HP 34401A User's Guide
Contents
1. Agilent 34401A Multimeter TUTORIAL pages from User s Guide Measurement Tutorial Measurement Tutorial The HP 34401A is capable of making highly accurate measurements In order to achieve the greatest accuracy you must take the necessary steps to eliminate potential measurement errors This chapter describes common errors found in measurements and gives suggestions to help you avoid these errors Thermal EMF Errors Thermoelectric voltages are the most common source of error in low level de voltage measurements Thermoelectric voltages are generated when you make circuit connections using dissimilar metals at different temperatures Each metal to metal junction forms a thermocouple which generates a voltage proportional to the junction temperature You should take the necessary precautions to minimize thermocouple voltages and temperature variations in low level voltage measurements The best connections are formed using copper to copper crimped connections The table below shows common thermoelectric voltages for connections between dissimilar metals Copper to Approx V C Copper lt 0 3 Gold 0 5 Silver 0 5 Brass 3 Beryllium Copper 5 Aluminum 5 Kovar or Alloy 42 40 Silicon 500 Copper Oxide 1000 Cadmium Tin Solder 0 2 Tin Lead Solder 5 The HP 34401A s input terminals are copper alloy 198 Chapter 7 Measurement Tutorial Loading Errors dc volts Loading Errors de volts Measurement2. change 204 Chapter 7 Measurement Tutorial Errors in High Resistance Measurements Errors in High Resistance Measurements When you are measuring large resistances significant errors can occur due to insulation resistance and surface cleanliness You should take the necessary precautions to maintain a clean high resistance system Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and dirty surface films Nylon and PVC are relatively poor insulators 109 ohms when compared to PTFE Teflon insulators 101 ohms Leakage from nylon or PVC insulators can easily contribute a 0 1 error when measuring a 1 MQ resistance in humid conditions DC Current Measurement Errors When you connect the multimeter in series with a test circuit to measure current a measurement error is introduced The error is caused by the multimeter s series burden voltage A voltage is developed across the wiring resistance and current shunt resistance of the multimeter as shown below Rs l Vs source voltage Rs DUT source resistance Vb multimeter burden voltage R multimeter current shunt 100 x Vb Error V S Teflon is a registered trademark of E I duPont deNemours and Co 205 Chapter 7 Measurement Tutorial True RMS AC Measurements True RMS AC Measurements True RMS responding multimeters like the HP 34401A measure the heating p
3. few precautions you can perform ac measurements at speeds up to 50 readings per second Use manual ranging to eliminate autoranging delays By setting the preprogrammed settling trigger delays to 0 each filter will allow up to 50 readings per second However the measurement might not be very accurate since the filter is not fully settled In applications where sample to sample levels vary widely the medium filter will settle at 1 reading per second and the fast filter will settle at 10 readings per second If the sample to sample levels are similar little settling time is required for each new reading Under this specialized condition the medium filter will provide reduced accuracy results at 5 readings per second and the fast filter will provide reduced accuracy results at 50 readings per second Additional settling time may be required when the dc level varies from sample to sample The multimeter s dc blocking circuitry has a settling time constant of 0 2 seconds This settling time only affects measurement accuracy when dc offset levels vary from sample to sample If maximum measurement speed is desired in a scanning system you may want to add an external dc blocking circuit to those channels with significant dc voltages present This circuit can be as simple as a resistor and a capacitor 214
4. loading errors occur when the resistance of the device under test DUT is an appreciable percentage of the multimeter s own input resistance The diagram below shows this error source Rs HI Vs ideal DUT voltage 1 Rs DUT source resistance So Ideal Ri multimeter input resistance TE Ri Meter 10 MQ or gt 10 Ga LO Error 100 x Rs i Rs Ri To reduce the effects of loading errors and to minimize noise pickup you can set the multimeter s input resistance to greater than 10 GQ for the 100 mVdc 1 Vdc and 10 Vdc ranges The input resistance is maintained at 10 MQ for the 100 Vdc and 1000 Vdc ranges Leakage Current Errors The multimeter s input capacitance will charge up due to input bias currents when the terminals are open circuited if the input resistance is 10 GQ The multimeter s measuring circuitry exhibits approximately 30 pA of input bias current for ambient temperatures from 0 C to 30 C Bias current will double x2 for every 8 C change in ambient temperature above 30 C This current generates small voltage offsets dependent upon the source resistance of the device under test This effect becomes evident for a source resistance of greater than 100 kQ or when the multimeter s operating temperature is significantly greater than 30 C ib multimeter bias current Error v ib X Rs HI O Rs DUT source resistance Ci
