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SCB-68 User Manual for Advanced Functions

1. vi v Le Le Q Eo rit ray S oRzoo G 9 90 99 00 90 9 0 0 0000 0 iat O ep 00000000000000000 O o o4 jo fooo000000 000000000 p ee Jl WR Oo SEO fi CO a oo 00 of l e lt RIA ea a o 68 NATIONAL SEE EN ROLES D bo 1 34 INSTRUMENTS D ORo O n KO a TRIO O A 67 12 o000000 1 K R6 F 900000000 9000000 o o orr aaa 33 0000000000 46 0666666 35 am G 0000000000 0000000 OBOE 04 O 166 6006006600 13 6600000 2 N Ras O fF RT G 39 0000090000 0000000 36 A ORNO a 0000000000 47 0000000 O 30 8 0000600000 0000000 O Semy nde t 650000000000 14 O 0000000 3 Of Ree 06000660000 0660000 o TEOS A o 31 0000000000 48 600000037 JB Sarap o Re 0000000000 0000000 OAA main O 64 000000006015 cod0000 4 fw ea 9000000000 9000000 Oem ree ko 30 5000000000 49 6600060 38 0000000000 0000000 5 gemah Ja kel O 63 0000000000 16 oo00000 5 iy Kom SG nye 29 0000000000 50 9000000 39 14 ost Se 0600000000 50 90006606 oeo yoga y 0000000000 0000000 oaot mim 6200009000000 17 cad0000 6l 308 0600000006 0000000 T Tan ad T ro 28 90000000000 51 o000000 40 0009 000 0000000 Teo Jal 0 619600 40018 ooooco0 7 aro y O 0000 000
2. V AGND Al lt i 8 gt Figure 4 30 SCB 68 Circuit Diagram for Differential Analog Input Highpass Filter e Single ended analog input highpass filter To build a single ended lowpass filter refer to Figure 4 31 Add the resistor to position B or D depending on the AI channel you are using Add the capacitor to position F or G depending on the AI channel you are using Refer to Table 4 1 for component positions for all analog input channels 5 V Al lt i gt Z G a 2 2 V AIGND Al lt i 8 gt Figure 4 31 SCB 68 Circuit Diagram for Single Ended Analog Input Highpass Filter on Al lt i gt SCB 68 User Manual for Advanced Functions 4 26 ni com Chapter 4 Adding Components for Special Functions Highpass Filtering Applications One of the most common applications for highpass filters for analog inputs is to use the filter to do AC coupling AC coupling can be achieved by creating a highpass filter with a very low cutoff frequency This filter allows most dynamic signals through while it blocks any DC offsets in the signal This can be used to increase the resolution with which you can measure a dynamic signal that is riding on top of an offset as shown in Figure 4 32 10V OV Time t
3. 44 D GND Figure 4 22 SCB 68 Circuit Diagram for Digital Trigger Input Lowpass Filter National Instruments Corporation 4 19 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Lowpass Filtering Applications The following sections list applications where lowpass filtering can be useful Analog Input Lowpass Filtering Applications The following applications benefit from lowpass filtering Noise filtering You can use a lowpass filter to highly attenuate the noise frequency on a measured signal For example power lines commonly add a noise frequency of 60 Hz Adding a filter with f lt 60 Hz at the input of the measurement system causes the noise frequency to fall into the stopband Referring to Equation 4 2 fix the resistor value at 10 kQ to calculate the capacitor value and choose a commercial capacitor value that satisfies the following relationship 1 C gt 5540 000 60 4 3 Antialiasing filtering Aliasing causes high frequency signal components to appear as a low frequency signal as Figure 4 23 shows Input Signal Sampled Points Reconstructed Signal Figure 4 23 Aliasing of a High Frequency Signal The solid line depicts a high frequency signal being sampled at the indicated points When these points are connected to reconstruct the waveform as shown by the do
4. Figure 4 32 Signal before Passing through Filter Without the AC coupling you would use the 10 V range or the 0 10 V range After passing through the filter the dynamic portion of the signal is retained and centered around 0 as shown in Figure 4 33 Time t Figure 4 33 Signal after Passing through Filter National Instruments Corporation 4 27 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions You can now reduce your range to 1 V to increase the resolution of the measurement Current Input Measurement Some DAQ devices cannot directly measure current This section describes how to add components for measuring current up to 20 mA The conversion from current to voltage is based on Ohm s Law summarized by the following equation V IxR where V is voltage Tis current and R is resistance By putting a resistor with a known value in series with the current and measuring the voltage produced across the resistor as shown in Figure 4 34 you can calculate the current flowing through the circuit bp e er Transducer R Vin Input e D Figure 4 34 Current to Voltage Electrical Circuit The application software must linearly convert voltage back to current The following equation demonstrates this conversion where the resistor is the denominator and Vi is the input voltage into the DAQ device SCB 68 User Manual for Advan
5. This icon denotes a caution which advises you of precautions to take to avoid injury data loss or a system crash Bold text denotes items that you must select or click in the software such as menu items and dialog box options Bold text also denotes parameter names Italic text denotes variables emphasis a cross reference or an introduction to a key concept Italic text also denotes text that is a placeholder for a word or value that you must supply Text in this font denotes text or characters that you should enter from the keyboard sections of code programming examples and syntax examples This font is also used for the proper names of disk drives paths directories programs subprograms subroutines device names functions operations variables filenames and extensions Text in this font denotes a specific platform and indicates that the text following it applies only to that platform Contents Chapter 1 Introduction Related Docuimentati Om ss iii aan E NGE seth eh a esto es ene ess 1 2 Chapter 2 Temperature Sensor and Thermocouple Using the Temperature Sensor eee aaa Ega TIDONG KN GA E BENG ENDHA DE aa Eee 2 1 Taking Thermocouple Measurements o0e00o0enoe naon naa na anana nana n anna eaaa nan aane 2 1 Temperature Sensor Output and ACCULAaCY saseseneenenennanann nanem nn anana naen nne n nen ennee 2 2 Thermocouple Sources Of FITOr s5as55 ec eeeesesseceeeseceseeeceseesece
6. Al GND Figure 4 10 Differential Connections for AC Coupled Floating Sources with Balanced Bias Resistors Refer to the Installing Bias Resistors section for information about installing bias resistors on the SCB 68 Installing Bias Resistors To install a single bias resistor on the negative line AID of a differential pair put the resistor in position D on the SCB 68 as shown in Figure 4 11 45V Al lt i gt A B dl o E gt nn Nn lt o C oo G S D i V AIGND Al lt i 8 gt Figure 4 11 Al Differential Configuration with Single Bias Resistor SCB 68 User Manual for Advanced Functions 4 12 ni com Chapter 4 Adding Components for Special Functions To install balanced bias resistors put resistors in positions B and D on the SCB 68 as shown in Figure 4 12 5 V Al lt i gt V AGND Al lt 8 gt Figure 4 12 Al Differential Configuration with Balanced Bias Resistors Filtering This section discusses lowpass and highpass filtering on the SCB 68 Lowpass Filtering This section discusses the following topics regarding lowpass filtering on the SCB 68 e One Pole Lowpass RC Filter e Selecting Components for Lowpass Filtering e Ad
7. Instrumentation Amplifier PGIA Al Measured Bias Current Return Paths IL i oe oe NI Al SENSE I O Connector O so 9 ie So m Voltage Input Multiplexers Al GND DAQ Device Configured in Differential Mode National Instruments Corporation 4 11 Figure 4 9 Differential Connections for Floating Signal Sources with Balanced Bias Resistors Both inputs of the NI PGIA require a DC path to ground in order for the NI PGIA to work If the source is AC coupled capacitively coupled the NI PGIA needs a resistor between the positive input and AI GND If the source has low impedance choose a resistor that is large enough not to significantly load the source but small enough not to produce significant input offset voltage as a result of input bias current typically 100 kQ to 1 MQ In this case connect the negative input directly to AI GND If the source has high output impedance balance the signal path as previously described using the same value resistor on both the positive and negative inputs be aware that there is some gain error from loading down the source as shown in Figure 4 10 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions AC Coupling DAQ Device Al AC Coupled 7 Floating y Signal Source o Al o Al SENSE o
8. 2009 National Instruments Corporation All rights reserved Important Information Warranty The SCB 68 is warranted against defects in materials and workmanship for a period of one year from the date of shipment as evidenced by receipts or other documentation National Instruments will at its option repair or replace equipment that proves to be defective during the warranty period This warranty includes parts and labor The media on which you receive National Instruments software are warranted not to fail to execute programming instructions due to defects in materials and workmanship for a period of 90 days from date of shipment as evidenced by receipts or other documentation National Instruments will at its option repair or replace software media that do not execute programming instructions if National Instruments receives notice of such defects during the warranty period National Instruments does not warrant that the operation of the software shall be uninterrupted or error free A Return Material Authorization RMA number must be obtained from the factory and clearly marked on the outside of the package before any equipment will be accepted for warranty work National Instruments will pay the shipping costs of returning to the owner parts which are covered by warranty National Instruments believes that the information in this document is accurate The document has been carefully reviewed for technical accuracy In the event that te
