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Chapter 2

Analog Input and Temperature Sensor Measurements

Using the Temperature Sensor

To accommodate thermocouples with DAQ devices, the SCB-68A has a temperature sensor for
cold-junction compensation (CJC), shown in Figure 1-2,

SCB-68A Printed Circuit Board

. To power the temperature sensor, set the switches for single-ended or differential

mode as described in the

Using the SCB-68A with MIO DAQ Devices

section of Chapter 1,

Getting Started with the SCB-68A

. This configuration also powers the signal conditioning area

and circuitry. Refer to Figure 2-1 for a diagram of the CJC circuitry on the SCB-68A.

Taking Thermocouple Measurements

You can measure thermocouples in differential or single-ended configuration:
•

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.

•

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

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. 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,

and enter the Info Code

rdfwn3

.

CJC with the SCB-68A 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-68A
away from drafts or other temperature gradients, such as those caused by heaters, radiators, fans,
and warm equipment.

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Содержание NI SCB-68A

Страница 1: ...DAQ NI SCB 68A User Manual 68 Pin Shielded Connector Block NI SCB 68A User Manual ni com manuals Deutsch Fran ais August 2012 375865A 01...

Страница 2: ...ses and current events National Instruments Corporate Headquarters 11500 North Mopac Expressway Austin Texas 78759 3504 USA Tel 512 683 0100 For further support information refer to the Technical Supp...

Страница 3: ...failure to follow the National Instruments installation operation or maintenance instructions owner s modification of the product owner s abuse misuse or negligent acts and power failure or surges fir...

Страница 4: ...HARM TO PROPERTY OR PERSONS INCLUDING THE RISK OF BODILY INJURY AND DEATH SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE TO AVOID DAMAGE INJURY OR D...

Страница 5: ...a loss or a system crash When this symbol is marked on a product refer to the Read Me First Safety and Electromagnetic Compatibility document for information about precautions to take bold Bold text d...

Страница 6: ...ential Connections with Floating Signal Sources 2 7 When to Use Non Referenced Single Ended NRSE Connections with Floating Signal Sources 2 7 When to Use Referenced Single Ended RSE Connections with F...

Страница 7: ...nput Measurement 2 32 Selecting a Resistor for Current Input Measurement 2 33 Adding Components for Current Input Measurement on Analog Input Signals 2 33 Attenuating Voltage 2 35 Selecting Components...

Страница 8: ...Digital Trigger Input Signals 4 6 PFI 0 Lowpass Filtering Applications 4 7 Attenuating Voltage 4 8 Selecting Components for Attenuating Voltage 4 9 Accuracy Considerations for Attenuating Voltage 4 9...

Страница 9: ...ents 68 pin or 100 pin DAQ device Figure 1 1 SCB 68A Parts Locator Diagram The SCB 68A features a general breadboard area for custom circuitry and through hole pads for interchanging electrical compon...

Страница 10: ...ough Mode section for more information What You Need to Get Started To set up and use your SCB 68A you need the following SCB 68A 68 pin shielded connector block kit s 1 containing the SCB 68A and SCB...

Страница 11: ...Install cover prior to use To avoid electrical shock do not remove SCB 68A covers unless you are qualified to do so Before removing the cover disconnect any live circuit from the connector block Repl...

Страница 12: ...and R21 8 Switches S2 1 S2 2 and S2 3 9 Screw Terminals 10 Printed Circuit Board Mount Screw 11 PFI 0 Pads 12 Analog Output Pads 13 Printed Circuit Board Mount Screw and Chassis Ground Lug CAUTION SEE...

Страница 13: ...0 25 in of insulation inserting the wires into the screw terminals and securely tightening the screws with the flathead screwdriver to a torque of 0 5 0 6 N m 4 5 in lb Caution To ensure the specified...

Страница 14: ...ature reference sensor and click OK d Click OK For more information about configuring the SCB 68A for a DAQ device refer to the Measurement Automation Explorer Help for NI DAQmx 13 Test specific devic...

Страница 15: ...ensor for cold junction compensation CJC to accommodate thermocouples switches S1 1 and S1 2 configure the temperature sensor for different analog input settings Switches S2 1 S2 2 and S2 3 provide po...

