Digital Multimeters (DMMs) and Precision DC Sources

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PXIe-4138 Isolation and Grounding

I am using an SMU (PXIe-4138) to force a 1A current through a conductive path with a total resistance around 2 Ohms. Part of that conductive path contains a slip-ring. The slip-ring rotates and experiences momentary, somewhat small (20m-100m Ohms), resistance changes. For a "bad" slip-ring the resistance changes can be large (1 Ohm)

 

I am digitizing the voltage created across the path (by the 1A current and ~2 Ohms) using a PXIe6363 analog input in a differential fashion across the path. I get better noise immunity using differential measurement as the environment is inherently noisy.

 

It appears that most SMUs have isolated (non-ground referenced) outputs. So it seems to me that I should have bias resistors (10k-100k range) present between AI+ and AIGND, and between AI- and AIGND. This is described here:

 

http://www.ni.com/white-paper/3394/en/

 

Do I need external bias resistors?

 

I seem to get normal looking data without them present. With them present (100k) I seem to ensure that the voltage will always be in the AI input's acceptable common mode range.

 

Using a diff measurement approach was a backup for us. It is a little bit of a pain to add the bias resistors, so I don't want to do that unless it is truly necessary.

 

Thanks!

 

Matt

 

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I accidentally labelled the differential inputs as A0+ and A0-. They are indeed analog inputs though.

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The 4138 reads back voltage. Is there a reason you are trying to read the voltage across the wire on a separate card instead of using the 4138 to report the voltage across the wire?

Eric H.
Staff Field Applications Engineer
National Instruments
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Hi Erich,

 

Thank you for your reply. We originally had some other things in our measurement sequence and interconnect that led us down the DAQ road. However, your point is a good one. We were using the differential input to the DAQ for noise considerations and because our code was already set up. I haven't done it using just the SMU, so maybe it performs just as well from a noise perspective. In any event, if we use the DAQ analog input, I think we need the resistors in there. I'll record some data w the SMU and let you know what we see vs the DAQ. Thanks again.

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