9. Documentation for your DAQ device at ni com manuals Measurement amp Automation Explorer Help DAQ Getting Started Guide NI KnowledgeBase at ni com kb NI Developer Zone at ni com zone SCB 68 User Guide included in your SCB 68 kit and also available at ni com manuals provides information about SCB 68 installation the temperature sensor and signal conditioning switch configuration analog input measurement connection and accessory fuse and power SCB 68 User Manual for Advanced Functions 1 2 ni com Temperature Sensor and Thermocouple This chapter covers the following temperature sensor and thermocouple related topics e Using the Temperature Sensor e Taking Thermocouple Measurements e Temperature Sensor Output and Accuracy e Thermocouple Sources of Error e Open Thermocouple Detection e Thermocouple Input Filtering Using the Temperature Sensor To accommodate thermocouples with DAQ devices the SCB 68 has a temperature sensor for cold junction compensation CJC shown in Figure 3 1 SCB 68 Printed Circuit Board Diagram To power the temperature sensor set switches S1 S2 and S3 for single ended or differential mode as described in the Using the SCB 68 with MIO DAQ Devices section of the SCB 68 User Guide This configuration also powers the signal conditioning area and circuitry Refer to Figure 4 1 Analog Input and Cold Junction Compensation Circuitry for a diagram of the CJC circuitry on the SCB 68 Taking T
10. This system affords the user protection for electromagnetic compatibility EMC and product safety You can obtain the DoC for your product by visiting ni com certification If you searched ni com and could not find the answers you need contact your local office or NI corporate headquarters Phone numbers for our worldwide offices are listed at the front of this manual You also can visit the Worldwide Offices section of ni com niglobal to access the branch office Web sites which provide up to date contact information support phone numbers email addresses and current events SCB 68 User Manual for Advanced Functions B 2 ni com Index Numerics 5 V signal adding power filters 4 38 power supply figure 4 38 A accuracy considerations for attenuating voltage 4 32 adding components 2 1 4 1 channel pad configurations 4 1 power filters 4 38 analog input attenuating voltage 4 33 differential 4 33 single ended 4 34 bias resistors 4 12 balanced 4 13 single 4 12 channel pad configuration 4 2 circuit diagram figure 4 2 component locations table 4 3 connecting signals 4 5 current input measurement 4 29 differential 4 29 single ended 4 30 highpass filtering 4 25 applications 4 27 differential 4 25 single ended 4 26 lowpass filtering 4 17 applications 4 20 differential 4 17 single ended 4 18 National Instruments Corporation open thermocouple detection differential 2 4 single en
11. 0000000 41 oeoo aw 27 0000000000 52 0000000 EA He 0000000000 0000000 D Gaza tte to 60 506006600019 d000000 8 O 09000000000 9000000 DELORS lo A o 26 9666000006 53 6600000 42 0 Rasp Ra 0000000000 0000000 Orm oO 59 0000000000 20 0090000 9 Ha TO RL6 F 0000000000 Ri Tara C1 of 25 0060066000 54 1 o 43 serra ene 0000000000 R2 A w mesak c A o 1586000000000 21 5 __FoBe L0 Ramo o MTC 0000000000 nok Sole yaa OEO manis 240000000000 55 PRs of O ngao Pio 57 999999099922 h ma o T a REL E 0000000000 k lo TELIS A kol 23 0000000000 56 45 0 Rarer REG 0000000000 0660600006 n SCB 68 02000000000 COPYRIGHT_199 aaa Veeck zZ 1 5 V Power Pads R20 and R21 5 Fuse 9 Analog Output Pads 2 Switches S3 S4 and S5 6 Switches S1 and S2 10 Analog Input Pads 3 68 Pin I O Connector 7 Screw Terminals 11 Temperature Sensor 4 Breadboard Area 8 PFI O Pads Figure 3 1 SCB 68 Printed Circuit Board Diagram SCB 68 User Manual for Advanced Functions 3 2 ni com Chapter 3 Soldering and Desoldering Components on the SCB 68 B Note Ifthe kit is missing any of the components in Figure 3 1 contact NI The SCB 68 ships with 0 Q resistors in the F and G positions You must remove the resistors to use the positions Use a low wattage soldering iron 20 to 30 W when soldering to the SCB 68 To desolder on the SCB 68 vacuum type tools work best Be c
12. and laboratory use EN 61326 IEC 61326 Class A emissions Basic immunity EN 55011 CISPR 11 Group 1 Class A emissions AS NZS CISPR 11 Group 1 Class A emissions FCC 47 CFR Part 15B Class A emissions ICES 001 Class A emissions 3 Note For the standards applied to assess the EMC of this product refer to the Online Product Certification section NA Note For EMC compliance operate this product according to the documentation CE Compliance C This product meets the essential requirements of applicable European Directives as follows National Instruments Corporation 2006 95 EC Low Voltage Directive safety 2004 108 EC Electromagnetic Compatibility Directive EMC A 3 SCB 68 User Manual for Advanced Functions Appendix A Specifications Online Product Certification Refer to the product Declaration of Conformity DoC for additional regulatory compliance information To obtain product certifications and the DoC for this product visit ni com certification search by model number or product line and click the appropriate link in the Certification column Environmental Management National Instruments is committed to designing and manufacturing products in an environmentally responsible manner NI recognizes that eliminating certain hazardous substances from our products is beneficial not only to the environment but also to NI customers For additional environmental information refer to the NI and the Enviro
13. equivalent source impedance The resistor puts the signal path nearly in balance so that about the same amount of noise couples onto both connections yielding better rejection of electrostatically coupled noise This configuration does not load down the source other than the very high input impedance of the NI PGIA National Instruments Corporation 4 9 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Floating Signal Source R is about 100 times source impedance of sensor DAQ Device Al P Al Al SENSE Al GND Figure 4 8 Differential Connections for Floating Signal Sources with Single Bias Resistor You can fully balance the signal path by connecting another resistor of the same value between the positive input and AI GND as shown in Figure 4 9 This fully balanced configuration offers slightly better noise rejection but has the disadvantage of loading the source down with the series combination sum of the two resistors If for example the source impedance is 2 kQ and each of the two resistors is 100 kQ the resistors load down the source with 200 KQ and produce a 1 gain error SCB 68 User Manual for Advanced Functions 4 10 ni com Chapter 4 Adding Components for Special Functions Floating Signal Vs Source Al OO Bias Resistors o Co see text lo cot ker cot
14. following suggested characteristics e AXL or RDL package e Tolerance of 20 e Maximum voltage of at least 25 V Adding Components for Lowpass Filtering Using the circuit shown in Figure 4 18 you can use a two component circuit to build a simple RC filter with analog input analog output or digital input Lowpass Filters on Analog Input Signals You can build a lowpass filter for the following analog input modes e Differential analog input lowpass filter To build a differential lowpass filter refer to Figure 4 19 Add the resistor to position F and the capacitor to position E Refer to Table 4 1 for component positions for all analog input channels National Instruments Corporation 4 17 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions 5 V Al lt i gt H eee eee eee E zene eee eee oa ce O C V AIGND Al lt i 8 gt Figure 4 19 SCB 68 Circuit Diagram for Differential Analog Input Lowpass Filter lt Single ended analog input lowpass filter To build a single ended lowpass filter refer to Figure 4 20 Add the resistor to position F or G depending on the AI channel you are using Add the capacitor to position B or D depending on the AI channel you are using Refer to Table 4 1 for component positions for a
15. on Registered users also receive access to the NI Discussion Forums at ni com forums NI Applications Engineers make sure every question submitted online receives an answer Standard Service Program Membership this program entitles members to direct access to NI Applications Engineers via phone and email for one to one technical support as well as exclusive access to on demand training modules via the Services Resource Center NI offers complementary membership for a full year after purchase after which you may renew to continue your benefits For information about other technical support options in your area visit ni com services or contact your local office at ni com contact Training and Certification Visit ni com training for self paced training eLearning virtual classrooms interactive CDs and Certification program information You also can register for instructor led hands on courses at locations around the world System Integration If you have time constraints limited in house technical resources or other project challenges National Instruments Alliance Partner members can help To learn more call your local NI office or visit ni com alliance B 1 SCB 68 User Manual for Advanced Functions Appendix B Technical Support and Professional Services Declaration of Conformity DoC A DoC is our claim of compliance with the Council of the European Communities using the manufacturer s declaration of conformity
16. 0 0 eee eee eters 4 7 When to Use Referenced Single Ended RSE Connections with Floating Signal Sources 00 0 eee eee 4 8 Using Differential Connections for Floating Signal Sources 4 9 Installing Bias R SIStOrS 455 aaa a NGE Tg Ga TE ENE Ega NGANG a Naga NA NG EN GE a Ta an Aa BANGAH 4 12 National Instruments Corporation v SCB 68 User Manual for Advanced Functions Contents Filtering sea aga NG NG a a Sedvs Rock NG aa EA vias ed ndash bees Maid aden Jaa vei a GN Bab 4 13 LOWpass Filteringi sisa asaba or aga eaaa aa a atan aga naak Bakaran Anan ena aka 4 13 One Pole Lowpass RC Filter 0aseonenenoenenennenaen nne eenen ene nenen 4 16 Selecting Components for Lowpass Filtering 0 0 0 eee 4 17 Adding Components for Lowpass Filtering 0e00a0e0enenoanenen 4 17 Lowpass Filtering Applications 00000eneeneneenaenanaenann anana anna 4 20 Highpass Filtering sonnis aa Na ENE TENANE esas UNANG gga YEN esas edd 4 23 One Pole Highpass RC Filter saonaoenennenenennen enam nane nane nene 4 24 Selecting Components for Highpass Filtering eee 4 25 Adding Components for Highpass Filtering eee 4 25 Highpass Filtering Applications 0ao0eoeaeeeoaa nean anana nana n anane 4 27 Current Input Measurement 253 5505065 nga naa a nan n NG DN TA BIS ea a TT a agak 4 28 Selecting a Resistor for Current Input Measurement 000eneneneeneneneno 4 29
