped up But at cold temperatures the device wouldn t start even with supplies of 15 Vi if the supplies came up slowly Worse yet if the device was running and you hi fth p power supply busses with certain positive or neg ative transients in s quence you could turn the part OFF and it might never turn ON again Needless t
373. t down that kind of thermal problem but in general there s not much to be gained unless parts are getting up near their max rated temperature or above 100 degrees Even plastic parts can be reliable at 100 degrees The ones I m familiar with are There s Nothing Like an Analog Meter Everybody knows that analog meters aren t as accurate as digital meters Except you can buy DVMs with a 0 8 accuracy analog meters better than that exist Anyway let s detail some problems with analog meters al 148 12 Roundup of Floobydust Even if an analog meter is accurately calibrated at full scale it may be less accu rate at smaller signals because of nonlinearity arising from the meter s inherent im perfections in its magnetic circuits You can beat that problem by making your own scale to correct for those nonlinearities Then there s the problem of friction and hysteresis The better meters have a taut band suspension which has negligible friction but most cheap meters don t Now as we have all learned you can neu tralize most of the effects of friction by gently rapping on tapping at or vibrating the meter It s a pain in the neck but when you re desperate it s good to know Even if you don t shake rattle or roll your meters you should be aware that they are position sensitive and can give a different reading if flat or upright or turned sideways The worst part about analog meters is that if y
374. t from oscillating A bead not only acts inductive but also acts lossy at high frequencies thus damping out ringing In general the choice of a bead is an empirical seat of the pants decision but designers who have a lot of experience in this area make good guesses This topic is one that I have not seen treated except perhaps one sentence at a time in any book or maga zine You ll just have to get a box of ferrite beads and experiment and fool around Transformers usually are susceptible to the same problems as inductors In addi tion the turns ratio may be wrong or the winding polarity might be incorrect And if your wire handling skills are sloppy you might have poor isolation from one winding to another Most ferrite materials are insulators but some are conductive So if you ve designed a toroidal transformer whose primary and secondary windings are on opposite sides of the toroid and you scrape off the core s insulating coating you could lose your primary secondary insulation If the insulating coating isn t good enough you might need to wrap tape over the core Fortunately it s easy to establish comparisons between a known good transformer and a questionable one If you apply the same input to the primaries of both trans formers you can easily tell if the secondaries are matched wound incorrectly or connected backwards If you re nervous about applying full line voltage to measure the voltages on a transformer don t
375. t led to this book and holds approximately ten US patents Recently he began a series of columns in Electronic Design magazine where he comments fortnightly on various aspects of linear and analog circuits Bob takes great delight in seeing his ideas embodied in the work of others For example one of his proudest accomplishments is a seismic preamplifier that he de signed for an aerospace company during his coffee break After many years of ser vice the amplifier was stil at work on the moon amplifying and telemetering moon quakes but its batteries may have recently expired Bob also designed a compact 3 ounce voltage to frequency module that was carried to the summit of Mt Everest where it was used to convert medical and scientific data for medical research with the 1980 American Medical Research Expedition from the University of California Medical School at La Jolla National has taken advantage of Bob s penchant for providing ideas that others can xi xi About the Author use In his role of senior scientist Bob s responsibilities besides designing voltage references and regulators temperature sensors and VFC ICs include consulting with co workers fielding applications questions that have stumped other engineers and reviewing colleagues designs In a similar vein Bob is a long time contributing editor who reviews design idea submissions of analog circuits for EDN magazine First Things First The Philos
376. t you can use an inverter as an amplifier by tying a few megohms from the input to the output At low voltages you can make it 122 10 The Analog Digital Boundary mediocre amplifier this way but when the supply voltage is above 6 V the power drain gets pretty heavy and the gain is low I don t recommend this approach for modern designs Many years ago people used to tie the outputs of DTL or open collector TTL gates together to form a wired OR gate This practice has fallen into disrepute as t sup posedly leads to problems with troubleshooting I don t know what other reason there is for not doing it except to avoid acting like a nerd However an open collector out put with a resistive pull up is slower than a conventional gate and wastes more power A couple engineers chided me that if you let TTL or DTL inputs float that may appear to work OK for a while but when you get all the signal busses packed in together the unused inputs may be driven to give a talse response not consistently but intermittently So it is bad practice to let your TTL inputs float It is also not quite correct to tie those inputs to the 5 V bus Tie them up toward 5 V through 1 k Then a momentary 7 V transient on the supply bus may do less harm less damage When digital circuit engineers have to drive a bus for a long distance say 20 or 30 inches they use special layouts so the bus will act like a 75 Q or 93 Q stripline They also add termi
377. tability Some variable resistors claim to have infinite resolution but if you apply 2 V across a variable resistor s ends and try to trim the wiper voltage to any or every millivolt in between you may find that there are some voltage levels you can t achieve So much for infinite resolution As a rule of thumb a good pot can usually be set to a resolution of 0 1 or every 2 mV in the previous case Thus counting on a settability of 0 2 is conservative Good settability includes not only being able to set the wiper to any desired posi tion but also having it stay there But I still see people advertising multi turn pots with the claimed advantage of superior settability The next time you need a pot with superior settability evaluate a multi turn pot and a single turn pot Set each one to the desired value tap the pots with a pencil and tell me which one stays put I nor mally expect a multi turn pot whether it has a linear or circular layout to be a factor Don t Exceed Your Pot s and V Ratings 31 L R 10M a Cc 0 3 pF gt 25M 25M 25M 2 5M 03 pF 03 pF 03 PF 03 pF b a t L HOMI c ee O08 PE OBB RB wei08 d Figure 3 3 You can reduce the capacitance ofa single resistor a by using several resistors i in series as shown in b assume the inductance ison jle This series resistor configuration has one fourth of the sii i esiston s fi ner jwency response as shown in d
378. table After curing oscillation most complaints about comparators are related to their common mode range We at National Semiconductor s applications engineering department get many queries from engineers who want to know if they can violate comparators common mode specs But they re not always happy with our answers I guess the complaints are partly the fault of the manufacturers for not being clear enough in their data sheets l By way of contrast most engineers know well that an op amp s common mode voltage range Vcy is defined provided that both inputs are at the same level This spec makes sense for an op amp because most operate with their inputs at the same level But in most cases a comparator s inputs are not at the same level As long as you keep both inputs within the comparator s specified common mode range the comparator s output will be correct But if one input is within the common mode range and the other is outside that range one of three things could happen depending on the voltages and the particular comparator involved For some input ranges you can overdrive the inputs and still get Common Mode Excursions Unpredictable 115 perfectly valid response for other input ranges you can get screwy response but cause no harm to the comparator and for others you ll instantly destroy the comparator For example for an LM339 type comparator running on a single 5 V supply if one of its inputs is in the 0 3 5
