Four PXIe-6356 connected to SCB-68A, Interference

[eloiamb]

Hello. I am running a PXIe-1095 with four slots filled PXIe-6356. Connected to teach slot is a SCB-68A. The analog inputs are reading voltage from battery cells. With one SCB-68A connected, the voltages read fine. With more than one connected, some of the analog inputs begin to act funny. They float at incorrect numbers. It seems like there is some interference internally to the PXI? We have tried changing slots, moving the SCB-68A around, etc. but, once two or more are connected, we no longer read all voltages correctly.

Any ideas?

Thank you for your time.

[santo_13]

Are you ensuring the actual signals you measure are within the common-mode and differential limits of the card? can you share some example wiring and data you collected?

[eloiamb]

Thank you for your kind reply. I believe they are within the limits. There are four PXIe-6356 placed inside a PXIe-1095 chassis. We are utilizing 8 analog input channels on an SCB-68A. The signal on each channel is <4 V.

I don't have a wiring diagram or anything like that but, the wire leads to the voltage source are about 50 ft in length. I'm worried there might be an impedance effect going on. But, with one SCB-68A, all signals are as expected, regardless of slot choice. Once two are connected, we start losing channels.

[santo_13]

You mentioned there are battery cells, are each of the cell isolated or part of a battery pack?

Do note that all the cards in a chassis share a common earth ground, this means, though your battery cell voltage is <4V but if it is part of a battery pack, the cell at the top will have a very high common mode voltage compared to the chassis and is not supported.

Ensure that none of the signals are above +/-11V from the chassis ground.