17. 17 RC6 R8 R9 AI 3 AI 11 AI3 R28 RC18 R29 RC19 RC7 R10 R11 AI 4 AI 12 AI 4 R30 RC20 R31 RC21 RC8 R12 R13 AI 5 AI 13 AI5 R32 RC22 R33 RC23 RC9 R14 R15 AI 6 AI 14 AI 6 R34 RC24 R35 RC25 RC10 R16 R17 AI7 AI 15 AI7 R36 RC26 R37 RC27 RC11 R18 R19 R denotes a socket for one component RC denotes sockets for two components to be connected in parallel National Instruments Corporation 4 3 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Conditioning Analog Output Channels Figure 4 3 shows the circuitry for both analog output channels on the SCB 68 R3 wia e MAMA AO 0 Screw Terminal RC3 Warnes AO GND Screw Terminal R2 ai e WS amp AO 1 Screw Terminal RC2 3 Z2 omnes b AO GND Screw Terminal Figure 4 3 Analog Output Circuitry Figure 4 4 illustrates the generic AO channel pad configuration Figure 4 4 Analog Output Channel Pad Configuration Table 4 2 correlates the component labels of the SCB 68 to component locations A and B for analog output channels 0 and 1 Table 4 2 Analog Output Channels Component Locations Channel A B AO 0 R3 RC3 AO 1 R2 RC2 R denotes a socket for one component RC denotes sockets for two components to be connected in parallel SCB 68 User Manual for Advanced Fun
18. 4 installing bias resistors 4 12 instrument drivers NI resources B 1 introduction 1 1 K KnowledgeBase B 1 L lowpass filtering 4 13 analog input 4 17 differential 4 17 single ended 4 18 analog output 4 19 applications 4 20 analog input 4 20 analog output 4 21 National Instruments Corporation 1 3 Index digital inputs 4 22 digital triggers 4 22 PFI 0 4 22 components adding 4 17 selecting 4 17 digital inputs 4 19 digital triggers 4 19 one pole lowpass RC filter 4 16 PFI 0 4 19 measurement 4 to 20 mA current 4 28 current input 4 28 National Instruments support and services B 1 non referenced single ended connections when to use with floating signal sources 4 7 0 one pole highpass RC filter 4 24 lowpass RC filter 4 16 open thermocouple detection 2 4 sources of error 2 5 P PFI 0 attenuating voltage 4 35 channel pad configuration 4 5 circuit diagram figure 4 5 lowpass filtering 4 19 applications 4 22 power filters 4 38 SCB 68 User Manual for Advanced Functions Index printed circuit board diagram figure 3 2 programming examples NI resources B 1 R referenced single ended connections when to use with floating signal sources 4 8 related documentation 1 2 removing the SCB 68 board from the base 3 1 S SCB 68 components adding 2 1 4 1 desoldering 3 1 soldering 3 1 documentation 1 2 introduction 1 1 modifications 3 1 pr
19. 828 g 1 lb 13 0z VO connector ioiaren nn One 68 pin male SCSI connector Screw terminals c cccesecesseeeeteeeees 68 Wire Bau Ber baia gaga sdn eag ah ites 14 30 AWG on Ka LA WA a aa WAN a A 0 5 0 6 N m 4 43 5 31 1b in Resistor sockets ccccceseseeeeseeeeeteeeees 0 032 to 0 038 in in diameter Maximum Working Voltage Maximum working voltage refers to the signal voltage plus the common mode voltage Channel to earth 00es0eeoeneeanenneno 30 Vims 42 Vpx 60 VDC Channel to channel eseeeeeeeseeseeeeee 30 Vims 42 Vpx 60 VDC Environmental The SCB 68 is intended for indoor use only Operating temperature 00 0 to 70 C Storage temperature eee eee 20 to 70 C Relative humidity eee 5 to 90 RH noncondensing SCB 68 User Manual for Advanced Functions A 2 ni com Appendix A Specifications Pollution Degree indoor use only 2 Maximum altitude 0eaaaneeeeanenene 2 000 meters Safety This product meets the requirements of the following standards of safety for electrical equipment for measurement control and laboratory use IEC 61010 1 EN 61010 1 UL 61010 1 CSA 61010 1 3 Note For UL and other safety certifications refer to the product label or the Online Product Certification section Electromagnetic Compatibility This product meets the requirements of the following EMC standards for electrical equipment for measurement control
20. Adding Components for Current Input Measurement 0 ee eee eters 4 29 Attenuating Voltage sie ss sesana a at san a E sae GUE INA i GNU E a GA sens cules a a te Naga 4 31 Selecting Components for Attenuating Voltage 0esen0eneenennaneenanaenenne 4 32 Accuracy Considerations for Attenuating Voltage 4 32 Adding Components for Attenuating Voltage 0 0 0 eee eeseereteeeeetees 4 33 Attenuating Voltage on Analog Input Signals eee 4 33 Attenuating Voltage on Analog Output Signals eee 4 35 Attenuating Voltage on Digital Inputs 000e00e0eneenennneneenen wane 4 35 Voltage DiIVId rS sasarapan an anaa aa ga EN EEN aseos ineo o A Kea pak Nan A aaRS aran 4 36 Voltage Dividers for Analog Input 00e00e0eneneennneneennnan nne 4 36 Voltage Dividers for Analog Output 0e00e0eenenenennennneneenen nane 4 37 Voltage Dividers for Digital Inputs eee eee eee eeeereeeeeeeees 4 37 Adding Power Filters jcc c cscsessiss cased aan ea na a agan Na Pa Kan a GAGANA aa gana aan ag aing 4 38 Appendix A Specifications Appendix B Technical Support and Professional Services Index SCB 68 User Manual for Advanced Functions vi ni com Introduction The SCB 68 is a shielded I O connector block with 68 screw terminals for easy signal connection to a National Instruments 68 pin or 100 pin DAQ device The SCB 68 features a general breadboard area for custom circuitry and sockets for intercha
21. CJC with the SCB 68 is accurate only if the temperature sensor reading is close to the actual temperature of the screw terminals Therefore when reading thermocouples keep the SCB 68 away from drafts or other temperature gradients such as those caused by heaters radiators fans and warm equipment Temperature Sensor Output and Accuracy The SCB 68 temperature sensor outputs 10 mV C and has an accuracy of 1 C You also can determine the temperature using the following formulas Tc 100 x V Tx Te 273 15 Tp ix Tc 32 where V is the temperature sensor output voltage and Tc Tx and T are the temperature readings in degrees Celsius Kelvin and Fahrenheit respectively SCB 68 User Manual for Advanced Functions 2 2 ni com Chapter 2 Temperature Sensor and Thermocouple Thermocouple Sources of Error When taking thermocouple measurements with the SCB 68 the possible sources of error are as follows National Instruments Corporation Compensation error Can arise from two sources inaccuracy of the temperature sensor and temperature differences between the temperature sensor and the screw terminals The temperature sensor on the SCB 68 is specified to be accurate to 1 C You can minimize temperature differences between the temperature sensor and the screw terminals by keeping the SCB 68 away from drafts heaters and warm equipment Linearization error A consequence of the polynomials being appro
22. DAQ SCB 68 User Manual for Advanced Functions 68 Pin Shielded Desktop Connector Block March 2009 7 NATIONAL 372551A 01 wW INSTRUMENTS Worldwide Technical Support and Product Information ni com National Instruments Corporate Headquarters 11500 North Mopac Expressway Austin Texas 78759 3504 USA Tel 512 683 0100 Worldwide Offices Australia 1800 300 800 Austria 43 662 457990 0 Belgium 32 0 2 757 0020 Brazil 55 11 3262 3599 Canada 800 433 3488 China 86 21 5050 9800 Czech Republic 420 224 235 774 Denmark 45 45 76 26 00 Finland 358 0 9 725 72511 France 01 57 66 24 24 Germany 49 89 7413130 India 91 80 41190000 Israel 972 3 6393737 Italy 39 02 41309277 Japan 0120 527196 Korea 82 02 3451 3400 Lebanon 961 0 1 33 28 28 Malaysia 1800 887710 Mexico 01 800 010 0793 Netherlands 31 0 348 433 466 New Zealand 0800 553 322 Norway 47 0 66 90 76 60 Poland 48 22 328 90 10 Portugal 351 210 311 210 Russia 7 495 783 6851 Singapore 1800 226 5886 Slovenia 386 3 425 42 00 South Africa 27 0 11 805 8197 Spain 34 91 640 0085 Sweden 46 0 8 587 895 00 Switzerland 41 56 2005151 Taiwan 886 02 2377 2222 Thailand 662 278 6777 Turkey 90 212 279 3031 United Kingdom 44 0 1635 523545 For further support information refer to the Technical Support and Professional Services appendix To comment on National Instruments documentation refer to the National Instruments Web site at ni com info and enter the info code feedback
23. ORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE TO AVOID DAMAGE INJURY OR DEATH THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES INCLUDING BUT NOT LIMITED TO BACK UP OR SHUT DOWN MECHANISMS BECAUSE EACH END USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION INCLUDING WITHOUT LIMITATION THE APPROPRIATE DESIGN PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION Conventions lt gt bold italic monospace Platform The following conventions are used in this manual Angle brackets that contain numbers separated by an ellipsis represent a range of values associated with a bit or signal name for example AO lt 3 0 gt The symbol leads you through nested menu items and dialog box options to a final action The sequence File Page Setup Options directs you to pull down the File menu select the Page Setup item and select Options from the last dialog box This icon denotes a note which alerts you to important information
24. a stairstep like signal is the input Volts V Time t National Instruments Corporation Figure 4 24 Lowpass Filtering of AO Signals Deglitching analog output signals Lowpass filters can be used to decrease glitches from an analog output signal When you use a DAC to generate a waveform you may observe glitches on the output signal These glitches are normal when a DAC switches from one voltage to another it produces glitches due to released charges The largest 4 21 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions glitches occur when the most significant bit of the DAC code changes You can build a lowpass deglitching filter to remove some of these glitches depending on the frequency and nature of the output signal To select a cutoff frequency for the deglitching filter refer to your DAQ device documentation for the maximum glitch duration PFI 0 Lowpass Filtering Applications Lowpass filters can function as debouncing filters to smooth noise on digital trigger input signals thus enabling the trigger detection circuitry of the DAQ device to understand the signal as a valid digital trigger A TTL Logic High Volts V TTL Logic Time t Figure 4 25 Digital Trigger Input Signal with a High Frequency Component Apply a lowpass filter to the signal to remove the high frequency component for a cleaner digital signal as Fi
