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National Instruments SCXI-1121 Calibration Procedure

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CALIBRATION PROCEDURE
™
SCXI -1121
Introduction
What Is Calibration?
Why Should You Calibrate?
How Often Should You Calibrate?
™
™
National Instruments
, NI
, ni.com
mentioned herein are trademarks or trade names of their respective companies.
370258B-01
This document contains information and step-by-step instructions for
calibrating the National Instruments (NI) SCXI-1121 module.
Calibration consists of verifying the measurement accuracy of a module
and adjusting for any measurement error. Verification is measuring the
performance of the module and comparing these measurements to the
factory specifications. During calibration, you supply and read voltage
levels using external standards, then you adjust the module calibration
constants. This circuitry compensates for any inaccuracies in the module,
and returns the accuracy of the module to the factory specifications.
The accuracy of electronic components drifts with time and temperature,
which can affect measurement accuracy as a module ages. Calibration
restores these components to their specified accuracy and ensures that the
module still meets NI standards.
The measurement requirements of your application determine how often
the SCXI-1121 module needs to be calibrated to maintain accuracy.
NI recommends that you perform a complete calibration at least once every
year. You can shorten this interval to 90 days or six months based on the
demands of your application.
™
™
™
, NI-DAQ
, and SCXI
are trademarks of National Instruments Corporation. Product and company names
© 2000, 2001 National Instruments Corp. All rights reserved.
September 2001

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Summary of Contents for National Instruments SCXI-1121

  • Page 1 How Often Should You Calibrate? The measurement requirements of your application determine how often the SCXI-1121 module needs to be calibrated to maintain accuracy. NI recommends that you perform a complete calibration at least once every year. You can shorten this interval to 90 days or six months based on the demands of your application.

  • Page 2: Equipment And Other Test Requirements

    You do not need any special software or documentation to calibrate the SCXI-1121 module. This calibration document contains all the information you need to complete the verification and adjustment procedures. If you would like more information on the module, refer to the SCXI-1121 User Manual. SCXI-1121 Calibration Procedure...

  • Page 3: Test Conditions

    Maintain the temperature between 18–28 °C. • Keep relative humidity below 80%. • Allow a warm-up time of at least 15 minutes for the SCXI-1121 module to ensure the measurement circuitry is at a stable operating temperature. Calibration The calibration procedure for the SCXI-1121 module consists of the following steps: Setting up the module for testing.