379. tected by built in clamp diodes For example you can make a pulse generator per Figure 10 3 but it s considered bad practice to drive the inputs hard into the rail and beyond as you will if the capacitance is more than 0 01 uF or the power supply voltage is higher than 6 V The circuits in Figure 10 4 do as good a job without overdriving the inputs One reader cautioned me that some LS TTL parts such as DM74LS86 and 74LS75s are very touchy when you pull their inputs below ground even micro mo mentarily and give false readings for a long time It sounds to me that there are prob ably currents being injected into tubs as with an LM339 Thus these are unhappy Vs D OP AMP OR COMPARATOR LM339 LM 324 OR SIMILAR DEVICE O OUTPUT Vs FOR CMOS R R Vs Vs l EXAMPLE Vg 5V V 5V Ry Ry 10k FOR TTL CHOOSE lt Vsl_ s 446 ma m R EXAMPLE Vs 5V Vs 5V R 4 7k Rz 2 2k FOR LTTL CHOOSE el tS 4 0 16 ma 2 1 EXAMPLE Vg5 5V V 5V Ay 4 7k Rz 3k Figure 10 2 Driving logic from an Op amp operating from the usual large supply voltages requires an atten uator between the amplifier and the logic IC The equations show how to calculate the atten uator ratios oe FT ne eon Dt eae Perfect Waveforms Don t Exist Vin 0 1 pF Vy Vz Vin 1M Vs Vi 1M V2 0 2 uF Ve EACH INVERTER 474C04 V4 a b oe Figure 10 3 This CMOS pulse generator a is not recommen
380. the applications section It indicates the novel and conventional ways to use a device Sometimes these applications are just little ideas to tweak a reador s mind After looking at a couple of appli Cations one can invent other ideas that are useful Some applications may be of no real interest or use In other cases an application circuit may be the compiete definition of the system s performance it can be the test circuit in which the specification limits are defined tested and guaranteed But in all other instances the performance of a typical application circuit is not guaranteed it is only typical In many circumstances the performance may de pend on external components and their Precision and matching Some manufacturers have added a phrase to their data sheets Applications for any circuits contained in this document are for illustration purposes only and the manufacturer makes no representation or warranty that such applications will be Suitable for the use indicated without further testing or modi fication in the future manufacturers may find it necessary to add disclaimers of this kind to avoid disappointing users with circuits that work well much of the time but cannot be easi ly guaranteed The applications section is also a good place to look for advice on quirks potential drawbacks or little details that may not be so little when a user wants to know if a davice will actually deliver the expected perfor
381. the unit can achieve its highest accuracy and a TO OSCILLOSCOPE VERTICAL PREAMPS b 20 or 30 pF TYP i f i d mT I l t i Figure 2 1 If an amplifier or a comparator is supposed to produce a square wave but the waveform looks like trace a or b how long should it take you to find the problem No time at all just turn the screw that adjusts the 10X probe s compensation so the probe s response is flat at ali frequencies c The schematic diagram of a typical 10 oscilloscope probe is shown in d 16 2 Choosing the Right Equipment a b Figure 2 2 When a fast square wave is supposed to be clean and fast settling but looks like a don t repair the square wave generator just set aside the probe s 6 inch ground lead c If you ground the probe directly at the ground point near the tip d special attachments that bring the ground out conveniently are available your waveforms will improve consider ably b PROBE INPUT i OPTIONAL R 0 Ry R2 R3 NOTES Q Q3 2N5486 or 2N5485 R R2 R 10M THIS 30M RESISTOR HAS APPROXIMATELY 0 08 pF OF CAPACITANCE a mized pri be s band speed Otherwise you ll be wasting time while the DVM autoranges For many analog circuits it s important to have a high impedance gt gt 10 000 MQ input that stays at high impedance up to 15 or 20 V the four DVMs meittioned above have this
382. things really well f they are at DC or just a little bit faster than DC I replied Well that s true but what s your point His point was that in RF circuits and many other kinds of fast circuits you should use capacitors and other components dressed Closely together so that the inductance is small and well controlled He is absolutely right the layout of a high speed fast settling or a high frequency circuit greatly affects its performance Capacitors for such circuits must be compact and not have long leads Ceramic and silvered mica capacitors are often used for that reason Every year billions of ceramic capacitors find their way into electronic products of all kinds There are basically three classes of these parts the high K and stable K types and the COG or NPO types The high K types such as those with a Z5U characteristic give you a lot of capacitance in a small space for example 10 pF in a 0 3 in square that is 0 15 in thick That s the good news The bad news is that the Capacitance of parts with this ZSU characteristic drops 20 below the room temperature value at 0 and 55 C it drops 60 below the room temperature value at 25 and 90 C Also the dielectric has a poor dissipation factor mediocre leakage and a mediocre voltage coefficient of capacitance Still none of these drawbacks prevents capacitors of this type from being used as bypass capacitors across the supply terminals of
383. tic Fields Reference l One problem recently illustrated the foibles of inductor design Our applications engineers had designed several DC DC converters to run off 5 V and to put out var ious voltages such as 15 V and 15 V DC One engineer built his converter using the least expensive components including a 16 cent 300 pH inductor wound on a ferrite rod Another engineer built the same basic circuit butused a toroidal inductor that cost almost a dollar Each engineer did a full evaluation of his converter both designs worked well Then the engineers swapped breadboards with each other The data on the toroid equipped converter was quite repeatable But they couldn t obtain repeatable measurements on the cheaper version After several hours of poking and fiddling the engineers realized that the rod shaped inductor radiated so much flux into the adjacent area that all measurements of AC voltage and current were affected With the toroid the flux was nicely contained inside the core and there were no problems making measurements The engineers concluded that they could tell you how to build the cheapest possible converter but any nearby circuit would be subject to such large magnetic fields that the converter might be useless When I am building a complicated precision test box I don t even try to build the power supply in the main box because I know that the magnetic fields from even the best power transformer will preclude low
384. time to tack solder down the ones that I am not actively pursuing In general though this technique is extremely valuable and I ve never seen it mentioned in any book Use it with my compliments It works with diodes resistors and transistors too Just make sure that solder flux doesn t prevent the spring loaded component lead from contacting the conductor And make sure that your finger doesn t add a lot of capacitance impedance or noise into the circuit If you do have this problem push on the component with the tip of a fingernail instead of a finger A fingernail adds less than 1 2 pF But Is This Really Troubleshooting When I passed the first draft of this chapter around to a few friends one guy asked Why are you telling us all these things about weird capacitors What does that have to do with troubleshooting I gave him the same answer I give you here If you had a mediocre coupling capacitor and you didn t realize that it could keep on leaking for many seconds or minutes longer than a good coupling capacitor would you wouldn t look for problems traceable to that capacitor I cannot foresee every problem you will have in a circuit but I can point out that similar looking components can have startlingly different characteristics You can t learn about these characteristics from looking in books or even in data sheets So if you get in trouble I m trying to suggest clues to look for to help you get out Co