25. areful to avoid damaging the component pads when desoldering Use only rosin core electronic grade solder because acid core solder damages the printed circuit device and components National Instruments Corporation 3 3 SCB 68 User Manual for Advanced Functions Adding Components for Special Functions This chapter describes how to condition signals by adding components to the open component locations of the SCB 68 This chapter describes the following signal conditioning applications e Installing Bias Resistors analog input e Filtering analog input analog output and digital input e Current Input Measurement analog input e Attenuating Voltage analog input analog output and digital input e Adding Power Filters UN Caution Add components at your own risk NI is not liable for any damage resulting from improperly added components In addition to the applications described in this chapter you can build many other types of signal conditioning using the component pads and the general purpose breadboard area of the SCB 68 Refer to Chapter 3 Soldering and Desoldering Components on the SCB 68 for more information about adding components and for soldering and desoldering instructions After building one of the applications described in this chapter or your custom circuitry refer to the Getting Started with the SCB 68 section of the SCB 68 User Guide for instructions about how to configure the SCB 68 in Measurement am
26. ation operation or maintenance instructions owner s modification of the product owner s abuse misuse or negligent acts and power failure or surges fire flood accident actions of third parties or other events outside reasonable control Copyright Under the copyright laws this publication may not be reproduced or transmitted in any form electronic or mechanical including photocopying recording storing in an information retrieval system or translating in whole or in part without the prior written consent of National Instruments Corporation National Instruments respects the intellectual property of others and we ask our users to do the same NI software is protected by copyright and other intellectual property laws Where NI software may be used to reproduce software or other materials belonging to others you may use NI software only to reproduce materials that you may reproduce in accordance with the terms of any applicable license or other legal restriction Trademarks National Instruments NI ni com and LabVIEW are trademarks of National Instruments Corporation Refer to the Terms of Use section on ni com legal for more information about National Instruments trademarks Members of the National Instruments Alliance Partner Program are business entities independent from National Instruments and have no agency partnership or joint venture relationship with National Instruments Patents For patents covering National Ins
27. ced Functions 4 28 ni com Chapter 4 Adding Components for Special Functions Selecting a Resistor for Current Input Measurement For best results when measuring current choose a resistor that has the following characteristics e Low wattage of approximately 0 125 W e Precision of at least 5 e Temperature stability e Tolerance of 5 e 232 Q suggested e AXL package suggested e Carbon or metal film suggested If you use the resistor described above you can convert a 20 mA current to 4 64 V by setting the device range to either 5 to 5 V or 0 to 5 V Adding Components for Current Input Measurement UN Caution Do not exceed 10 V at the analog inputs NI is not liable for any device damage or personal injury resulting from improper connections You can build a one resistor circuit for measuring current at the single ended or differential inputs of the SCB 68 e Differential analog inputs To build a one resistor circuit that measures current at the differential inputs of the SCB 68 add the resistor to position E for each differential channel pair that is used Leave the 0 Q resistors in place for positions F and G Refer to Table 4 1 for component positions for all analog input channels Calculate the current according to the following equation V m Rg J National Instruments Corporation 4 29 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Al lt
28. characteristics Figures 4 27 and 4 28 show the Bode Plots for the ideal filter and the real filter respectively and indicate the attenuation of each transfer function A Passband Gain Stopband fy Log Frequency Figure 4 27 Transfer Function Attenuation for an Ideal Filter National Instruments Corporation 4 23 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Gain Passband Stopband d Transition Region t Li fe Log Frequency Figure 4 28 Transfer Function Attenuation for a Real Filter Instead of having a gain of absolute zero for frequencies less than f the real filter has a transition region between the passband and the stopband a ripple in the passband and a stopband with a finite attenuation gain One Pole Highpass RC Filter Figure 4 29 shows the transfer function of a simple series circuit consisting of a resistor R and capacitor C when the voltage across R is assumed to be the output voltage V C e o Vin R 3 Vout e e Figure 4 29 Simple RC Highpass Circuit The transfer function is a mathematical representation of a one pole highpass filter with a time constant of 1 27RC SCB 68 User Manual for Advanced Functions 4 24 ni com Chapter 4 Adding Components for Special Functions Use Equation 4 4 to design a lowpass filter for a simple resistor and capacit
29. chnical or typographical errors exist National Instruments reserves the right to make changes to subsequent editions of this document without prior notice to holders of this edition The reader should consult National Instruments if errors are suspected In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it EXCEPT AS SPECIFIED HEREIN NATIONAL INSTRUMENTS MAKES NO WARRANTIES EXPRESS OR IMPLIED AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE CUSTOMER S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART OF NATIONAL INSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING FROM LOSS OF DATA PROFITS USE OF PRODUCTS OR INCIDENTAL OR CONSEQUENTIAL DAMAGES EVEN IF ADVISED OF THE POSSIBILITY THEREOF This limitation of the liability of National Instruments will apply regardless of the form of action whether in contract or tort including negligence Any action against National Instruments must be brought within one year after the cause of action accrues National Instruments shall not be liable for any delay in performance due to causes beyond its reasonable control The warranty provided herein does not cover damages defects malfunctions or service failures caused by owner s failure to follow the National Instruments install
30. circuitry by connecting a high value resistor between the positive input and 5 V A resistor of a few MQ or more is sufficient but a high value resistor allows you to detect an open or defective thermocouple NA Note Refer to Chapter 3 Soldering and Desoldering Components on the SCB 68 for more information about adding components and for soldering and desoldering instructions Differential analog input open thermocouple detection Use position A to connect a high value resistor between the positive input and 5 V Leave the 0 Q resistors at positions F and G in place for each channel used Refer to Table 4 1 Analog Input Channels Component Locations for component positions for all analog input channels Al lt i gt OD Al lt 8 gt Figure 2 1 Differential Analog Input Open Thermocouple Detection Single ended analog input open thermocouple detection Use position A for one channel and C for the next channel when you connect a high value resistor between the positive input and 5 V Leave the 0 Q resistors at positions F and G in place for each channel used Refer to Table 4 1 Analog Input Channels Component Locations for component positions for all analog input channels SCB 68 User Manual for Advanced Functions 2 4 ni com Chapter 2 Temperature Sensor and Thermocouple 5 V Al l
31. ctions 4 4 ni com Chapter 4 Adding Components for Special Functions Conditioning PFI 0 Figure 4 5 shows the shows the digital input channel configuration for PFI 0 on the SCB 68 PFI O Al START TRIG R1 PFI O AI START TRIG I O Pin 11 2 AWV 4 Screw Terminal RC1 DGND I O Pin 44 b D GND Screw Terminal Figure 4 5 Digital Trigger Circuitry Figure 4 6 illustrates the digital input channel configuration for PFI 0 PFI O R1 M AN SJ saa ES RC1 a aan D GND Figure 4 6 Digital Input Channel Pad Configuration Connecting Analog Input Signals Table 4 3 summarizes the recommended input configuration for both types of signal sources National Instruments Corporation 4 5 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Table 4 3 Analog Input Configuration Floating Signal Sources Not Connected to Ground Referenced Building Ground Signal Sources Examples Example e Ungrounded thermocouples e Plug in instruments with ge deh i non isolated outputs e Signal conditioning with P AI Ground Reference D a Setting e Battery devices Differential DIFF Signal Source DAQ Device Signal Source DAQ Device co Al gt Al R Al Al l S Al GND a a Non Referenced Signa
