National Instruments PCI-6023E - Manual (Page 81)

Chapter 5 Calibration
PCI-6023E/6024E/6025E User Manual 5-2 © National Instruments Corporation
vary with time and temperature. It is better to self-calibrate when the board
is installed in the environment in which it will be used.
Self-Calibration
Your board can measure and correct for almost all of its calibration-related
errors without any external signal connections. Your National Instruments
software provides a self-calibration method. This self-calibration process,
which generally takes less than a minute, is the preferred method of
assuring accuracy in your application. Initiate self-calibration to minimize
the effects of any offset, gain, and linearity drifts, particularly those due to
warmup.
Immediately after self-calibration, the only significant residual calibration
error could be gain error due to time or temperature drift of the onboard
voltage reference. This error is addressed by external calibration, which is
discussed in the following section. If you are interested primarily in relative
measurements, you can ignore a small amount of gain error, and
self-calibration should be sufficient.
External Calibration
Your board has an onboard calibration reference to ensure the accuracy of
self-calibration. Its specifications are listed in Appendix A,
Specifications .
The reference voltage is measured at the factory and stored in the EEPROM
for subsequent self-calibrations. This voltage is stable enough for most
applications, but if you are using your board at an extreme temperature or
if the onboard reference has not been measured for a year or more, you may
wish to externally calibrate your board.
An external calibration refers to calibrating your board with a known
external reference rather than relying on the onboard reference.
Redetermining the value of the onboard reference is part of this process and
the results can be saved in the EEPROM, so you should not have to perform
an external calibration very often. You can externally calibrate your board
by calling the NI-DAQ calibration function.
To externally calibrate your board, be sure to use a very accurate external
reference. The reference should be several times more accurate than the
board itself.
PCI.book Page 2 Wednesday, September 16, 1998 9:09 AM