385. tion at 10 kHz If he made all the changes at once the performance might improve but if we weren t sure which changes made the improvement we wouldn t be learning much would we Systems and Circuits When a system is designed it is usually partitioned into subsections that are assigned to different people or groups to engineer Two very important ingredients in such a system are Planning and Communication If the partitioning was done unfairly then some parts of the system might be excessively easy to design and other parts sub stantially impossible We ve all seen that happen so we must be careful to prevent it from happening to our systems For if all the subsystems work except for one the whole project will probably fail Figure 12 4 Figure 12 5 152 12 Roundup of Floobydust If you re worried that some foolish person will ruin a circuit by misadjusting a trimming po tentiometer you can foil the bungler with this snip trim network The procedure for trim ming Vout to 22 V within tolerance is as follows If Vout is higher than 23 080 V snip out R if not don t e then if Vou is higher than 22 470 V snip out Rg if not don t e then if Vout is higher than 22 160 V snip out Rs if not don t Obviously you can adapt this scheme to almost any output voitage Choosing the break points and resistor values is only a little bit tricky Vout TO TTL b T a One of my pet pe
386. tioned in the previous chapter Since the voltage gain is defined as Ay gm X Z computing it is often a trivial task You may have to adjust this simple equation in certain cases For instance if you include an emitter degeneration resistor R the effective transconductance falls to 1 R 2m Ay is also influenced by temperature changes bias shifts in the emitter current hidden impedances in parallel with the load and the finite output impedance of the transistor Remember higher beta devices can have much worse output impedance than normal Also be aware that although the transconductance of a well biassed bipolar tran sistor is quite predictable beta usually has a wide range and is not nearly as predictable So you have to watch out for adverse shifts in performance if the beta gets too low or too high and causes shifts in your operating points and biasses Field Effect Transistors For a given operating current field effect transistors normally have much poorer gm than bipolar transistors do You ll have to measure your devices to see how much lower Additionally the Vgs of FETs can cover a very wide range thus making them harder to bias than bipolars JFETs Junction Field Effect Transistors became popular 20 years ago because you could use them to make analog switches with resistances of 30 Q and lower JFETs also help make good op amps with lower input currents than bipolar devices at least at moderate or cool temperatur
387. tions you may want to connect antireversal diodes across a or in a series with b a power supply s pins Or maybe both pected signals to their source Again any conducted interference should be at least 20 dB below requirements 7 No matter how you measure the near field radiation from your design such radiation should be at least 20 dB below the 3 meter radiation limit To make these measure ments you ll need a shielded screened test room es S Thomas L Fischer Pacific West Electronics Costa Mesa CA You recommend using antite isal diodesiaoross a power supply input to protect cirouitry against re i id power supply s lead see Figure 13 4 However ifia power supply does get hooked up backwards high currents will flow through the diode n ruin them Remember the diodes are a part of the circuitry too fi ommend connecting the diodes in series with the input pins Figure 13 4b Now the board has protection but with virtually no diode current Marvin Smith Harbor City CA Dear Mr Smith DERE l Eaa re You are cof ness T was delinquent in not mentioning them Forexampl it uting the diode in series with the correct path may be appropriate Then if reversal happens the battery won t be crowbarred and you will avoid damage to the battery and it environs However if you have a 5 V bus a diode in series with the supply would both waste a large fraction of the total voltage and possibly spoil
388. tively if you aren t sure whether the circuit is running from 12 V or 12 V and you start making contradictory statements And if I ask when the device started working badly don t tell me At 3 25 PM I m looking for clues such as About two minutes after I put it in the 125 C oven or Just after I connected the 4 Q load So just as we can all learn a little more about troubleshooting we can all learn to watch for the clues that are invaluable for fault diagnosis Methodical Logical Plans Ease Troubleshooting Even a simple problem with a resistive divider offers an opportunity to concoct an intelligent troubleshooting plan Suppose you had a series string of 128 1 kQ resistors See Figure 1 2 If you applied 5 V to the top of the string and O V to the bottom you would expect the midpoint of the string to be at 2 5 V If it weren t 2 5 V but actually V you could start your troubleshooting by checking the voltage on each resistor working down from the top one by one But that strategy would be absurd Check the voltage at say resistor 96 the resistor which ts halfway up from the midpoint to the top Then depending on whether that test is high low or reasonable try at 112 or 80 at 5 8 or 7 8 of the span then at 120 or 104 or 88 or 72 branching along in a sort of binary search that would be much more effective With just a few trials about seven you could find where a resistor was broken
389. tly the gain error and your curve of CMRR vs frequency will look just as bad as the Bode plot That is because if you used the circuit of Figure 8 3 that s just what you will be seeing There are still a few op amp data sheets where the CMRR curve is stated to be the same as the Bode plot The National LF400 and LF401 are two examples next year we will correct those curves to show that the MRR is actually much higher than the gain at 100 or 1000 H7 National is nor by the way the anly company to have this kind of absurd error in some of their data sheets Ah tets avoid that floating scope let s drive the sine wave generator into the pe bet MEE EGE Fades QRS OETA AGEA LOLS midpoint of the power supply and ground the scope and ground point A Figure 8 4 Then we ll get the true CMRR because the output will stay near ground it won t have to swing right Wrong The circuit function has not changed at all only the viewpoint of the observer has changed The output does have to swing referred to any power supply so this still gives the same wrong answer You may say that you asked for the CMRR as a function of frequency but the answer you get is in most cases the curve of gain vs frequency What about as an alternative the well known scheme shown in Figure 8 5 where an extra servo amplifier closes the loop and does not require the op amp output to do any swinging That s OK at DC it is fine for DC testing and f
390. to meet a a rush contract we had to build circuits on boards made i in our own lab ning sor and ton opens we anship king too mine that s when the foil a lift th iert alin the admirabl tenen of solder to bridke things together which i is wonderful i in 1 most instances becomes disastrous XXXPC CEM CEM 3 G 10 F4486 PR 4 GT 999 GX 527 HE 3008 3003 quart Generic 4il Kiw amp o t paper based ptteniitics popr mechan PG Ae re ical strength Generic 45 0025 PR AO 5X10 130 Standard economical Generic 4 7 0 020 1 X108 5X10 130 Similar to CEM 1 but can be punched Generic 4 75 0023 5 XO 4 X 108 130 Comparable to CEM 3 Oak 3 5 0 02 1x 109 3 X10 amp N A Flexible Generic 49 0 018 1X 108 5X 107 130 Similar to CEM 3 but fire retardant per UL 94 V 0 Keene 2 5 0 0010 tx ag 1x 107 260 Teflon is good f r high temp high speed Keene LS GOOI a eGHa 1X10 xd 260 Characterized for high frequency Fechnoph 44 0 020 2X 10 5x 10 4250 0 000 hours Polyimide Fechnopls LO 0 004 5 x 10 X 10 ISO 0O00 hours Comparable to CEM 3 34 5 Preventing Material and Assembly Problems SHIELD amp BOX Vaouust Re Ww POLARITY SELECTOR O SWITCH O 100k METER 50 OR 100 pA SCALE ZERO CENTER IS PREFERRED NOTES A YaLMC660N OR tL MC662N Q 2N4250 OR SIMILAR TRANSISTOR 6v Q2 2N930 OR SIMILAR TRANSISTOR Figure 5 1 You can use this c
