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National Instruments Corporation 5-1 PCI-6023E/6024E/6025E User Manual
5
Calibration
This chapter discusses the calibration procedures for your board. If you are
using the NI-DAQ device driver, that software includes calibration
functions for performing all of the steps in the calibration process.
Calibration refers to the process of minimizing measurement and output
voltage errors by making small circuit adjustments. For these boards, these
adjustments take the form of writing values to onboard calibration DACs
(CalDACs).
Some form of board calibration is required for all but the most forgiving
applications. If you do not calibrate your board, your signals and
measurements could have very large offset, gain, and linearity errors.
Three levels of calibration are available to you and described in this chapter.
The first level is the fastest, easiest, and least accurate, whereas the last
level is the slowest, most difficult, and most accurate.
Loading Calibration Constants
Your board is factory calibrated before shipment at approximately 25° C to
the levels indicated in Appendix A, Specifications. The associated
calibration constants—the values that were written to the CalDACs to
achieve calibration in the factory—are stored in the onboard nonvolatile
memory (EEPROM). Because the CalDACs have no memory capability,
they do not retain calibration information when the board is unpowered.
Loading calibration constants refers to the process of loading the CalDACs
with the values stored in the EEPROM. NI-DAQ software determines
when this is necessary and does it automatically. If you are not using
NI-DAQ, you must load these values yourself.
In the EEPROM there is a user-modifiable calibration area in addition to
the permanent factory calibration area. This means that you can load the
CalDACs with values either from the original factory calibration or from a
calibration that you subsequently performed.
This method of calibration is not very accurate because it does not take into
account the fact that the board measurement and output voltage errors can
PCI.book Page 1 Wednesday, September 16, 1998 9:09 AM