Страница 16: ...E mode AI 8 is available on a screw terminal 5 V power provided to signal conditioning area of the accessory Refer to Figure 1 4 for a detailed diagram MIO with differential temperature sensor mode Mo...

Страница 17: ...and print the panel mounting template PDF attached in the KnowledgeBase document SCB 68A Panel Mounting Template Go to ni com info and enter the Info Code scb68amounting to locate the KnowledgeBase 2...

Страница 18: ...rewdriver as shown in Figure 1 5 Note The threaded holes on the SCB 68A for DIN rail mounting should not be used more than five times Unscrewing and reinstalling the DIN rail clip will produce a compr...

Страница 19: ...0 in lb Soldering and Desoldering Components on the SCB 68A Some applications require you to make modifications to the SCB 68A usually in the form of adding components to the printed circuit device So...

Страница 20: ...68A ships with surface mount 0 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 68A To desol...

Страница 21: ...ion For more information about using the SCB 68A with your DAQ device refer to the following resources Documentation for your DAQ device at ni com manuals Measurement Automation Explorer Help NI DAQmx...

Страница 22: ...ts 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 68A Refer...

Страница 23: ...CB 68A to condition 16 AI channels Figure 2 1 shows the analog input and CJC circuitry on the SCB 68A Figure 2 1 Analog Input and Cold Junction Compensation Circuitry AI 0 I O Pin 68 S1 1 CJC Not Used...

Страница 24: ...2 illustrates the basic AI channel configuration You can use AI i and AI i 8 as either a differential channel pair or as two single ended channels Figure 2 2 Analog Input Channel Circuitry for AI i a...

Страница 25: ...sitions B and D for grounded sources as shown in Figure 2 3 Build any signal conditioning circuitry requiring a ground reference in the custom breadboard area using AI SENSE as the ground reference in...

Страница 26: ...ations Channel Positions A B C D E F and G Single Ended Differential AI 0 AI 8 AI 0 SC0 AI 1 AI 9 AI 1 SC1 AI 2 AI 10 AI 2 SC2 AI 3 AI 11 AI 3 SC3 AI 4 AI 12 AI 4 SC4 AI 5 AI 13 AI 5 SC5 AI 6 AI 14 AI...

Страница 27: ...lated outputs Differential DIFF Non Referenced Single Ended NRSE Referenced Single Ended RSE Refer to the documentation for your DAQ device for descriptions of the RSE NRSE and DIFF modes analog input...

Страница 28: ...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 Referen...

Страница 29: ...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 differenc...

Страница 30: ...se 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 F...

Страница 31: ...source but small enough not to produce significant input offset voltage as a result of input bias current typically 100 k to 1 M In this case connect the negative input directly to AI GND If the sour...

Страница 32: ...2 8 NRSE Connections for Floating Signal Sources All of the bias resistor configurations discussed in the Using Differential Connections for Floating Signal Sources section apply to the NRSE bias resi...

Страница 33: ...d to the same building power system is typically between 1 and 100 mV but the difference can be much higher if power distribution circuits are improperly connected If a grounded signal source is incor...

Страница 34: ...In the single ended modes more electrostatic and magnetic noise couples into the signal connections than in DIFF configurations The coupling is the result of differences in the signal path Magnetic co...

Страница 35: ...Referenced Signal Sources 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...

Страница 36: ...ve input of the NI PGIA Therefore the ground point of the signal connects to the negative input of the NI PGIA Any potential difference between the device ground and the signal ground appears as a com...

Страница 37: ...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 d...

Страница 38: ...consequence of the polynomials being approximations of the true thermocouple output The linearization error depends upon the degree of polynomial used Measurement error The result of inaccuracies in...

Страница 39: ...pen or defective thermocouple Note Refer to the Soldering and Desoldering Components on the SCB 68A section of Chapter 1 Getting Started with the SCB 68A for more information about adding components a...

Страница 40: ...e 2 13 Single Ended Analog Input Open Thermocouple Detection on AI i If the thermocouple opens the voltage measured across the input terminals rises to 5 V a value much larger than any legitimate ther...