391. to my old friend Bruce Seddon Starting 30 years ago he helped me appreciate some of the niceties of worst case design They never did teach that at school so you have to have a wise old timer to learn it from Bruce was never too busy to lend an ear and a helping hand and if I never got around to saying thank you well 30 years is a long time to be an ingrateful lazy bum but now s the time to say Thank you Bruce I want to express my appreciation to the 40 odd friends who helped review the drafts of these articles correct my mistakes and suggest additions Special thanks go to Jim Moyer Tim Regan Dennis Monticelli Larry Johnson and to Dan Strassberg at EDN who contributed significant technical ideas that were beyond my experience I also want to thank Cindy Lewis of Sun Circuits Inc Santa Clara CA for her help in preparing the table of PC board materials in Chapter 5 Credit goes to Mineo Yamatake for his elegant thermocouple amplifier design Steve Allen Peggi Willis Al Neves and Fran Hoffart for their photography and Erroll Dietz as Key Grip and Carlos Huerta as Gaffer Thanks also to Hendrick Santo and to the people at Natasha s Attic in San Jose for their help in engineering assembling and styling the Czar s Uniform And kudos to each of the EDN editors who slaved over my copy Julie Anne Schofield Anne Watson Swager Charles H Small and Dan Strassberg in addition to Carol S Lewis at HighText Publications in
392. to see if output is ringing see Pease s Principle Check resistive and reactive load Check the input _ Test with a lower input frequency or size 7 Check resistor markings and tolerances Check for oscillations across working range Swap in a known good amplifier Swap bad amplifier into a good circuit Amplifier gets hot Tum off power measure ohms Put in a signal thru R see if output moves etc etc a Iaea IUU Now what is the best thing about this table That it will solve all your op amp problems Hell no You can surely bump into circuits and problems that I have never seen that I have never even envisioned circuits that need more help than this table will give Well is it because it gives you some general approaches that can be used for any circuit That is a good idea and this is definitely of some value but that is not the most valuable thing Okay what is the most valuable thing about this table The most valuable thing is that you can make up your own troubleshooting tables You don t have to be perfect or brilliant or unerring You don t have to keep perfect notes You don t have to make a plan of action and follow it exactly one after another sequentially You don t even have to write your plans down although that is usually a good idea You don t have to do any one thing except to think occasionally If you do some thinking in a skeptical way you can guess solutions
393. to where half the problems have occurred over the years in inter connections John D Loop Research Engineer BellSouth Atlanta GA Rap s comment I don t find quite that much use for an ohmmeter but I agree that there are many different ways to find a problem J think I solve about half of my problems by reading a schematic a data book a customer s request a spec sheet a set of test results and test conditions So I guess you could say that half my problems come from bad connections in the way of communicating RAP Reference Pease Robert A Protection Circuit Cuts Voltage Loss Electronic Design June 14 1990 p 77 14 Real Circuits and Real Problems Congratulations You are the proud owner of a brand new Varoom Automobile It has been built with the highest old world craftsmanship and the finest computerized engineering to assure you of many miles of trouble free driving Nothing can go wronX Just in case of some problems Varoom Motor Co is pleased to include the fol lowing table for troubleshooting Troubleshooting Table Problem Car will not run Indication of Problem _ Solution CC Ash tray is full Use Ashtray Empty computer procedure Fuel gauge reads Empty Purchase fuel Fuel does not reach engine Replace Fuel Injection Computer Spark does not ignite fuel Replace Ignition Computer Console display shows Have vehicle and checkbook towed Computer Malfunctio
394. told me of the time he was using an electrolytic capacitor as an audio coupling capac itor with 2 V of reverse bias Because of the reverse bias it was producing all sorts of low frequency noise and jitter So excess noise is often a clue that something is going wrong perhaps it is trying to tell you about a misapplication or a part in stalled backwards Extended Foil Offers Extensive Advantages Figure 4 1 Another aspect of the film capacitor is whether or not it uses extended foil con struction The leads of many inexpensive wound foil capacitors are merely connected to the tip ends of the Jong strip of metal foil However in an extended foil capacitor the foils extend out on each side to form a direct low resistance low inductance path to the leads Eora MON This construction is swell suited for capacitors that must provide I low ESR equiva lent seri s resistance in applications such as high frequency filters Then if you substituted a capacitor without extended foil the filter 8 performance would be dras tically degraded DESE So there are y al mettiods of construction n atid several dielectrics that are impor tant considerations for most capacitor applications Tf an aggressive purchasing agent wants to do some Substituting to improve cost or availability the components engi neer or design engineet T haveto doa lot of work to make sure that the substitu tion won t cause p pl
395. tor on end as in an old transistor radio if it has any dissipation itmight get a lot hotter on one end than the other Many precision wirewound and film resistors have low Seebeck coefficients in the range 0 3 to 1 5 pV C But avoid tin oxide resistors which can have a thermocouple effect as large as 100 WEG Afiyou are going t specify a resistor for a critical appli cation where thermocouple errors could degrade circuit performance check with the manufacturer 0 role So you ought to know that resistors can n present challenging troubleshooting prob lems Rather than re inventing the wheel every time try to learn from people with experience When Is a Resistor Not just a Resistor When it s a fuse Obviously when a low value resistor is fed too much current and fails open that is sometimes a useful function and the multi million dollar fuse industry thus serves to protect us from trouble But the fuses themselves can cause a little trouble They don t always blow exactly when we wish they would As Ian Sinclair put it in his book Passive Components a User s Guide If you thought 2 Mr Sinclair s book has a lot of good information in it about all kinds of passive components and thoroughly recommend it see Ref 1 for more information 34 3 Getting Down to the Component Level that a A fuse would blow when the current exceeded A then you have not been heavily involved in choosing fuses Ref 1
396. tors current too much 31 32 dry failure See wiper current too little power too much 30 31 resolution 30 31 settability 30 31 wiper current too little 32 wiper current too much 32 resistors fixed 26 31 121 122 133 136 139 types of fixed resistors 27 carbon composition 27 28 30 109 121 carbon film 27 28 conceptual computer model 144 145 204 206 208 diffused 29 82 83 high tempco See temperature compensating types low tempco 28 29 metal film 26 28 28 monolithic See diffused networks See thin film networks Nichrome 29 Precision films low tempco 27 28 155 RNSSD or RN6OD 26 28 Sichrome 29 30 temp compensating 27 29 32 thick film or cermet 28 thin film networks 24 28 type HS See wirewound high speed wire wound high speed type 28 29 Index wire wound power 27 139 wire wound precision 27 28 29 149 158 with spirals 27 28 28 series connection of R to get low capacitance 30 31 resistor substitution box 18 19 See also under RC substitution boxes resistor failures or problems capacitance 30 31 cracks 32 33 find by nose 32 inductance 139 ohmmeters 33 open circuits 32 33 Seebeck effect 33 thermocouples See Seebeck effect too much current 33 wrong value 26 176 185 206 RF shielding 162 163 rheostat See resistors adjustable ringing 44 45 56 99 100 162 See also under Pease s Principle and
