NI9215: Measuring common mode voltage - Oscillating voltage issue

[johndoe1920]

Hello NI members,

I am measuring the common mode voltage right before the shunt resistor to the system ground as shown in the attached pictures. I see that the amplitude of the voltage is oscillating. I checked with an oscilloscope and a multimeter. In both the cases the voltage is held  constant at ~1.24V. I am not doing anything fancy. Just using the DAQ assistant and trying to see how the voltage looks like. I am attaching the video file as well. Could you please tell me what the issue in here is? and why my voltage is oscillating with NI 9215 but not with an oscilloscope and a multimeter.

Thank you for your time.

[OA1]

Hi Johndoe,

Have you tried connecting AI- to COM?

[johndoe1920]

Thank you OA1 for pointing me to that direction. I did not connect AI0- to COM nor tested that. How should I connect them together? Directly or with some large resistance? 

Does the datasheet provide any guidance on this? I could not find more details in there. 

[johndoe1920]

The getting started guide does provide more details. Thank you so much. Will test it and get back.