Страница 41: ...B 68A as shown in Figure 2 14 Leave the 0 resistors at positions F and G in place for each channel used Figure 2 14 AI Differential Configuration with Single Bias Resistor To install balanced bias res...

Страница 42: ...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 frequ...

Страница 43: ...its output Instead of having a gain of absolute zero for frequencies greater than fc the real filter has a transition region between the passband and the stopband a ripple in the passband and a stopb...

Страница 44: ...2 19 and 2 20 show the difference in response to a square wave between an ideal and a real filter respectively Figure 2 19 Response of an Ideal Filter to a Square Wave Input Signal Figure 2 20 Respon...

Страница 45: ...r for a simple resistor and capacitor circuit where the values of the resistor and capacitor alone determine fc 2 1 where G is the DC gain and s represents the frequency domain Selecting Components fo...

Страница 46: ...on E Refer to Table 2 1 for component positions for all analog input channels The SCB 68A ships with a surface mount 0 resistor in the F position You must remove the resistor to use the position Figur...

Страница 47: ...i 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 measu...

Страница 48: ...d 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...

Страница 49: ...the Bode Plots for the ideal filter and the real filter respectively and indicate the attenuation of each transfer function Figure 2 25 Transfer Function Attenuation for an Ideal Filter Figure 2 26 Tr...

Страница 50: ...imple resistor and capacitor circuit where the values of the resistor and capacitor alone determine fc 2 4 where G is the DC gain and s represents the frequency domain Selecting Components for Highpas...

Страница 51: ...2 1 for component positions for all analog input channels The SCB 68A ships with a surface mount 0 resistor in the F position You must remove the resistor to use the position Figure 2 28 SCB 68A Circ...

Страница 52: ...r 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 sig...

Страница 53: ...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 base...

Страница 54: ...nvert 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 on Analog Input Signals Caution Do not exceed 10 V at the ana...

Страница 55: ...nputs of the SCB 68A add the resistor to position B or D depending on the channel being used Leave the 0 resistors in place for channel positions F and G respectively Refer to Table 2 1 for component...

Страница 56: ...ifications Figure 2 35 shows how to use a voltage divider to attenuate the output signal of the transducer Figure 2 35 Attenuating Voltage with a Voltage Divider The voltage divider splits the input v...

Страница 57: ...uating Voltage To set up the resistors complete the following steps 1 Select the value for R2 10 k is recommended 2 Use Equation 2 6 to calculate the value for R1 Base the R1 calculation on the follow...

Страница 58: ...esistor in the F and G positions You must remove the resistors to use these positions Figure 2 36 SCB 68A Circuit Diagram for Differential Analog Input Attenuation Install resistors in positions E F a...

Страница 59: ...ere RB or D is the resistance of the resistor in position B or D and RF or G is the resistance of the resistor in position F or G Analog Input Voltage Dividers When calculating the values for R1 and R...

Страница 60: ...National Instruments 2 39 NI SCB 68A User Manual Zin is the new input impedance Refer to the device specifications for the input impedance of your device Zin R1 R2 Input Impedance R2 Input Impedance...

Страница 61: ...ation 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 Setting up the SCB 6...

Страница 62: ...h frequency stopband signals Lowpass filters do not attenuate signals with frequencies below the cut off frequency or low frequency passband signals Ideally lowpass Table 3 1 Analog Output Channels Co...

Страница 63: ...tion Figure 3 3 Transfer Function Attenuation for an Ideal Filter Figure 3 4 Transfer Function Attenuation for a Real Filter The cut off frequency fc is defined as the frequency beyond which the gain...

Страница 64: ...ut whereas a real filter causes some ringing in the signal as the higher frequency components of the signal are delayed Figure 3 5 Square Wave Input Signal Figures 3 6 and 3 7 show the difference in r...

Страница 65: ...tage across R is assumed to be the output voltage Vm Figure 3 8 Simple RC Lowpass Filter The transfer function is a mathematical representation of a one pole lowpass filter with a time constant of Use...

Страница 66: ...ilm suggested Choose a capacitor that has the following suggested characteristics AXL or RDL package Tolerance of 20 Maximum voltage of at least 25 V Adding Components for Lowpass Smoothing Filters on...

Страница 67: ...s 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 glit...