397. tors are another possible cause of oscillation trouble due to their wide range of DC gain and AC response A switching regulator IC on the other hand is not as likely to cause oscillations because its response would normally be faster than the loop s frequency But the IC is never absolved until proven blameless For this reason you should have an extra module with sockets installed just for evaluating these funny little problems with differing sup pliers variant device types and marginal ICs You might think that the sockets stray capacitances and inductances would do more harm than good but in practice you can learn more than you lose When Is an Oscillation Not an Oscillation We still get a phone call every month or so from somebody complaining about a 120 Hz oscillation on one of our circuits It s a good thing we do because one of our applications engineers was mentioning such a case recently and I realized I had forgotten to mention this type of oscillation so this paragraph got plopped into the text at the last minute If I hadn t remembered to include this class of oscillation I would have been terribly embarrassed Now how can an op amp be oscillating at 60 or 120 Hz Well it is not impossible for an op amp or regulator to oscillate at this frequency but it is extremely unlikely What is surely happening is that there is some noise at power line frequency getting into the circuit There are four m
398. trageous promises Still we all agree that computers promise some real advantages if only we can overcome their adversities and problems In many cases if you have trouble with the simulation of an analog circuit or system you troubleshoot the simulation just as you would the circuit itself You get voltage maps at various times and temperatures you insert various stimuli watch to see what s happening and modify or tweak the circuit Just like a real circuit But just like the Mario brothers you can encounter problems in Computerland 1 You might actually have a bad circuit 2 You might forget to ask the computer the right question 3 You might have mistyped a value or instruction or something The easiest mistake of this sort is to try to add a 3 3 M resistor into your circuit SPICE thinks you mean 3 3 milliohms not megohms This problem has hooked almost everybody I know I solved it by using 3300 K 3300 kQ in SPICE or I may just type out 3300111 4 You might have a bad model for a transistor or device I ve seen a typographical error in the program listing of a transistor s model tie a project in knots for months 5 You might have neglected to include Strays such as substrate capacitance PC board capacitance or something that most people forget tlead inductance 6 You might get a failure to converge or an excessive run time Or the computer might balk because the program is ta
399. ts whom are you going to get angry at Cause a test result to vary considerably compared to a guaranteed tested specification If you have to work in an unspecified range you should keep a store of tested good ICs in a safe as insurance If a new batch comes in and tests bad you ll have some backup devices I recall a complaint from a user of LM3046 transistor arrays A fraction of the parts failed to log accurately over a wide range The bad ones turned Rely Only on Guaranteed Specs 105 out to have a beta of 20 at a collector current of 50 pA versus a beta of 100 for the good ones I convinced the user that keeping a few hundred of these inexpensive parts in a safe yes literally would be a lot cheaper than getting the manufacturer to sort out high B devices Op amps and other linear ICs can also have errors due to thermal tails These tails occur when the change of heat in one output transistor causes a thermal gradient to sweep across the chip This change occurs gradually often over milliseconds and causes uneven heating of input transistors or other sensitive circuits Many high power circuits and precision circuits such as the LM317 LM350 LM338 LM396 LM333 and LM337 have been tested for many years for thermally caused error These tests aren t performed only on power ICs but also on precision references such as LM368 and LM369 and on instrument grade op amps In fact a recent article by a Tektronix
400. ture gradients 78 vacuum tubes xt 22 40 42 88 147 188 vapox 7 variable capacitors See capacitors adjustable variable resistors See resistors ad justable Variac 19 Varoom 172 VFC See under voltage to frequency converters video display terminal 155 virtual ground 97 Vishay 155 voltage gain See under operational amplifiers or transistors voltage references band gap type 71 135 154 206 207 broadband noise 135 180 Zener based 71 135 See also Zener diodes buried zeners 71 135 147 long term stability 29 71 135 147 voltage regulator ICs linear 130 177 181 183 185 177 181 191 193 See also switch mode regulators adjustable 152 153 177 181 177 178 191 193 heat sinks needed 135 output capacitor needed for negative regulators 135 136 output impedance 136 191 193 Index i output ringing vs load current 99 Widlar Robert J 36 1 88 B 118 136 191 193 Widlarize rn i Pease s Principle 99 wires and cables 6 10 15 20 22 25 user friendly 135 50 62 64 153 coax x cable 62 with external boost transistor 136 137 wt gimmicks 20 62 problems with voltage regulators inadequate supply filtering 37 38 137 inadequate heat sink 135 inductive loads 137 oscillation ringing 99 118 136 over voltage 137 ton speaker cable 63 popcorn noise 137 soe Teflon insulated 62 63 voltage map 20 106 126 144 1565
401. ultancously verify that the output is have an interestin still oscillating ADE ye Many of the techiniques T use dependon whether the circuit is one I ve never tried before or one that I see all the time Sometimes I find an unbelievable situation and I make sure that I understand what s going on before I just squash the problem and Proceed to the next After all if I m fooling myself I Teally ought to find out how or why so I won t do it again x If the op amp exhibits a DC error or a peg I first check with my scope to see that there s no oscillation Then I bring in my 5 digit DVM and scribble down a voltage map on a copy of the schematic On the first pass I m likely to just keep the numbers in my head to see if I can do a quick diagnosis of a problem that s obvious such as a bad power supply or a ground wire that fell off or a missing resistor Failing that I start writing meticulous notes to help look for a more insidious problem I look at the numbers on the schematic and try to guess the problem What failure could cause that References 10 Different Methods Uncover Different Errors 107 set of errors A resistor of the wrong value A short An open Then I try to cook up a test to confirm my theory Sometimes I have to disconnect things but I try to mini mize that Sometimes adding a resistor or voltage or current will yield the same re sult and it s much easier than disconnecting components
402. ultiplexer As mentionedialteady a multiplexer can draw big transients if you suddenly con nect it across big signals at low impedances So be careful not to overdo operating a multiplexer inthis manner as excessive current could flow and cause damage or confusion It s well known that multiplexers like most other forms of analog switches are imperfect dac to leakages oniresistance and response time But they are popular and won t give you much trouble until you turn the power supplies OFF and keep the signals going I recall that in the past few years at least one or two manufacturers Hay new designs that could survive some fairly tough over voltages with the power removed I m not sure what the designs in volved other than adding thin film resistors and diode clamps on the inputs ahead of the FET switches But if you add discrete resistors ahead of any mono lithic multiplexer s inputs the resistors can help the multiplexer survive the loss of power One other problem with multiplexers is that you don t have a whole lot of con trol over the break befor make margin And if you should want make before break action I don t think it s an available option So sometimes you may have to roll your own multiplexer If your signal leve k than 1 5 V p p you may be able to use the popular CD4051 and CD4053 sultiplexens and the CD4066 CMOS analog switches which are inexpensive and quick an usual