32. ded 2 4 thermocouple input filtering 2 5 voltage dividers 4 36 analog output attenuating voltage 4 35 channel pad configurations 4 4 circuit diagram figure 4 4 component locations table 4 4 lowpass filtering applications 4 21 smoothing filters 4 19 voltage dividers 4 37 applications highpass filtering 4 27 lowpass filtering 4 20 attenuating voltage 4 31 accuracy considerations 4 32 analog input 4 33 differential 4 33 single ended 4 34 analog output 4 35 components adding 4 33 selecting 4 32 digital inputs 4 35 PFI 0 4 35 voltage dividers 4 36 bias resistors 4 12 balanced 4 13 single 4 12 l 1 SCB 68 User Manual for Advanced Functions Index C channel pad configuration analog input 4 2 analog output 4 4 digital inputs 4 5 digital triggers 4 5 PFI 0 4 5 circuit diagrams 5 V power supply figure 4 38 analog input figure 4 2 analog output figure 4 4 cold junction compensation figure 4 2 digital inputs figure 4 5 digital trigger figure 4 5 PFI 0 figure 4 5 cold junction compensation CJC circuit diagram figure 4 2 components adding attenuating voltage 4 33 current input measurement 4 29 highpass filtering 4 25 lowpass filtering 4 17 locations analog input table 4 3 analog output table 4 4 selecting attenuating voltage 4 32 current input measurement 4 29 highpass filtering 4 25 lowpass filtering 4 17 connecting analog input signals 4 5
33. ding Components for Lowpass Filtering e Lowpass Filtering Applications Lowpass filters highly or completely attenuate signals with frequencies above the cut off frequency or high frequency stopband signals Lowpass filters do not attenuate signals with frequencies below the cut off frequency or low frequency passband signals Ideally lowpass filters have a phase shift that is linear with respect to frequency This linear phase shift delays signal components of all frequencies by a constant time independent of frequency thereby preserving the overall shape of the signal National Instruments Corporation 4 13 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions In practice lowpass filters subject input signals to a mathematical transfer function that approximates the characteristics of an ideal filter By analyzing the Bode Plot or the plot that represents the transfer function you can determine the filter characteristics Figures 4 13 and 4 14 show the Bode Plots for the ideal filter and the real filter respectively and indicate the attenuation of each transfer function A Passband Gain Stopband fg Log Frequency Figure 4 13 Transfer Function Attenuation for an Ideal Filter A Gain Passband Stopband Transition p gt Region _ b fo Log Frequency Figure 4 14 Transfer Function Attenuation for a Real F
34. ding Components for Special Functions A 2 S Le gt Time t Figure 4 17 Response of a Real Filter to a Square Wave Input Signal One Pole Lowpass RC Filter Figure 4 18 shows the transfer function of a simple series circuit consisting of a resistor R and capacitor C when the voltage across R is assumed to be the output voltage Vn R e VW e Vin c Vm e e Figure 4 18 Simple RC Lowpass Filter The transfer function is a mathematical representation of a one pole lowpass filter with a time constant of 1 27RC Use Equation 4 1 to design a lowpass filter for a simple resistor and capacitor circuit where the values of the resistor and capacitor alone determine f 7 G z Ms TT anROs WI where G is the DC gain and s represents the frequency domain SCB 68 User Manual for Advanced Functions 4 16 ni com Chapter 4 Adding Components for Special Functions Selecting Components for Lowpass Filtering To determine the value of the components in the circuit fix R 10 kQ is reasonable and isolate C from Equation 4 1 as follows 1 2nRf 4 2 The cut off frequency in Equation 4 2 is fo For best results choose a resistor that has the following characteristics e Low wattage of approximately 0 125 W e Precision of at least 5 e Temperature stability e Tolerance of 5 e AXL package suggested e Carbon or metal film suggested Choose a capacitor that has the
35. ers Refer to the SCB 68 User Guide for information about the 5 V power lines and SCB 68 fuse replacement A 470 Q series resistor R21 is part of the power filter for the 5 V power on the SCB 68 Due to the nature of the filter design as the filtered 5 V is loaded the voltage supplied to the SCB 68 circuitry and screw terminal 8 decreases Pad R20 shown in Figure 3 1 SCB 68 Printed Circuit Board Diagram is in parallel with R21 You can install a resistor if needed to decrease the overall resistance used in the filter and reduce the loading effect However completely shorting R20 bypasses the filter while capacitively coupling D GND to AI GND and AO GND and is not recommended Caution NIis not liable for any device damage resulting from improper use of the SCB 68 and the DAQ device Figure 4 44 shows the power supply circuitry on the SCB 68 XF1 Clip 5 V Screw Terminal gt a ie ACC Not Powered R20 5V 2 NC Optional I O Pin 8 ACC Powered R21 D GND p MAMA ep 15V D GND Screw Terminal ae C2 C6 C4 WO Pin 7 2 NO 10 uF 0 i uF G0 pF 0 1 pF gt WwW NY AI GND Non MIO AI GND Screw Terminal A NC WO Pin 56 MIO AI SCB 68 User Manual for Advanced Functions Figure 4 44 5 V Power Supply 4 38 ni com Specifications This appendix lists the SCB 68 specifications These specifications are typical at 25 C unless oth
36. erwise noted General Number of screw terminals 68 all I O signals are available at screw terminals Temperature sensor ACCULACY sisah eats alesse aa Baga NAN D 1 0 C over a 0 to 110 C range QUE PUL sisanne peranane aa naas na 10 mV C UN Caution Do not connect hazardous voltages 242 V 60 VDC to the SCB 68 Power Requirement Power consumption at 5 VDC 5 Typical punnan 1 mA with no signal conditioning installed Maxim sns 800 mA from host computer NA Note The power specifications pertain to the power supply of the host computer when using internal power or to the external supply connected at the 5 V screw terminal when using external power The maximum power consumption of the SCB 68 is a function of the signal conditioning components installed and any circuits constructed on the general purpose breadboard area If the SCB 68 is powered from the host computer the maximum 5 V current draw which is limited by the fuse is 800 mA National Instruments Corporation A 1 SCB 68 User Manual for Advanced Functions Appendix A Specifications Fuse Manfactutter iseiti Littelfuse part number 235 800 or equivalent Amp re rating sosisini 800 mA SIZE R TE 5 20 mm Voltage rating 0a0eoneneneneenenenen nane 250 V Nominal resistance n 0 195 Q Physical Dimensions including feet 18 1 x 15 2 x 4 5 cm 7 1 x 6 0 x 1 8 in W dhi orenera a inet cents
37. floating signal sources 4 7 conventions used in the manual iv current input measurement 4 28 adding components 4 29 analog input 4 29 differential 4 29 single ended 4 30 selecting a resistor 4 29 SCB 68 User Manual for Advanced Functions D desoldering 3 1 diagnostic tools NI resources B 1 differential connections bias resistors balanced 4 13 single 4 12 current input measurement 4 29 highpass filtering 4 25 lowpass filtering 4 17 open thermocouple detection 2 4 using with floating signal sources 4 9 when to use with floating signal sources 4 7 digital inputs attenuating voltage 4 35 channel pad configuration 4 5 circuit diagram figure 4 5 lowpass filtering 4 19 applications 4 22 voltage dividers 4 37 digital triggers circuit diagram figure 4 5 lowpass filtering 4 19 applications 4 22 documentation 1 2 conventions used in the manual iv NI resources B 1 drivers NI resources B 1 E examples NI resources B 1 F filtering highpass 4 23 lowpass 4 13 power 4 38 thermocouple input 2 5 ni com floating signal sources connecting 4 7 description 4 7 using in differential mode 4 9 when to use in differential mode 4 7 in NRSE mode 4 7 in RSE mode 4 8 H help technical support B 1 highpass filtering 4 23 analog input 4 25 applications 4 27 differential 4 25 single ended 4 26 components adding 4 25 selecting 4 25 one pole highpass RC filter 4 2
38. ground reference point or return signal e The signal leads travel through noisy environments e Two analog input channels AI and AI are available for the signal Differential signal connections reduce noise pickup and increase common mode noise rejection Differential signal connections also allow input signals to float within the common mode limits of the NI PGIA Refer to the Using Differential Connections for Floating Signal Sources section for more information about differential connections When to Use Non Referenced Single Ended NRSE Connections with Floating Signal Sources Only use NRSE input connections if the input signal meets the following conditions e The input signal is high level greater than 1 V e The leads connecting the signal to the device are less than 3 m 10 ft Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions National Instruments Corporation 4 7 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions In the single ended modes more electrostatic and magnetic noise couples into the signal connections than in differential configurations The coupling is the result of differences in the signal path Magnetic coupling is proportional to the area between the two signal conductors Electrical coupling is a function of how much the electric field differs between the two cond
39. gure 4 26 shows Volts V Time t Figure 4 26 Lowpass Filtering of Digital Trigger Input Signals SCB 68 User Manual for Advanced Functions 4 22 ni com Chapter 4 Adding Components for Special Functions B Note Due to the filter order the digital trigger input signal is delayed for a specific amount of time depending on the filter you use before the DAQ device senses the signal at the trigger input Highpass Filtering This section discusses the following topics regarding highpass filtering on the SCB 68 e One Pole Highpass RC Filter e Selecting Components for Highpass Filtering e Adding Components for Highpass Filtering e Highpass Filtering Applications Highpass filters highly or completely attenuate signals with frequencies below the cut off frequency or low frequency stopband signals Highpass filters do not attenuate signals with frequencies above the cut off frequency or high frequency passband signals The cut off frequency f is defined as the frequency below which the gain drops 3 dB Figure 4 27 shows how an ideal filter causes the gain to drop to zero for all frequencies less than f Thus f does not pass through the filter to its output In practice highpass filters subject input signals to a mathematical transfer function that approximates the characteristics of an ideal filter By analyzing the Bode Plot or the plot that represents the transfer function you can determine the filter
