PXIe-4080 Noise and Offset when measuring thermocouple

[RyanMi]

I am trying to use a PXIe-4080 to measure thermocouple level signals to validate a sensing circuit (sensor). I have a thermocouple simulator to provide the rough voltage levels, and then I want to compare the values measured by the PXIe-4080 and my sensor. When I have my sensor hooked up to the simulator it reads the correct value (within a few uV) and has low noise. When I connect the negative probe of the PXIe-4080 to the negative terminal of the sensor the reading shifts by ~10uV and a lot of noise shows up (~+/- 20uV). This does not go away when I connect the positive probe. This does not seem normal, since the PXIe-4080 probes should be isolated and measuring differentially.

I don't believe this has anything to do with my specific sensor because if I use an Agilent 34401A DMM I do not see this effect.

I am using the Soft Front Panel, I have tried 10MOhm and 10GOhm input impedance to no avail.

Any ideas of what to check or what could be wrong?

[A-Guz-Man]

Hi RyanMi,

Here are some ideas:

You could check Table 1 of the following document: http://www.ni.com/white-paper/3344/en/

And check this other two:

http://zone.ni.com/reference/en-XX/help/370384T-01/dmm/noise/

http://zone.ni.com/reference/en-XX/help/370384T-01/dmm/dc_noise_rejection/

Hope this helps!

[RyanMi · Accepted solution]

Hi A-Guz-Man,

Thanks for the suggestions. Unfortunately the noise/offset is well suppressed in the PXIe-4080 output but is induced into our sensor output when we attach the cables from the PXIe-4080 (but not the Agilent 34410a)

What we eventually ended up trying is putting a capacitor across the lines and that seemed to clear up the issue.