403. und plane has a low inductance path to the main chassis even if this path goes through an edge connector Incorporate a T or LRC filter on all input and output lines The resistor should always be on the side leading away from the hot circuit It should be the largest value pos sible as great as 1 kQ This resistor will damp or de Q any resonant circuit that the interconnect lines form as well as filter the noise You must choose the resistor and capacitor carefully so as not to adversely affect the desired signal on the line The capacitor should be a ceramic disk with a value between 10 pF and 0 01 uF depending on the signal source Probe each filtered line to confirm that only required signals are present Probe each PC board to locate the areas of maximum radiation Experiment with metal foil tape to determine where a metal shield will be most effective Install a temporary shield soldered to the ground plane to verify effectiveness You should examine purchased items such as disks and power supplies for I O line filters and radiation containment A commercially available filter or a suitable substitute should filter your circuit s input to check that any conducted interference is at least 20 dB below requirements Monitor your circuit s I O lines during normal operation and track down any unex Letters to Bob 163 15V 15V a GROUND GROUND m FT 15V 15V mne T S a b i Figure 13 4 Depending on your applica
404. ure the correct temperature with virtually no electrical or thermal effect on the circuit Figure 2 10 shows a thermocouple amplifier with designed in cold junction compen sation Some people have suggested that an LM35 temperature sensor IC Figure 2 11 is a simple way to measure temperature and so it is But if you touch or solder an LM35 in its TO 46 package to a resistor or a device in a TO 5 or TO 3 case the LM35 will increase the thermal mass and its leads will conduct heat away from the device whose temperature you are trying to measure Thus your measurements will be less accurate than if you had used a tiny thermocouple with small wires Choosing the Right Equipment 25 6V OR 9V LM35CAZ or LM35CAH GND TO ANALOG OR DIGITAL VOLTMETER 10 mVi C Figure 2 11 The LM35CAZ is a good simple convenient general purpose temperature sensor But be ware of using it to measure the temperature of very small objects or in the case of extreme temperature gradients it would then give you less accurate readings than a tiny thermo couple with small wires 27 Little filters in neat metal boxes to facilitate getting a good signal to noise ratio when you want to feed a signal to a scope They should be set up with switch selectable cut off frequencies and neat connectors If in your business you need sharp roll offs well you can roll your own Maybe even with op amps and batteries You figure out what you need Usua
405. us yet are Instructional and life saving Once a friend was in the throes of a knock down drag out struggle to troubleshoot a fast A D converter He had tried many experiments but a speed problem eluded him He asked me if he should try a socket for a critical When Is a Connector Not a Connector 6i high speed component At first I was aghast But after I thought about it and real ized that the socket would add barely 1 pF of capacitance I said Well OK it may not do much harm The addition of the socket led to the realization that the speed problem was criti cally correlated with that component and the problem was soon solved The socket that might have caused terrible strays actually caused almost no harm and in fact facilitated the real troubleshooting process If nothing you do leads in an encouraging direction and you have a half baked notion to tell your technician to install a socket that may be the best id a you have all day The socket may do very little harm and could lead to many experiments which might give you the vital clue that puts you on the track of the realiculpriti dst ifi properly applied I evaluated some recently that had less than 5 fA of a caet when I guarded them vote a they can 0 r switches to valk to digital logic circuits anti Also when there isieven a moderate amount of voltage uk ell out the need for some series RC net work to put across the contacts to he
406. ut I am stillin favor of genuine intelligence Conversely people who rely on Artificial Intelligence are able to solve some kinds of problems but you can never be sure if they can accommodate every kind of Genuine Stupidity as well as Artificial Stupidity That isthe kind that is made up especially to prove that Artificial Intelligence works just great I won t argue that the computer isn t a natural for this job it will probably be cost effective and it won t be absent minded But I am definitely nervous because if computers do all the routine work soon there will be nobody left to do the thinking when the computer gives up and admits it is stumped I sure hope we don t let the computers leave the smart troubleshooting people without jobs whether the object is circuits or people My concern is shared by Dr Nicholas Lembo the author of a study on how physi cians make diagnoses which was published in the New England Journal of ZIGGY Copyright 1988 Ziggy amp Friends Dist by Usnversal Press Syndicate All rights reserved i2 First Things First Medicine He recently told the Los Angeles Times With the advent of all the new technology physicians aren t all that much interested in bedside medicine because they can order a 300 to 400 test to tell them something they could have found by listening An editorial accompanying the study commented sadly The present trend may soon leave us with a
407. ut that s usually not a problem just wait a few hours and you ll have plenty Murphy s Law implies that if you are not prepared for trouble you will get a lot of it Conversely if you have done all your homework you may avoid most of the possible trouble I ve tried to give you some insights on the Philosophy of how to troubleshoot Don t believe that you can get help on a given problem from only one specific person In any particular case you can t predict who might provide the solution Conversely when your buddy is in trouble and needs help give it a try you could turn out to be a hero And even if you don t guess correctly when you do find out what the solution is you ll have added another tool to your bag of tricks When you have problems try to think about the right plan to attack and nail down the problem When you have intermittent problems those are the nastiest types we even have some advice for that case It s cleverly hidden in Chapter 12 So if you do your philosophy homework it may make life easier and better for you You ll be able not only to solve problems but maybe even to avoid problems That sounds like a good idea to me Dostal Jiri Operational Amplifiers Elsevier Scientific The Netherlands 1981 also Elsevier Scientific Inc 655 Avenue of the Americas NY NY 10010 212 989 5800 113 in 1990 2 Smith John I Modern Operational Circuit Design John Wiley amp Sons
408. ut the DC output s bad load regulation as the regulator is forced into and out of oscilla tion Whoever said you don t need an oscilloscope to check out DC problems Regulators Are Almost Foolproof 137 When these customers ask for help I not only explain how to stop the oscillation but I give them Pease s Principle see the box Pease s Principle in Chapter 8 How ever these days most engineers find it s better to use a bigger regulator LM350 at 3 A LM338 at 5 A because if you just add on an external transistor you cannot protect it from overheating Consequently the external power transistor has lost favor 7 Too Much Voltage Leads to Regulator Death f You can kill any regulator with excessive voltage So if you re driving inductive loads or if your circuit has an inductive source make sure to have a place for the current to go when the normal load path changes For example if you re using the LM350 as a simple battery charger with only a few microfarads of filter capacitor on the input a short between the output and ground is usually disastrous When the regulator tries to draw an increasing amount of current from the transformer and then goes into current limit the inductance of the transformer will give you marvelous 80 _V transients which then destroy the M350 The solution is to put 1000 pF rather than just 1 or 10 wR across oinp 0 j ing regulators with output noise of about 0 01 of the