  • Page 4: Setting Up The Module

    Setting Up the Module Complete the following steps to set up the SCXI-1121 module for verification: Remove the grounding screw from the module. Remove the cover on the module to access the potentiometers. Refer to Figure 1 for an illustration of steps 1 and 2.
  • Page 5 3 Side Plate Figure 2. Side Plate Removal and Module Installation The SCXI-1121 module does not need to be connected to a data acquisition (DAQ) device. Leave the configuration of the digital jumpers W41–W45 and W46 unchanged because they do not affect this procedure.
  • Page 6 15 Half-Bridge Completion Jumpers 7 Rear Signal Connector 16 Input Null Adjust Potentiometers 8 Product Name, Assembly Number, Revision Letter 17 Excitation Mode Jumpers 9 SCXIbus Connector 18 Grounding Screw Figure 3. SCXI-1121 Parts Locator Diagram SCXI-1121 Calibration Procedure ni.com...
  • Page 7 Table 2 for the jumper reference designator of the channel gain stage you want to change, and Figure 3 for the location of this jumper. Move the jumper to the position indicated in Table 3. © National Instruments Corporation SCXI-1121 Calibration Procedure...
  • Page 8 Configuring the Filter Jumpers Each input channel also has two user-configurable filter stages. The SCXI-1121 module ships in the 4 Hz position. Refer to Table 4 to find the correct jumper setting for the desired cutoff frequency. Figure 3 shows the locations of the jumper blocks on the SCXI-1121 module.
  • Page 9 Set both jumpers in the same mode for correct operation of the excitation channel. Refer to Table 5 to determine how to set up the module in the mode you want. The SCXI-1121 module ships to you in the voltage mode.
  • Page 10 Configuring the Excitation Level Each excitation channel of the SCXI-1121 module has two different current or voltage levels. You can set a given channel to one of the following level modes: • In the current mode—0.150 or 0.450 mA •...
  • Page 11 Verifying the Operation of the Module The verification procedure determines how well the SCXI-1121 module is meeting its specifications. You can use this information to select the appropriate calibration interval for your application. Refer to the Setting Up the Module section for information on how to configure the channel gain, channel filter, and excitation level.
  • Page 12 Table 7. SCXI-1121 Front Connector Pin Assignments (Continued) Pin Number Column A Column B Column C EGND1 CH2+ CH2– EX2+ EX2– EGND2 CH3+ CH3– EX3+ EX3– EGND3 RSVD SCAL RSVD MTEMP CGND DTEMP NC—No connect SCXI-1121 Calibration Procedure ni.com...
  • Page 13 45 46 47 48 49 50 Figure 4. SCXI-1121 Rear Connector Pin Assignments Set the calibrator voltage to the value specified by the Test Point entry listed in Table 12. Read the resulting output voltage on the DMM. If the output voltage result falls between the Upper Limit and the Lower Limit values, the module passed the test.
  • Page 14 Compare the DMM reading to the limits for excitation shown in Table 8. If the reading falls between the Upper Limit and the Lower Limit values, the module passed the test. Table 8. SCXI-1121 Voltage Excitation Limits Test Point (V) Upper Limit (V) Lower Limit (V) 3.333...
  • Page 15 Compare the DMM reading to the limits for excitation shown in Table 9. If the reading falls between the Upper Limit and Lower Limit values, the module passed the test. Table 9. SCXI-1121 Current Excitation Limits Test Point (mA) Upper Limit (mA) Lower Limit (mA) 0.150...
  • Page 16 Connect the DMM leads to the excitation outputs as closely as possible to the resistor body. Adjust the excitation voltage potentiometer until the voltage reading falls between 3.337 and 3.329 V. Refer to Figure 3 for the potentiometer location and Table 11 for the potentiometer reference designator. SCXI-1121 Calibration Procedure ni.com...
  • Page 17 0.450900–0.449100 mA. Refer to Figure 3 for the potentiometer location and Table 11 for the potentiometer reference designator. Repeat steps 1 through 5 for all remaining channels. You have completed adjusting the current excitation channels. © National Instruments Corporation SCXI-1121 Calibration Procedure...
  • Page 18 Verifying the Operation of the Module section. Verifying the adjusted values ensures that the SCXI-1121 module is operating within its specifications after adjustments. If the SCXI-1121 module fails after calibration, return it to NI for repair or Note replacement. SCXI-1121 Calibration Procedure...

  • Page 19: Specifications

    Specifications Table 12 contains test specifications for the SCXI-1121 module. If the device was calibrated within the last year, the output from the module should fall between the Upper Limit and Lower Limit values. Table 12. SCXI-1121 Specifications 4Hz filter setting...
  • Page 20 Table 12. SCXI-1121 Specifications (Continued) 4Hz filter setting 10kHz filter setting Upper Lower Upper Lower Gain Test Point (V) Limit (V) Limit (V) Limit (V) Limit (V) 0.0000 0.005141 –0.005141 0.006542 –0.006542 –2.2500 –4.465708 –4.534292 –4.464307 –4.535693 0.9000 4.534293 4.465707 4.535706...
  • Page 21 Table 12. SCXI-1121 Specifications (Continued) 4Hz filter setting 10kHz filter setting Upper Lower Upper Lower Gain Test Point (V) Limit (V) Limit (V) Limit (V) Limit (V) 1000 0.0045 4.542321 4.457679 4.546501 4.453499 1000 0.0000 0.013170 –0.013170 0.017350 –0.017350 1000 –0.0045...