National Instruments E Series - Manual (Page 82)

Chapter 5 Calibration
DAQCard E 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 DAQCard measurement and output voltage
errors can vary with time and temperature. It is better to self-calibrate
when the DAQCard is installed in the environment in which it will be
used.
Self-Calibration
Your DAQCard 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 you
can use. 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 DAQCard 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 DAQCard
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
DAQCard.
An external calibration refers to calibrating your DAQCard 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