409. virtually every digital IC in the whole world That s a lot of capacitors These ceramic capacitors have a feature that is both an advantage and a drawback a typical ESR of 0 1 or lower So when a digital IC tries to draw a 50 mA surge of current for a couple of nanoseconds the low ESR is a good feature It helps to prevent spikes on the power supply bus To get good bypassing and low inductance you must of course install the ceramic capacitors with minimum lead length However when you have 10 ICs in a row and 10 ceramic bypass capacitors you ve got a long LC resonator Figure 4 2 with the power supply bus acting as a low loss inductor between each pair of bypass capacitors When repetitive pulses excite this resonator ringing of rather large amplitude can build up and cause an excessively noisy power supply bus This can be especially troublesome if the signal Etc t Etc Figure 4 2 Low ESR ina decoupling capacitor is a two edged sword Though a capacitor with low ESR stabilizes the supply bus when the ICs draw short duration current spikes the low dissipation factor encourages ringing by allowing the decoupling capacitance to resonate with the bus inductance One good cure is to place electrolytic capacitors such as C across the bus C s ESR of approximately Q damps the ringing ESR Friend or Foe 45 rep rates are close to the resonant frequency of the LC network And remember that the Z5U capacitors
410. way as soon as its input hits the threshold and its fast output responds without waiting for the slower response of the LM311 As mentioned in Chapter 9 this AC coupled hysteresis decays and has no effect on the accuracy of the oscillator The circuit of Figure D 3 is very similar but is adapted for comparators with an ECL output such as A6685 The LM311 s threshold is changed and the amount of AC hysteresis is maintained by changing the impedance Oscillation occurs at 0 4 MHz No spurious oscillations have been observed although as with any fast circuit a thoughtful layout is mandatory 5V SIMPLIFIED CIRCUIT 100 Figure D I The basic concept of a self oscillating test circuit is familiar 194 D Testing Fast Comparators for Voltage Offset 0 02 MYLAR DISC L TTL OUTPUT EXTENDED CERAMIC a 4 7K 100pF Figure D 2 In practice you need precision output levels as shown here 10K 10K T Ot I f 1N914 s 4 99K 7 0 2 0 02 MYLAR DISC FEXITENDED CERAMIC FOR 100 74C04 51 Figure D 3 The circuit in Figure D 2 is for TTL type comparators for ECL ou circuit is suitable tput comparators this 195 Appendix E Vp vs lp on Various Diodes List of Diodes A SR306 Schottky rectifier 1N87G germanium HP5082 2811 Schottky Big old stud rectifier 3814 rectifier 1N4001 silicon rectifier IN4148 1N914 HER 103 ultra fast rectifier 1N645 FD300 low leakage diode oO L
411. what s really going on When I work with digital ICs I would be easily confused if I did not sketch the actual waveforms of the ICs to show their relationships to each other So I sketch these waveforms on large sheets of quadrille paper 1 4 in grid to produce some thing I call a choreography because it maps out what I want all the signals to do and exactly where and when I require them to dance or pirouette When the system gets big and scary I sometimes tape together two or three or four sheets horizontally and as many sheets as I need vertically Needless to say I am not very popular when I drag one of these monsters up to the copying machine and try to figure out how to make a copy Figure 10 3b is a small example NOTE when I first published this Figure 10 3b in EDN magazine in 1989 the sketch was printed with an error some of the pulses were positioned at the wrong time And did EDN make the error Not at all drew it wrong and the error wasn t caught until after publication when a kid engineer suggested it might be erroneous He was right How embarrassing It would have been even worse if a whole lot of people had called to correct me That just goes to show if you stand ona big soapbox and rant and holler people will often think you know what you are talking about They stop looking for mistakes and that s a mistake Bigwigs make mistakes and wanna be big wigs too Embarrassing Maybe the guys who design r
412. whole new generation of young physicians who have no confidence in their own ability to make worthwhile bedside diagnoses Troubleshooting is still an art and it is important to encourage those artists The Computer Is Your Helper and Friend I read in the San Francisco Chronicle Ref 5 about a case when SAS the Scandinavian airline implemented an Expert System for its mechanics Management knew something was wrong when the quality of the work started decreasing It found the system was so highly mechanized that mechanics never questioned its judgment So the mechanics got involved in its redesign They made more decisions on the shop floor and used the computer to augment those decisions increasing productivity and cutting down on errors A computer can never take over everything said one mechanic Now there are greater demands on my judgment my job is more interesting What can I add Just be thoughtful Be careful about letting the computers take over No Problems No Problem Just Wait References L Now let s skip ahead and presume we have all the necessary tools and the right re ceptive attitude What else do we need What is the last missing ingredient That reminds me of the little girl in Sunday School who was asked what you have to do to obtain forgiveness of sin She shyly replied First you have to sin So to do trou bleshooting first you have to have some trouble B
413. will be slow to regulate It s possible to avoid this problem by adding a 470 kQ resistor from the top of C to the input supply This resistor pulls C up to a high level where it cannot interfere with operation after start up Even after you add the resistor this circuit would be in trouble especially if the input power shuts off briefly It takes many seconds to discharge C a 5 pF capac itor and after a brief power outage no soft start capability would be available A good fix is to install a diode across the 470 kQ resistor to discharge C quickly when the input supply voltage drops This gives you a chance for soft start on the restart I m not saying that this circuit is a good worst case design you d have to prove that with engineering and tests But it s not as bad as it was Meanwhile we have at least added these components to our LM3524 data sheet Toys Illustrate Some Basics My first encounter with a start up circuit happened at a pretty young age I remember the old toy that resided in a box with a big ON OFF toggle switch on its front When your curiosity led you to flip the switch ON a motor would begin to whir the top of the box would rise up and a mechanical hand would reach out The hand would throw the switch to the OFF position and then retreat back into the box The cover would close and the whirring would cease What a charming way to represent a start up or in this case a shut down function When I was
414. with slow op amps a wide bandwidth scope is important because some transistors in slow appli cations can oscillate in the range of 80 or 160 MHz and you should be able to see these little screams Of course when working with fast circuits you may need to commandeer the lab s fastest scope to look for glitches Sometimes a peak to peak automatic triggering mode is helpful and time saving Be sure you know how all the controls work so you don t waste much time with setup and false triggering prob lems Two or three scope probes They should be in good condition and have suitable 14 Choosing the Right Equipment 15 hooks or points Switchable 1 X 10X probes are useful for looking at both large and very small signals You should be aware that 1X probes only have a 16 or 20 MHz bandwidth even when used with a 100 MHz scope When you use 10X probes be sure to adjust the capacitive compensation of the probe by using the square wave calibrator per Figure 2 I Failure to do so can be a terrible time wasting source of trouble Ideally you ll want three probes a at your disposal so that you can have onefor the trigger input and one for each channel For general purpose troubleshooting the probes should have a long ground wire but for high speed waveforms you ll need to change to a short ground wire Figure 2 2 The shorter ground wires not only give you better frequency response and step response for your signal but
