Analog Input Read Drifting in Test Panel After Given Period

Updated Nov 9, 2018

Reported In

Hardware

  • NI-9205

Issue Details

I'm reading a constant analog input to my NI module in a test panel in NI-MAX, and after a given time period of good readings, the input drifts and then becomes constant at a different, incorrect value. My inputs use differential grounding. Why is this happening?

Solution

The problem is likely the result of a violation of the common-mode voltage of your NI module. This specification can be found in the datasheet of your device (example pulled from NI-9205 datasheet is seen below).
 

Even if the differential input is lower than the common mode voltage - in this case +/- 10.4 V, each channel making up that differential must also be within +/- 10.4 V of COM. If either of your inputs are close to this limit, a little bit of noise can cause the signal to rail in one direction. 

Additional Information

You can read more about common-mode voltage in our tutorial on How to Measure Voltage and in our whitepaper on Grounding Considerations.

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