How to correct Bode Analyzer behavior with RC circuits

[Cavarval]

The circuit on the picture is not grounded. Can you wire the AI1 - to AIGND (pin 4 to pin 18) and repeat the test?

I think I can get an ELVIS II board myself and run a quick test tomorrow to compare our results.

[drneil]

I can try that.

Should the AIGND be used as the ground for the FGEN signal?  Under what conditions?

Aren't the AI channelsconsidered to be differential, in which case the ground shouldn't matter, correct?

[Cavarval]

Should the AIGND be used as the ground for the FGEN signal?  Under what conditions?

     No, the FGEN is internally grounded. Once you wire your AI - to the AIGND, the ground reference for the FGEN and the AI - will be the same.

Aren't the AI channelsconsidered to be differential, in which case the ground shouldn't matter, correct?

     The channels are differential, but differential requires a ground. That's why pins 2 and 4 must be connected to the AIGND pin 18. If the ground is not wired you have an RSE configuration (Reference Single Ended). This can be used to measure floating sources like batteries.

You can find more detailed explanations in the following documents:

http://www.ni.com/white-paper/3344/en/

http://www.ni.com/white-paper/7113/en/