415. with switch mode regula tors _ design problems 138 139 layout problems 138 7 loop stability 109 ed for current probes 15 network analyzer for 109 Pease s Principle 99 soft start circuit 37 141 139 141 step response 109 radiated and transmitted noise 138 when to design a switcher when not to 138 switch mode power supplies See switch mode regulators switches analog See under analog switches switches mechanical 61 62 95 problems of switches refer to problems of relays pages 61 62 switching power supplies See switch mode regulators symptoms 2 4 9 11 26 32 34 106 See also clues technician 1 3 5 6 8 9 14 26 32 36 38 48 51 60 61 119 136 149 151 155 159 173 193 Tektronix 79 105 Teledyne Philbrick vii xi 8 76 107 203 Teledyne Components vii telephone 9 20 155 television receivers 10 42 162 163 temperature 23 temperature cycling 26 55 57 170 temperature meter thermometer 24 25 63 73 temperature sensors 24 25 83 111 165 See also under thermo couples thermistors temperature excessive 42 59 74 78 82 83 105 finger as sensor 83 infrared image detector with TV display 83 Tempilag 165 test points See under probe points testability design for 7 thermal limit circuits 105 106 no thermal limit 117 oscillation in thermal limit 106 thermal probe 156 159 thermal response 24 82 104 105 111 147 149 155 156 165 lests f
416. with the circuit is to use a broad sheet of single sided copper clad board Placed copper side down and witha ground wire soldered to the copper it provides an alternate ground plane To prevent electrostatic discharge ESD damage to CMOS circuits you ll need a wrist strap to ground your body through MQ Safety equipment When working on medium or high power circuits that might explode with considerable power in the case of a fault condition you should be wearing safety goggles or glasses with safety lenses Keep a fire extinguisher nearby too A suitable hot soldering iron If you have to solder or unsolder heavy busses from broad PC board traces use a big enough iron or gun For small and delicate traces around ICs a small tip is essential And be sure that the iron is hot enough An easy way to delaminate a trace or pad whether you want to or not is to heat it for too long a time which might happen if your iron weren t big enough or hot enough The old Heathkit warnings not to use a hot iron became obsolete along with the germanium transistor In some cases a grounded soldering iron is required in others a portable ungrounded or rechargeable soldering iron is ideal Make sure you know whether your iron is grounded or floating Tools for removing solder such as solder wick or a solder sucker You should be comfortable with whatever tools you are using a well practiced technique is some times critical for getting
417. y want to add RC filters RLC filters or active filter buffers with precision operational amplifiers to cut down on the noise being injected into your circuit There is a little RC filter shown in Figure 2 4 of Chapter 2 that is useful for keeping the noises of the DVM from kicking back Or you might want to go to an analog meter which as we dis cussed on a previous page do not have any tendency to oscillate or put out noise An analog meter with a battery powered preamplifier will not generate much noise at all by comparison to a DVM Signal Sources While I m on the subject of instruments I really enjoy using a good function gener ator to put out sines and triangle waves and square waves and pulses I love my old Wavetek 191 But I certainly don t expect the signals to be absolutely undistorted all these waveforms will distort a little especially at high frequencies So if I want my function generator to give me a clean sine wave I put its output through an active filter at low frequencies or an LC filter at high frequencies If I want a clean crisp square wave I will put the signal through a clipping amplifier or into a diode limited attenuator Figure 12 3 If I want a cleaner triangle than the function generator will give me I just make a triangle generator from scratch But a function generator lets me down when some absent minded person pushes one button too many and the output stops Usually that absent minded pers
418. ylene unit of the same value expect the new capacitor s characteristics to be a lot better than those of the old capacitor If the tail gets a lot smaller either my plan to use polyester was not a good one or this particular polyester capacitor is much worse than usual I s me to check But usually Fd expect to find that the polypropylene capacitor does make the circul perform much better than the polyester capacitor did and Fd conclude that something else must be causing the problem 48 4 Getting Down to the Component Level Capacitor Problems For either of these techniques to work it is helpful to have a large stock of assorted capacitors In our lab we have several cartons of used but not too badly beaten up components left over from old experiments One is a box of small mica and ceramic capacitors one holds various electrolytic capacitors and one is a tray of assorted wound film capacitors These boxes are extremely valuable because if I need an odd type or an odd value I can usually fish in one of those cartons and find some thing close Or I can find some capacitors that give the right value if I parallel two or three of them I can use these capacitors per the add it on or the substitution method to find out what my unhappy circuit is trying to tell me In addition I keep a couple of Teflon capacitors in my file cabinet for when I need a super good capacitor A technique that nobody talks about but is as old as the hills
419. ys have less shunt capacitance than others However the main point is that if you need a resistance with low shunt capacitance you can connect lower value resis tors in series and if you evaluate several different manufacturers resistors you may find a pleasant surprise Variable Resistors and Pots As with the fixed resistors discussed so far there are many kinds and types of variable resistors such as trimming potentiometers potentiometers and rheostats These resistors are made with many different resistive elements such as carbon cermet conductive plastic and wire As with fixed resistors be careful of inexpen sive carbon resistors which may have such poor TC that the manufacturer avoids any mention of it on the data sheet These carbon resistors would have a poor TC when used as a rheostat but might have a good TC when used as a variable voltage divider or a potentiometer Recently I ran an old operational amplifier where the offset trim pot had a range of 100 mV Yet for 4 hours in a row the amplifier s offset held better than 10 uV That s an amazing 0 01 stability for a carbon composition pot On the other hand some of the cermet resistors have many excellent characteristics but are not recommended for applications that involve many hundreds of wiper cycles For example a cermet resistor would be inappropriate for a volume control on a radio The major problem area for variable resistors is their resolution or set
420. zzles instantaneously the case may be as hot as 140 C Alternatively you can buy an infrared imaging detector for a price of several thousand dollars and you won t burn your fingers You will get beautiful color images on the TV screen and contour maps of isothermal areas You will learn a lot from those pictures About twice a vear wish I could borrow or rent one 84 7 Identifying and Avoiding Transistor Problems i iy Figure 7 4 When using high power amplifiers there are certain problems you just never have if you use a big enough heat sink This heat sink s thermal resistance is lower than 0 5 C W Photo copyright Peggi Willis Fabrication Structures Make a Difference Another thing you should know when using bipolar power transistors is that there are two major fabrication structures the epitaxial base and the planar structure pio neered by Fairchild Semiconductor Figure 7 5 Ref 4 See my comments a couple paragraphs down concerning the obsolete single diffused transistors Transistors fabricated with the epi base structure are usually more rugged and have a wider safe operating area Planar devices feature faster switching speeds and higher frequency response but aren t as rugged as the epi base types You can compare the two types by looking at the data sheets for the Motorola 2N3771 and the Harris 2N5039 The 2N50339 planar device has a current gain bandwidth 10 times greater than the 2N3771 epi base device

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