National Instruments SCXI -1122 User Manual

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National Instruments SCXI -1122 User Manual Manual pdf 43 page image

© National Instruments Corporation 5-1 SCXI-1122 User ManualChapter 5CalibrationThis chapter discusses the calibration procedures for the SCXI-1122.OverviewThe onboard calibration hardware that calibrates the SCXI-1122, consists of an EEPROM tostore the calibration constants and an auto-zero channel on the amplifier input selector. Whenthe auto-zero channel is selected, the input relay multiplexer is disconnected from the amplifierinputs and the isolation amplifier inputs are connected to its ground reference.Auto-zeroing is a method for nulling error sources that compromise the quality of measurements.Auto-zeroing determines the amount of offset at the output of the SCXI-1122 at a given gain ofthe amplifier. It is recommended to perform auto-zeroing at the start of an experiment for eachgain to be used to eliminate error due to drift in the amplifier internal circuitry and increase theaccuracy of the measurement. Notice that the auto-zero path is different from the analog inputpath; therefore, even after auto-zeroing, a residual input offset still exists and has a value of lessthan 6 μV. Refer to Appendix A, Specifications, for further details.You can store this offset in the onboard EEPROM for future use and for automatic calibrationwhen you are using National Instruments software. The EEPROM also stores correction factorsfor each gain of the SCXI-1122 as well as for the excitation channels. If you are not usingNational Instruments software, refer to the SCXI-1122 Register-Level Programmer Manual ifyou need a more a detailed description of the EEPROM.When using National Instruments software such as NI-DAQ, LabVIEW, and LabWindows, andyou are using the factory-determined calibration constants, you do not need to read the followingsection; continue reading the Excitation Calibration section. You need to read the followingsection only if you are using National Instruments software and you intend to determine newcalibration constants.Calibration ProcedureCalibration Equipment RequirementsAccording to standard practice, the equipment used to calibrate the SCXI-1122 should be 10times as accurate as the SCXI-1122. Practically speaking, calibration equipment with four timesthe accuracy of the item under calibration is generally considered acceptable. To calibrate theSCXI-1122, you need the following equipment.• For the amplifier gains, you need a voltmeter with the following specifications:- Accuracy ±0.002% standard±0.08% sufficient- Range -10 to +10 V