5. multimeter input capacitance For DCV ranges i EE Ideal i O lb AS Ci Meter 0 1V 1V 10V Ci lt 700 pF 100V 1000V Ci lt 50 pF For all ACV ranges lt 50 pF LO l 199 Chapter 7 Measurement Tutorial Rejecting Power Line Noise Voltages Rejecting Power Line Noise Voltages A desirable characteristic of integrating analog to digital A D converters is their ability to reject spurious signals Integrating techniques reject power line related noise present with dc signals on the input This is called normal mode rejection or NMR Normal mode noise rejection is achieved when the multimeter measures the average of the input by integrating it over a fixed period If you set the integration time to a whole number of power line cycles PLCs of the spurious input these errors and their harmonics will average out to approximately zero The HP 34401A provides three A D integration times to reject power line frequency noise and power line frequency harmonics When you apply power to the multimeter it measures the power line frequency 50 Hz or 60 Hz and then determines the proper integration time The table below shows the noise rejection achieved with various configurations For better resolution and increased noise rejection select a longer integration time Integration Time Digits NPLCs 60 Hz 50 Hz NMR 4 Fast 0 02 400 us 400 us 4 Slow 1 16 7ms 20 ms 60 dB 5 Fast 0 2 3 ms 3 ms
6. range per C and is automatically removed when you change functions or ranges When manual ranging to a new range in an overload condition the internal offset measurement may be degraded for the selected range Typically an additional 0 01 of range error may be introduced This additional error is automatically removed when you remove the overload condition and then change functions or ranges 210 Chapter 7 Measurement Tutorial Low Level Measurement Errors Low Level Measurement Errors When measuring ac voltages less than 100 mV be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources An exposed test lead will act as an antenna and a properly functioning multimeter will measure the signals received The entire measurement path including the power line act as a loop antenna Circulating currents in the loop will create error voltages across any impedances in series with the multimeter s input For this reason you should apply low level ac voltages to the multimeter through shielded cables You should connect the shield to the input LO terminal Make sure the multimeter and the ac source are connected to the same electrical outlet whenever possible You should also minimize the area of any ground loops that cannot be avoided A high impedance source is more susceptible to noise pickup than a low impedance source You can reduce the high frequency impedance of a source by placi
7. will also occur if you attempt to measure the frequency or period of an input following a dc offset voltage change You must allow the multimeter s input dc blocking capacitor to fully settle before making frequency measurements Making High Speed DC and Resistance Measurements The multimeter incorporates an automatic zero measurement procedure autozero to eliminate internal thermal EMF and bias current errors Each measurement actually consists of a measurement of the input terminals followed by a measurement of the internal offset voltage The internal offset voltage error is subtracted from the input for improved accuracy This compensates for offset voltage changes due to temperature For maximum reading speed turn autozero off This will more than double your reading speeds for dc voltage resistance and dc current functions Autozero does not apply to other measurement functions 213 Chapter 7 Measurement Tutorial Making High Speed AC Measurements Making High Speed AC Measurements The multimeter s ac voltage and ac current functions implement three different low frequency filters These filters allow you to trade off low frequency accuracy for faster reading speed The fast filter settles in 0 1 seconds and is useful for frequencies above 200 Hz The medium filter settles in 1 second and is useful for measurements above 20 Hz The slow filter settles in 7 seconds and is useful for frequencies above 3 Hz With a
8. 5 Slow 10 167ms 200 ms 60 dB 61 Fast 10 167ms 200 ms 60 dB 6 Slow 100 1 67 sec 2 sec 60 dB 200 Chapter 7 Measurement Tutorial Common Mode Rejection CMR Common Mode Rejection CMR Ideally a multimeter is completely isolated from earth referenced circuits However there is finite resistance between the multimeter s input LO terminal and earth ground as shown below This can cause errors when measuring low voltages which are floating relative to earth ground Vi float voltage l Rs DUT source resistance Ideal imbalance Meter Ri multimeter isolation I l I Vtest resistance LO Earth Ci multimeter input capacitance 200 pF LO Earth Vi x Rs Error V R A Noise Caused by Magnetic Loops If you are making measurements near magnetic fields you should take the necessary precautions to avoid inducing voltages in the measurement connections You should be especially careful when working near conductors carrying large currents Use twisted pair connections to the multimeter to reduce the noise pickup loop area or dress the test leads as close together as possible Loose or vibrating test leads will also induce error voltages Make sure your test leads are tied down securely when operating near magnetic fields Whenever possible use magnetic shielding materials or physical separation to reduce problem magnetic
9. displays 0 ohms with the leads shorted together Power Dissipation Effects When measuring resistors designed for temperature measurements or other resistive devices with large temperature coefficients be aware that the multimeter will dissipate some power in the device under test If power dissipation is a problem you should select the multimeter s next higher measurement range to reduce the errors to acceptable levels The following table shows several examples DUT Range Test Current Power at Full Scale 100 Q 1 mA 100 pW 1 ka 1mA 1 mW 10 ka 100 pA 100 pW 100 ka 10 uA 10 pW 1 Ma 5 yA 30 pW 10 Ma 500 nA 3 uW Settling Time Effects The HP 34401A has the ability to insert automatic measurement settling delays These delays are adequate for resistance measurements with less than 200 pF of combined cable and device capacitance This is particularly important if you are measuring resistances above 100 KQ Settling due to RC time constant effects can be quite long Some precision resistors and multi function calibrators use large parallel capacitors 1000 pF to 0 1 uF with high resistor values to filter out noise currents injected by their internal circuitry Non ideal capacitances in cables and other devices may have much longer settling times than expected just by RC time constants due to dielectric absorption soak effects Errors will be measured when settling after the initial connection and after a range
