National Instruments VXI-MIO Series - Manual (Page 93)

Chapter 5 Calibration
VXI-MIO Series User Manual 5-2  National Instruments Corporation
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 module measurement and output voltage
errors can vary with time and temperature. It is better to self-calibrate
when the module is installed in the environment in which it will be used.
Self-Calibration
Your VXI-MIO Series module 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
for you. 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 ensure that you 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. External calibration addresses this error,
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 VXI-MIO Series module 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 module 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 module.
An external calibration refers to calibrating your module 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. Externally calibrate your