National Instruments SCXI -1122 - Manual (Page 43)

© National Instruments Corporation 5-1 SCXI-1122 User Manual
Chapter 5
Calibration
This chapter discusses the calibration procedures for the SCXI-1122.
Overview
The onboard calibration hardware that calibrates the SCXI-1122, consists of an EEPROM to
store the calibration constants and an auto-zero channel on the amplifier input selector. When
the auto-zero channel is selected, the input relay multiplexer is disconnected from the amplifier
inputs 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 of
the amplifier. It is recommended to perform auto-zeroing at the start of an experiment for each
gain to be used to eliminate error due to drift in the amplifier internal circuitry and increase the
accuracy of the measurement. Notice that the auto-zero path is different from the analog input
path; therefore, even after auto-zeroing, a residual input offset still exists and has a value of less
than 6
µ
V. Refer to Appendix A, Specifications, for further details.
You can store this offset in the onboard EEPROM for future use and for automatic calibration
when you are using National Instruments software. The EEPROM also stores correction factors
for each gain of the SCXI-1122 as well as for the excitation channels. If you are not using
National Instruments software, refer to the SCXI-1122 Register-Level Programmer Manual if
you need a more a detailed description of the EEPROM.
When using National Instruments software such as NI-DAQ, LabVIEW, and LabWindows, and
you are using the factory-determined calibration constants, you do not need to read the following
section; continue reading the Excitation Calibration section. You need to read the following
section only if you are using National Instruments software and you intend to determine new
calibration constants.
Calibration Procedure
Calibration Equipment Requirements
According to standard practice, the equipment used to calibrate the SCXI-1122 should be 10
times as accurate as the SCXI-1122. Practically speaking, calibration equipment with four times
the accuracy of the item under calibration is generally considered acceptable. To calibrate the
SCXI-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