Страница 68: ...R2 causing the voltage on each resistor to be noticeably lower than Vin Use Equation 3 3 to determine the Vm that the DAQ device measures 3 3 Use Equation 3 4 to determine the overall gain of a voltag...

Страница 69: ...of approximately 0 125 W Precision of at least 5 Temperature stable Tolerance of 5 AXL package suggested Carbon or metal film suggested Verify that R1 and R2 drift together with respect to temperatur...

Страница 70: ...d R2 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 3 13 shows the electrical circuit you use to calc...

Страница 71: ...section of Chapter 1 Getting Started with the SCB 68A for more information about adding components and for soldering and desoldering instructions After building one of the applications described in th...

Страница 72: ...and 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 respe...

Страница 73: ...he 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 Real filters have some nonlinearity in their phase r...

Страница 74: ...gures 4 6 and 4 7 show the difference in response to a square wave between an ideal and a real filter respectively Figure 4 6 Response of an Ideal Filter to a Square Wave Input Signal Figure 4 7 Respo...

Страница 75: ...le resistor and capacitor circuit where the values of the resistor and capacitor alone determine fc 4 1 where G is the DC gain and s represents the frequency domain Selecting Components for Lowpass Fi...

Страница 76: ...e circuit shown in Figure 4 8 you can use a two component circuit to build a simple RC filter with digital input For PFI 0 add the resistor to position A and the capacitor to position B Refer to Figur...

Страница 77: ...4 10 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 Figure 4 11 shows Figure 4...

Страница 78: ...R1 and R2 causing the voltage on each resistor to be noticeably lower than Vin Use Equation 4 3 to determine the Vm that the DAQ device measures 4 3 Use Equation 4 4 to determine the overall gain of a...

Страница 79: ...en attenuating voltage choose a resistor that has the following characteristics Low wattage of approximately 0 125 W Precision of at least 5 Temperature stable Tolerance of 5 AXL package suggested Car...

Страница 80: ...5 Digital Input Voltage Dividers If you use the Vin voltage of Figure 4 12 to feed TTL signals you must calculate Vin so that the voltage drop on R2 does not exceed 5 V Caution A voltage drop exceedin...

Страница 81: ...protected by a 1 A self resetting fuse shown in Figure 1 2 SCB 68A Printed Circuit Board Diagram Shorting pin 8 to ground trips the fuse on the SCB 68A Pin 14 is not fuse protected on the SCB 68A Sho...

Страница 82: ...8 decreases Pad R20 shown in Figure 1 2 SCB 68A 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 re...

Страница 83: ...al supply connected at the 5 V screw terminal when using external power The maximum power consumption of the SCB 68A is a function of the signal conditioning components installed and any circuits cons...

Страница 84: ...tromagnetic Compatibility This product meets the requirements of the following EMC standards for electrical equipment for measurement control and laboratory use EN 61326 1 IEC 61326 1 Class A emission...

Страница 85: ...oducts in an environmentally responsible manner NI recognizes that eliminating certain hazardous substances from our products is beneficial to the environment and to NI customers For additional enviro...

Страница 86: ...ervice Program SSP with the purchase of most software products and bundles including NI Developer Suite NI also offers flexible extended contract options that guarantee your SSP benefits are available...

Страница 87: ...rt and Professional Services 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 number...

Страница 88: ...8 single ended 2 19 thermocouple input filtering 2 19 voltage dividers 2 38 analog output attenuating voltage 3 9 channel pad configurations 2 5 circuit diagram figure 3 1 component locations table 3...

Страница 89: ...ls NI resources B 1 differential connections bias resistors balanced 2 20 single 2 20 current input measurement 2 33 highpass filtering 2 30 lowpass filtering 2 25 open thermocouple detection 2 18 usi...

Страница 90: ...s analog input 2 26 analog output 3 7 digital inputs 4 7 digital triggers 4 7 PFI 0 4 7 components adding for analog input 2 25 adding for analog output 3 6 adding for digital filtering 4 6 selecting...

Страница 91: ...when to use non referenced single ended connections with floating signal sources 2 7 when to use referenced single ended connections with floating signal sources 2 8 software NI resources B 1 solderi...

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