10. er supplies There are situations however where you might want to know the ac dc true RMS value You can determine this value by combining results from dc and ac measurements as shown below You should perform the dc measurement using at least 10 power line cycles of integration 6 digit mode for best ac rejection ac dc A ac de Crest Factor Errors non sinusoidal inputs A common misconception is that since an ac multimeter is true RMS its sinewave accuracy specifications apply to all waveforms Actually the shape of the input signal can dramatically affect measurement accuracy A common way to describe signal waveshapes is crest factor Crest factor is the ratio of the peak value to RMS value of a waveform For a pulse train for example the crest factor is approximately equal to the square root of the inverse of the duty cycle as shown in the table on the previous page In general the greater the crest factor the greater the energy contained in higher frequency harmonics All multimeters exhibit measurement errors that are crest factor dependent Crest factor errors for the HP 34401A are shown in the specifications in chapter 8 Note that the crest factor errors do not apply for input signals below 100 Hz when using the slow ac filter 207 Crest Factor continued Example Chapter 7 Measurement Tutorial Crest Factor Errors non sinusoidal inputs You can estimate the measurement error due to signal c
11. field sources 201 Chapter 7 Measurement Tutorial Noise Caused by Ground Loops Noise Caused by Ground Loops When measuring voltages in circuits where the multimeter and the device under test are both referenced to a common earth ground a ground loop is formed As shown below any voltage difference between the two ground reference points Vground causes a current to flow through the measurement leads This causes errors such as noise and offset voltage usually power line related which are added to the measured voltage The best way to eliminate ground loops is to maintain the multimeter s isolation from earth do not connect the input terminals to ground If the multimeter must be earth referenced be sure to connect it and the device under test to the same common ground point This will reduce or eliminate any voltage difference between the devices Also make sure the multimeter and device under test are connected to the same electrical outlet whenever possible Viest _ _ Ri lead resistance Ri multimeter isolation resistance Vground voltage drop on ground bus 202 Chapter 7 Measurement Tutorial Resistance Measurements Resistance Measurements The HP 34401A offers two methods for measuring resistance 2 wire and 4 wire ohms For both methods the test current flows from the input HI terminal and then through the resistor being measured For 2 wire ohms the voltage d
12. ng a capacitor in parallel with the multimeter s input terminals You may have to experiment to determine the correct capacitor value for your application Most extraneous noise is not correlated with the input signal You can determine the error as shown below Voltage Measured Vn 2 Noise Correlated noise while rare is especially detrimental Correlated noise will always add directly to the input signal Measuring a low level signal with the same frequency as the local power line is a common situation that is prone to this error 211 Chapter 7 Measurement Tutorial Common Mode Errors Common Mode Errors Errors are generated when the multimeter s input LO terminal is driven with an ac voltage relative to earth The most common situation where unnecessary common mode voltages are created is when the output of an ac calibrator is connected to the multimeter backwards Ideally a multimeter reads the same regardless of how the source is connected Both source and multimeter effects can degrade this ideal situation Because of the capacitance between the input LO terminal and earth approximately 200 pF for the HP 34401A the source will experience different loading depending on how the input is applied The magnitude of the error is dependent upon the source s response to this loading The multimeter s measurement circuitry while extensively shielded responds differently in the backward input case due to
13. nput capacitance 100 pF plus cable capacitance 209 Chapter 7 Measurement Tutorial Measuremenis Below Full Scale Measurements Below Full Scale You can make the most accurate ac measurements when the multimeter is at full scale of the selected range Autoranging occurs at 10 and 120 of full scale This enables you to measure some inputs at full scale on one range and 10 of full scale on the next higher range The accuracy will be significantly different for these two cases For highest accuracy you should use manual range to get to the lowest range possible for the measurement High Voltage Self Heating Errors If you apply more than 300 Vrms self heating will occur in the multimeter s internal signal conditioning components These errors are included in the multimeter s specifications Temperature changes inside the multimeter due to self heating may cause additional error on other ac voltage ranges The additional error will be less than 0 02 and will dissipate in a few minutes Temperature Coefficient and Overload Errors The HP 34401A uses an ac measurement technique that measures and removes internal offset voltages when you select a different function or range If you leave the multimeter in the same range for an extended period of time and the ambient temperature changes significantly or if the multimeter is not fully warmed up the internal offsets may change This temperature coefficient is typically 0 002 of
