National Instruments E Series , Other Considerations

Chapter 5 Calibration
 National Instruments Corporation 5-3 DAQCard E Series User Manual
perform an external calibration very often. You can externally calibrate
your DAQCard by calling the NI-DAQ calibration function.
To externally calibrate your DAQCard, use a very accurate external
reference. The reference should be several times more accurate than the
DAQCard itself. For example, to calibrate a 12-bit DAQCard, the
external reference should be at least ± 0.005% ( ± 50 ppm) accurate. To
calibrate a 16-bit DAQCard, the external reference should be at least
± 0.001% ( ± 10 ppm) accurate.
Other Considerations
The CalDACs adjust the gain error of each analog output channel by
adjusting the value of the reference voltage supplied to that channel.
This calibration mechanism is designed to work only with the internal
10 V reference. Thus, in general, it is not possible to calibrate the analog
output gain error when using an external reference. In this case, it is
advisable to account for the nominal gain error of the analog output
channel either in software or with external hardware. See Appendix A,
Specifications , for analog output gain error information.