40. hermocouple Measurements You can measure thermocouples in differential or single ended configuration e Differential configuration has better noise immunity Use bias resistors when the DAQ device is in differential input mode as described in the Installing Bias Resistors section of Chapter 4 Adding Components for Special Functions National Instruments Corporation 2 1 SCB 68 User Manual for Advanced Functions Chapter 2 Temperature Sensor and Thermocouple e Single ended configuration has twice as many inputs For single ended configuration set your DAQ device for referenced single ended RSE input mode The maximum voltage level thermocouples generate is typically only a few millivolts You should use a DAQ device with high gain for best resolution For more information about thermocouple measurements refer to the NI Developer Zone tutorial Taking Thermocouple Temperature Measurements To access this document go to ni com info and enter the info code rdtttm The DAQ device must have a ground reference because thermocouples are floating signal sources For more information about floating signal sources refer to the Connecting Analog Input Signals section of Chapter 4 Adding Components for Special Functions For more information about field wiring refer to the NI Developer Zone document Field Wiring and Noise Considerations for Analog Signals To access this document go to ni com info and enter the info code rdfwn3
41. i gt e OZ en V AIGND Al lt 8 gt Figure 4 35 Measuring Current with Differential Analog Inputs Single ended analog inputs To build a one resistor circuit that measures current at the single ended analog inputs of the SCB 68 add the resistor to position B or D depending on the channel being used Leave the 0 Q resistors in place for channel positions F and G respectively Refer to Table 4 1 for component positions for all analog input channels Calculate the current according to the following equation Vm I RgoD where Rg o p is the resistance of the resistor in position B or D Al lt i gt LY O O F L NA o G o Al lt i 8 gt Figure 4 36 Measuring Current with Single Ended Analog Input Al lt gt SCB 68 User Manual for Advanced Functions 4 30 ni com Chapter 4 Adding Components for Special Functions Attenuating Voltage Transducers can generate more than 10 VDC per channel but DAQ devices cannot read more than 10 VDC per input channel Therefore you must attenuate output signals from the transducer to fit within the DAQ device specifications Figure 4 37 shows how to use a voltage divider to attenuate the output signal
42. ilter The cut off frequency f is defined as the frequency beyond which the gain drops 3 dB Figure 4 13 shows how an ideal filter causes the gain to drop to zero for all frequencies greater than f Thus f does not pass through the filter to its output Instead of having a gain of absolute zero for frequencies greater than f the real filter has a transition region between the passband and the stopband a ripple in the passband and a stopband with a finite attenuation gain SCB 68 User Manual for Advanced Functions 4 14 ni com Chapter 4 Adding Components for Special Functions Real filters have some nonlinearity in their phase response causing signals at higher frequencies to be delayed longer than signals at lower frequencies and resulting in an overall shape distortion of the signal For example when the square wave shown in Figure 4 15 enters a filter an ideal filter smooths the edges of the input whereas a real filter causes some ringing in the signal as the higher frequency components of the signal are delayed Volts V Time t Figure 4 15 Square Wave Input Signal Figures 4 16 and 4 17 show the difference in response to a square wave between an ideal and a real filter respectively Volts V Time t Figure 4 16 Response of an Ideal Filter to a Square Wave Input Signal National Instruments Corporation 4 15 SCB 68 User Manual for Advanced Functions Chapter 4 Ad
43. inted circuit board diagram figure 3 2 removing the board from the base 3 1 special functions 2 1 4 1 specifications A 1 signals connecting analog input 4 5 floating sources 4 7 single bias resistor 4 12 single ended connections attenuating voltage 4 34 current input measurement 4 30 highpass filtering 4 26 lowpass filtering 4 18 open thermocouple detection 2 4 when to use non referenced single ended connections with floating signal sources 4 7 when to use referenced single ended connections with floating signal sources 4 8 SCB 68 User Manual for Advanced Functions l 4 software NI resources B 1 soldering and desoldering 3 1 equipment 3 1 guidelines 3 2 sources of error open thermocouple detection 2 5 specifications A 1 support technical B 1 T technical support B 1 temperature sensor accuracy 2 2 output 2 2 thermocouples 2 1 4 13 input filtering 2 5 open thermocouple detection 2 4 differential analog input 2 4 single ended analog input 2 4 sources of error 2 5 temperature sensor output and accuracy 2 2 training and certification NI resources B 1 troubleshooting NI resources B 1 V voltage attenuation 4 31 voltage dividers 4 36 voltage dividers 4 36 analog input 4 36 analog output 4 37 digital inputs 4 37 W Web resources B 1 ni com
44. iver Soldering iron and solder Long nose pliers Components specific to your application Removing the SCB 68 Board from the Base Refer to Figure 1 1 SCB 68 Parts Locator Diagram while completing the following steps to remove the SCB 68 from the base 1 2 National Instruments Corporation Disconnect the 68 pin cable from the SCB 68 if connected Remove the shielding screws on either side of the top cover with a Phillips 1 screwdriver then open the box Loosen the strain relief screws with a Phillips 2 screwdriver Remove the signal wires from screw terminals with a flathead screwdriver Remove the device mount screws with a Phillips 1 screwdriver 3 1 SCB 68 User Manual for Advanced Functions Chapter 3 Soldering and Desoldering Components on the SCB 68 6 Remove the 68 pin connector screws with a flathead screwdriver 7 Tilt the SCB 68 up and pull it out To reinstall the SCB 68 reverse the order of the steps Soldering and Desoldering Guidelines As you solder and desolder components on the SCB 68 refer to Figure 3 1
45. l Source DAQ Device Signal Source DAQ Device Single Ended NRSE ALS Al Pan OE e AI GND 28 AI GND Referenced Single Ended RSE Signal Source DAQ Device Al NOT RECOMMENDED Signal Source DAQ Device E d Ground loop potential Va Vg are added to measured signal Refer to the documentation for your DAQ device for descriptions of the RSE NRSE and DIFF modes analog input signal sources and software considerations SCB 68 User Manual for Advanced Functions 4 6 ni com Chapter 4 Adding Components for Special Functions Connecting Floating Signal Sources What Are Floating Signal Sources A floating signal source is not connected to the building ground system but has an isolated ground reference point Some examples of floating signal sources are outputs of transformers thermocouples battery powered devices optical isolators and isolation amplifiers An instrument or device that has an isolated output is a floating signal source When to Use Differential Connections with Floating Signal Sources Use differential input connections for any channel that meets any of the following conditions e The input signal is low level less than 1 V e The leads connecting the signal to the device are greater than 3 m 10 ft e The input signal requires a separate
46. ll analog input channels 3 Note Filtering increases the settling time of the instrumentation amplifier to the time constant of the filter used Adding RC filters to scanning channels greatly reduces the practical scanning rate since the instrumentation amplifier settling time can be increased to 10T or longer where T R C NA Al lt b A WY o B FO p F D NAMA E al ai PE SIANA E EEEN E A Fe TS C 6 ot D V AGND Al lt i 8 gt Figure 4 20 SCB 68 Circuit Diagram for Single Ended Analog Input Lowpass Filter on Al lt i gt SCB 68 User Manual for Advanced Functions 4 18 ni com Chapter 4 Adding Components for Special Functions Lowpass Smoothing Filters on Analog Output Signals To build a lowpass filter for analog output put a resistor in position A and a capacitor in position B as shown in Figure 4 21 Refer to Table 4 2 for component positions for both analog output channels AO GND lt 4 AO Figure 4 21 SCB 68 Circuit Diagram for Analog Output Lowpass Filter Lowpass Digital Filters on Digital Trigger Input Signals For PFI 0 add the resistor to position R1 and the capacitor to position RC1 Refer to Figure 4 22 for the digital input channel pad configuration PFIO
47. nging electrical components These sockets or component pads allow filtering 4 to 20 mA current input measurement open thermocouple detection and voltage attenuation The open component pads allow you to easily add signal conditioning to the analog input AJ analog output AO and PFI 0 signals of a 68 pin or 100 pin DAQ device 1 Quick Reference Label 5 Shielding Screws 8 Strain Relief Screws 2 Top Cover 6 68 Pin I O Connector 9 Strain Relief Hardware 3 68 Pin Connector Screws 7 Base 10 SCB 68 Board Assembly 4 Lock Washers Figure 1 1 SCB 68 Parts Locator Diagram National Instruments Corporation 1 1 SCB 68 User Manual for Advanced Functions Chapter 1 Introduction This document contains information about advanced functions of the SCB 68 Refer to the following chapters for detailed information Chapter 2 Temperature Sensor and Thermocouple features information about using the temperature sensor taking thermocouple measurements open thermocouple detection and thermocouple input filtering Chapter 3 Soldering and Desoldering Components on the SCB 68 Chapter 4 Adding Components for Special Functions features information about installing bias resistors filtering current input measurement attenuating voltage and adding power filters Appendix A Specifications Related Documentation For more information about using the SCB 68 with your DAQ device refer to the following resources