14. otential of an applied voltage Unlike an average responding measurement a true RMS measurement is used to determine the power dissipated in a resistor The power is proportional to the square of the measured true RMS voltage independent of waveshape An average responding ac multimeter is calibrated to read the same as a true RMS meter for sinewave inputs only For other waveform shapes an average responding meter will exhibit substantial errors as shown below Waveform Crest Factor Average Shape C F AC RMS AC DC RMS Responding Error eae i E l i 1 414 ee ine Calibrated for O error ka a Ei The multimeter s ac voltage and ac current functions measure the ac coupled true RMS value This is in contrast to the ac dc true RMS value shown above Only the heating value of the ac components of the input waveform are measured dc is rejected For sinewaves triangle waves and square waves the ac and ac dc values are equal since these waveforms do not contain a dc offset Non symmetrical waveforms such as pulse trains contain dc voltages which are rejected by ac coupled true RMS measurements 206 Chapter 7 Measurement Tutorial Crest Factor Errors non sinusoidal inputs An ac coupled true RMS measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets For example this situation is common when measuring ac ripple present on dc pow
15. rest factor as shown below Total Error Error sine Error crest factor Error bandwidth Error sine error for sinewave as shown in chapter 8 Error crest factor crest factor additional error as shown in chapter 8 Error bandwidth estimated bandwidth error as shown below c F2 F C F signal crest factor Bandwidih Bop A F input fundamental frequency 4n x BW BW multimeter s 3 dB bandwidth 1 MHz for the HP 34401A Calculate the approximate measurement error for a pulse train input with a crest factor of 3 and a fundamental frequency of 20 kHz For this example assume the multimeter s 90 day accuracy specifications 0 05 0 03 Total Error 0 08 0 15 1 4 1 6 208 Chapter 7 Measurement Tutorial Loading Errors ac volts Loading Errors ac volts In the ac voltage function the input of the HP 34401A appears as a 1 MQ resistance in parallel with 100 pF of capacitance The cabling that you use to connect signals to the multimeter will also add additional capacitance and loading The table below shows the multimeter s approximate input resistance at various frequencies Input Frequency Input Resistance 100 Hz 1 MQ 1 kHz 850 ka 10 kHz 160 ka 100 kHz 16 ka For low frequencies 100 x R Error o 4 Mo S Additional error for high frequencies Error 100 x 1 V1 22x Fx Rs x Cin Rs source resistance F input frequency Cin i
16. rop across the resistor being measured is sensed internal to the multimeter Therefore test lead resistance is also measured For 4 wire ohms separate sense connections are required Since no current flows in the sense leads the resistance in these leads does not give a measurement error The errors mentioned earlier in this chapter for dc voltage measurements also apply to resistance measurements Additional error sources unique to resistance measurements are discussed on the following pages 4 Wire Ohms Measurements The 4 wire ohms method provides the most accurate way to measure small resistances Test lead resistances and contact resistances are automatically reduced using this method Four wire ohms is often used in automated test applications where long cable lengths numerous connections or switches exist between the multimeter and the device under test The recommended connections for 4 wire ohms measurements are shown below See also To Measure Resistance on page 17 R Veter Ideal nN ltest Meter 1 203 Chapter 7 Measurement Tutorial Removing Test Lead Resistance Errors Removing Test Lead Resistance Errors To eliminate offset errors associated with the test lead resistance in 2 wire ohms measurements follow the steps below 1 Short the ends of the test leads together The multimeter displays the test lead resistance 2 Press Null from the front panel The multimeter
17. slight differences in stray capacitance to earth The multimeter s errors are greatest for high voltage high frequency inputs Typically the multimeter will exhibit about 0 06 additional error for a 100 V 100 kHz reverse input You can use the grounding techniques described for de common mode problems to minimize ac common mode voltages see page 201 AC Current Measurement Errors Burden voltage errors which apply to de current also apply to ac current measurements However the burden voltage for ac current is larger due to the multimeter s series inductance and your measurement connections The burden voltage increases as the input frequency increases Some circuits may oscillate when performing current measurements due to the multimeter s series inductance and your measurement connections 212 Chapter 7 Measurement Tutorial Frequency and Period Measurement Errors Frequency and Period Measurement Errors The multimeter uses a reciprocal counting technique to measure frequency and period This method generates constant measurement resolution for any input frequency The multimeter s ac voltage measurement section performs input signal conditioning All frequency counters are susceptible to errors when measuring low voltage low frequency signals The effects of both internal noise and external noise pickup are critical when measuring slow signals The error is inversely proportional to frequency Measurement errors
Download Pdf Manuals
Related Search