Other Considerations - National Instruments AT-MIO/AI E Series User Manual

Chapter 5 Calibration
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. For example, to calibrate a 12-bit board,
the external reference should be at least 0.005% ( 50 ppm) accurate.
To calibrate a 16-bit board, 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.
National Instruments Corporation 5-3 AT-MIO/AI E Series User Manual