48. nment Web page at ni com environment This page contains the environmental regulations and directives with which NI complies as well as other environmental information not included in this document Waste Electrical and Electronic Equipment WEEE EU Customers At the end of their life cycle all products must be sent to a WEEE recycling center For more information about WEEE recycling centers and National Instruments WEEE initiatives visit ni com environment weee htm Dt BEFAR mis ish BEDE CHE ROHS OOQ PARA National instruments 44t TH e TA AF Ah TBA EF EAT eR ROHS KF National Instruments MEH ROHS TAMARA A WK ni com environment rohs_china For information about China RoHS compliance go to ni com environment rohs_china SCB 68 User Manual for Advanced Functions A 4 ni com Technical Support and Professional Services Visit the following sections of the award winning National Instruments Web site at ni com for technical support and professional services National Instruments Corporation Support Technical support at ni com support includes the following resources Self Help Technical Resources For answers and solutions visit ni com support for software drivers and updates a searchable KnowledgeBase product manuals step by step troubleshooting wizards thousands of example programs tutorials application notes instrument drivers and so
49. of the transducer R4 e MA Vin R2 Vm e e Figure 4 37 Attenuating Voltage with a Voltage Divider The voltage divider splits the input voltage V between two resistors R and Ro causing the voltage on each resistor to be noticeably lower than Vin Use Equation 4 6 to determine the V that the DAQ device measures Vn Yak 4 6 m MAR R 4 6 Use Equation 4 7 to determine the overall gain of a voltage divider circuit Va R Vi R R 4 7 The accuracy of Equation 4 7 depends on the tolerances of the resistors that you use UN Caution The SCB 68 is not designed for any input voltages 242 V even if a user installed voltage divider reduces the voltage to within the input range of the DAQ device Input voltages 242 V can damage the SCB 68 any devices connected to it and the host computer Overvoltage can also cause an electric shock hazard for the operator National Instruments Corporation 4 31 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Selecting Components for Attenuating Voltage To set up the resistors complete the following steps 1 Select the value for R 10 KQ is recommended 2 Use Equation 4 6 to calculate the value for R Base the R calculation on the following values e Maximum Vi you expect from the transducer e Maximum voltage lt 10 VDC that you want to input to the DAQ device Accuracy Considerations for At
50. ons SCB 68 User Manual for Advanced Functions 4 8 ni com Chapter 4 Adding Components for Special Functions Using Differential Connections for Floating Signal Sources It is important to connect the negative lead of a floating source to AI GND either directly or through a bias resistor Otherwise the source can float out of the maximum working voltage range of the NI PGIA and the DAQ device returns erroneous data The easiest way to reference the source to AI GND is to connect the positive side of the signal to AI and connect the negative side of the signal to AI GND as well as to AI without using resistors This connection works well for DC coupled sources with low source impedance lt 100 Q DAQ Device Al Floating Signal Source o Al Inpedance lt 100Q o Al SENSE oj Al GND Figure 4 7 Differential Connections for Floating Signal Sources without Bias Resistors However for larger source impedances this connection leaves the differential signal path significantly off balance Noise that couples electrostatically onto the positive line does not couple onto the negative line because it is connected to ground This noise appears as a differential mode signal instead of a common mode signal and thus appears in your data In this case instead of directly connecting the negative line to AI GND connect the negative line to AI GND through a resistor that is about 100 times the
51. or circuit where the values of the resistor and capacitor alone determine f G TS ag 4 4 where G is the DC gain and s represents the frequency domain Selecting Components for Highpass Filtering To determine the value of the components in the circuit fix R 10 kQ is reasonable and isolate C from Equation 4 4 as follows ad 2TRf 4 5 The cutoff frequency in Equation 4 5 is f For best results choose a resistor that has the following characteristics e Low wattage of approximately 0 125 W e Precision of at least 5 e Temperature stability e Tolerance of 5 e AXL package suggested e Carbon or metal film suggested Choose a capacitor that has the following suggested characteristics e AXL or RDL package e Tolerance of 20 e Maximum voltage of at least 25 V Adding Components for Highpass Filtering Using the circuit shown in Figure 4 29 you can use a two component circuit to build a simple RC filter with an analog input e Differential analog input highpass filter To build a differential lowpass filter add the resistor to position E and the capacitor to position F as shown in Figure 4 30 Refer to Table 4 1 for component positions for all analog input channels National Instruments Corporation 4 25 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions 5 V Al lt i gt OD
52. owing equation to determine the gain of the circuit Rg Rg Rp Ro National Instruments Corporation 4 33 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Single ended analog input attenuators To build a two resistor circuit for attenuating voltages at the single ended analog inputs of the SCB 68 refer to Figure 4 39 Refer to Table 4 1 for component positions for all analog input channels AY Al lt b A Ta HA aa H O Leezak mE S O AE TEA o C oo D d V AIGND Al lt i 8 gt Figure 4 39 SCB 68 Circuit Diagram for Single Ended Analog Input Attenuation on Al lt i gt Install resistors in positions B and F or positions D and G depending on the channel you are using on the SCB 68 Use the following equation to calculate the gain of the circuit G Rpg orD Rg orD Rr org where Rg o p is the resistance of the resistor in position B or D and RF orc is the resistance of the resistor in position F or G SCB 68 User Manual for Advanced Functions 4 34 ni com Chapter 4 Adding Components for Special Functions Attenuating Voltage on Analog Output Signals To build a two resistor circuit for attenuating voltages at the AO 0 and AO 1 pins on the SCB 68 refer to the pad positions in Figure 4 40 Refer to Table 4 2 for component
53. p Automation Explorer MAX You can create virtual channels in MAX to create a custom scale or map your voltage ranges to the type of transducer that you use Channel Pad Configurations When you use the SCB 68 with a 68 pin or 100 pin MIO DAQ device you can use the component pads on the SCB 68 to condition 16 AI channels two AO channels and PFI 0 National Instruments Corporation 4 1 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Conditioning Analog Input Channels Figure 4 1 shows the analog input and CJC circuitry on the SCB 68 5 V AlO Screw Terminal I O Pin 56 CJC Not Used Al GND User Configurable 5 V ALO e S5 A I O Pin 68 gt CJC Used C3 Q1 0 1 uF R38 ot WY 1 WF T 5V ego A iY Ras WA Als f Screw RSE CJC 7 NA Terminal Al 8 or Non MIO ANAN I O Pin 56 I O Pin 34 S4 i WO Pin 34 DIFF CJC i H T Rci3 AIGND Al User Contfigurable Figure 4 1 Analog Input and Cold Junction Compensation Circuitry Figure 4 2 illustrates the AI channel configuration You can use AI lt i gt and AI lt i 8 gt as either a differential channel pair or as two single ended channels To use the SCB 68 with ground referenced single ended inputs do not use the open positions that connect the input to AI GND positions B and D for grounded sou
54. positions for both analog output channels AO GND lt 4 AO Figure 4 40 SCB 68 Circuit Diagram for Analog Output Attenuation Install resistors in positions A and B and determine the gain according to Equation 4 8 Rg ge 4 8 Rpt Ry ol Attenuating Voltage on Digital Inputs To build a two resistor circuit for attenuating voltages at the PFI 0 pin on the SCB 68 refer to the pad positions in Figure 4 41 PFI O 44 D GND Figure 4 41 SCB 68 Circuit Diagram for Digital Input Attenuation National Instruments Corporation 4 35 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Voltage Dividers Use positions R1 and RC1 for PFI 0 and determine the gain according to Equation 4 9 G _ RCI 4 9 RC1 R1 You can build voltage dividers for the analog inputs analog outputs and digital inputs of the SCB 68 Voltage Dividers for Analog Input When calculating the values for R and Ro consider the input impedance value from the point of view of V as shown in Figure 4 42 Input Impedance Figure 4 42 Input Impedance Electrical Circuit The following equation shows the relationship among all of the resistor values Z R R2x input Impedance p Rp OS UU in R Input Impedance Zin 1s the new inpu
55. rces as shown in Figure 4 2 Build any signal conditioning circuitry requiring a ground reference in the custom breadboard area using AI SENSE as the ground reference instead of building the circuitry in the open component positions SCB 68 User Manual for Advanced Functions 4 2 ni com Chapter 4 Adding Components for Special Functions B Note Some versions of the SCB 68 have 0 Q resistors hardwired in the factory default positions In such cases to move these resistors to and from the factory default positions you must solder and desolder on the SCB 68 circuit card assembly When soldering refer to Chapter 3 Soldering and Desoldering Components on the SCB 66 a Y Al lt i gt O A Cat ft OT H E l 222 717 9 G O 0 4 6 O D 6 V AGND Al lt i 8 gt Figure 4 2 Analog Input Channel Pad Configuration for Al lt i gt and Al lt 48 gt Table 4 1 correlates the component labels of the SCB 68 to component locations A through G for differential channels 0 through 7 Table 4 1 Analog Input Channels Component Locations Channel Single Ended Differential A B C D E F G AIO AI 8 AI0 R22 RC12 R23 RC13 RC4 R4 R5 AI 1 AI 9 All R24 RC14 R25 RCIS RCS R6 R7 AI 2 AI 10 AI2 R26 RC16 R27 RC
56. seesecesesseeseeeaeeneeeasenseeaes 2 3 Open Thermocouple Detection 5 5ases erang anaa KN EN EN SENENGE svesuenesa sete aee GEN 2 4 Thermocouple Input Filtering eee anaa Ngan aaa aana angen a 2 5 Chapter 3 Soldering and Desoldering Components on the SCB 68 Soldering Equap ment cess ies coe shes sheesh dee os NENGGE NANGEN NAN GR cules dense Pa AN ENY pan Sagara Dyah 3 1 Removing the SCB 68 Board from the Base eee eecesesseeeeeseeeseeseeeseeseeeseeneeeaes 3 1 Soldering and Desoldering Guidelines 00asen0enanenennenaennnannn naen anane n nane n ene 3 2 Chapter 4 Adding Components for Special Functions Channel Pad Configurations 20 00 00 ci eeeeceeseeseeseceseeseeseeeaeceseesecesecsesseeeaesaeeaeseeseaseeeeaes 4 1 Conditioning Analog Input Channels eee ee eeeeseeseeeeeseeeseeseeneeeseenaes 4 2 Conditioning Analog Output Channels 600esen0eoeneneoenneneenennen enem enen ene 4 4 Conditioning PREDO soren aie e pak Ta a a Na ANE EN Ga NG a RE 4 5 Connecting Analog Input Signals 000 eee eee eecceseceeeceeesecseesaeeseeesesseeeaecseeeaseneesaes 4 5 Connecting Floating Signal Sources 00 eee ee eee ese eeeeeseeeeeeseeeeeaeeeenseeaeenaes 4 7 What Are Floating Signal S0urces s0o0eseeoenenennnnene nn nnen e neneen 4 7 When to Use Differential Connections with Floating Signal Sources 3 eA Av hii BA ee a 4 7 When to Use Non Referenced Single Ended NRSE Connections with Floating Signal Sources 0
57. t i gt DO V AGND Al lt i 8 gt Figure 2 2 Single Ended Analog Input Open Thermocouple Detection on Al lt gt If the thermocouple opens the voltage measured across the input terminals rises to 5 V a value much larger than any legitimate thermocouple voltage You can create a bias current return path by using a 100 kQ resistor between the negative input and AI GND Thermocouple Input Filtering To reduce noise you can connect a simple one pole RC lowpass filter to the analog inputs of the SCB 68 Refer to the Lowpass Filtering section of Chapter 4 Adding Components for Special Functions for more information National Instruments Corporation 2 5 SCB 68 User Manual for Advanced Functions Soldering and Desoldering Components on the SCB 68 Some applications require you to make modifications to the SCB 68 usually in the form of adding components to the printed circuit device 3 Note Some versions of the SCB 68 have 0 Q resistors hardwired in the factory default positions In such cases to move these resistors to and from the factory default positions you must solder and desolder on the SCB 68 circuit card assembly Soldering Equipment To solder components on the SCB 68 you need the following DODD Q Phillips 1 and 2 screwdrivers 0 125 in flathead screwdr
58. t impedance Refer to the device specifications for the input impedance of your device SCB 68 User Manual for Advanced Functions 4 36 ni com Chapter 4 Adding Components for Special Functions Voltage Dividers for Analog Output When you use the circuit shown in Figure 4 37 for analog output the output impedance changes Thus you must choose the values for R and R3 so that the final output impedance value is as low as possible Refer to the device specifications for the output impedance for your device Figure 4 43 shows the electrical circuit you use to calculate the output impedance Ry Zout AS e lt Output 3 Ro Impedance Figure 4 43 Electrical Circuit for Determining Output Impedance The following equation shows the relationship between R Ro and Zout where Z iS the old output impedance and Z 2 is the new output impedance R XR Zout R R Z out2 Z out Voltage Dividers for Digital Inputs If you use the Vi voltage of Figure 4 37 to feed TTL signals you must calculate Vi so that the voltage drop on R does not exceed 5 V UN Caution A voltage drop exceeding 5 V on R can damage the internal circuitry of the DAQ device NI is not liable for any device damage resulting from improper use of the SCB 68 and the DAQ device National Instruments Corporation 4 37 SCB 68 User Manual for Advanced Functions Chapter 4 Adding Components for Special Functions Adding Power Filt
59. tenuating Voltage For best results when attenuating voltage choose a resistor that has the following characteristics e Low wattage of approximately 0 125 W e Precision of at least 5 e Temperature stable e Tolerance of 5 e AXL package suggested e Carbon or metal film suggested Verify that R and R drift together with respect to temperature otherwise the system may consistently read incorrect values SCB 68 User Manual for Advanced Functions 4 32 ni com Chapter 4 Adding Components for Special Functions Adding Components for Attenuating Voltage You can build a circuit for attenuating voltages at the analog inputs analog outputs and digital inputs of the SCB 68 Attenuating Voltage on Analog Input Signals You can build a two or three resistor circuit for attenuating voltages at the single ended analog inputs and differential analog inputs of the SCB 68 e Differential analog input attenuators To build a three resistor circuit for attenuating voltages at the differential analog inputs of the SCB 68 refer to Figure 4 38 Refer to Table 4 1 for component positions for all analog input channels 5 V Al lt i gt Y AGND Al lt i 8 gt Figure 4 38 SCB 68 Circuit Diagram for Differential Analog Input Attenuation Install resistors in positions E F and G of the chosen differential channel pair Use the foll
60. truments products technology refer to the appropriate location Help Patents in your software the patents txt file on your media or the National Instruments Patent Notice at ni com patents WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS 1 NATIONAL INSTRUMENTS PRODUCTS ARE NOT DESIGNED WITH COMPONENTS AND TESTING FOR A LEVEL OF RELIABILITY SUITABLE FOR USE IN OR IN CONNECTION WITH SURGICAL IMPLANTS OR AS CRITICAL COMPONENTS IN ANY LIFE SUPPORT SYSTEMS WHOSE FAILURE TO PERFORM CAN REASONABLY BE EXPECTED TO CAUSE SIGNIFICANT INJURY TO A HUMAN 2 IN ANY APPLICATION INCLUDING THE ABOVE RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY COMPUTER HARDWARE MALFUNCTIONS COMPUTER OPERATING SYSTEM SOFTWARE FITNESS FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION INSTALLATION ERRORS SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES TRANSIENT FAILURES OF ELECTRONIC SYSTEMS HARDWARE AND OR SOFTWARE UNANTICIPATED USES OR MISUSES OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED SYSTEM FAILURES ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS INCLUDING THE RISK OF BODILY INJURY AND DEATH SHOULD NOT BE RELIANT SOLELY UPON ONE F
61. tted line the signal appears to have a lower frequency Any signal with a frequency greater than one half of its sample rate is aliased and incorrectly analyzed as having a SCB 68 User Manual for Advanced Functions 4 20 ni com Chapter 4 Adding Components for Special Functions frequency below one half the sample rate This limiting frequency of one half the sample rate is called the Nyquist frequency To prevent aliasing remove all signal components with frequencies greater than the Nyquist frequency from input signals before those signals are sampled Once a data sample is aliased it is impossible to accurately reconstruct the original signal To design a lowpass filter that attenuates signal components with a frequency higher than half of the Nyquist frequency substitute the half Nyquist value for the f value in Equation 4 3 3 Note NI PCI PXI 6115 6120 6289 Devices Only NI PCI PXI 6115 6120 and NI PCI PXI 6289 devices provide filters and may not need antialiasing filters implemented at the SCB 68 terminal block Refer to your device documentation for more information Analog Output Lowpass Filtering Applications The following applications benefit from lowpass filtering Protection for external circuitry Lowpass filters can smooth stairstep like curves on AO signals If the curves are not smoothed the AO signals can be a hazard for some external circuitry connected to it Figure 4 24 shows the output of a lowpass filter when
62. uctors With this type of connection the NI PGIA rejects both the common mode noise in the signal and the ground potential difference between the signal source and the device ground Refer to the documentation for your DAQ device for more information about NRSE connections When to Use Referenced Single Ended RSE Connections with Floating Signal Sources Only use RSE input connections if the input signal meets the following conditions e The input signal can share a common reference point AI GND with other signals that use RSE e The input signal is high level greater than 1 V e The leads connecting the signal to the device are less than 3 m 10 ft Differential input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions In the single ended modes more electrostatic and magnetic noise couples into the signal connections than in differential configurations The coupling is the result of differences in the signal path Magnetic coupling is proportional to the area between the two signal conductors Electrical coupling is a function of how much the electric field differs between the two conductors With this type of connection the NI PGIA rejects both the common mode noise in the signal and the ground potential difference between the signal source and the device ground Refer to the documentation for your DAQ device for more information about RSE connecti
63. ximations of the true thermocouple output The linearization error depends upon the degree of polynomial used Measurement error The result of inaccuracies in the DAQ device These inaccuracies include gain offset and noise Accuracy can be calculated from the DAQ device specifications For best results you must use a well calibrated DAQ device NI recommends that you run self calibration on your DAQ device frequently to reduce error Thermocouple wire error The result of inconsistencies in the thermocouple manufacturing process These inconsistencies or nonhomogeneities are the result of defects or impurities in the thermocouple wire The errors vary depending on the thermocouple type and the gauge of wire used but an error of 2 C is typical For more information about thermocouple wire errors and more specific data consult the thermocouple manufacturer Noise error Error due to inherent system noise Use the average of a large number of samples to obtain the most accurate reading Noisy environments require averaging more samples for greater accuracy white noise 5 resulting noise number of samples For best results use the average of at least 100 readings to reduce the effects of noise typical absolute accuracies should then be about 2 C 2 3 SCB 68 User Manual for Advanced Functions Chapter 2 Temperature Sensor and Thermocouple Open Thermocouple Detection You can build open thermocouple detection

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