Current measurements between 4071 and 2575 sometimes not correct

[cswieson]

I'm using PXI-4071 and a PXIe-2575 in handshaking mode with a scan list.

The DMM is measuring current and the MUX is in 1x196 mode.

When running the scan list, I rarely (randomly) get a very small measurement in the nA to uA (when it should be 4-20mA). I'll run the scan list again and that channel will be correct, so it doesn't seem to correspond to a channel.
I've tried increasing the settling time of both the DMM and the MUX, but neither have had any effect.

Any help would be great.

Thanks,

Caleb Swieson

[cswieson]

One more point. I am reading the measurements from the DMM while it is still running through the scan list (so I can process the data as it's being measured). At the moment this is where I feel like the issue might be, but have no idea if and why that would be.

[Frank-L]

Cswieson,

Is the relay board relatively old? If a board is close to end of life, there is a chance that the relay will sporatically not close and/or open. If the relay is stuck open, the DMM might measure very low current because no current is flowing (and only noise is coming through).

[cswieson]

Hi Frank,

No, the board is only a year old. I checked the relay counts and the most counts is still <10,000. Most are less than 1000.

Caleb

[cswieson · Accepted solution]

After further testing, we have determined it was actually the devices being measured that were outputting the very small currents. So it wasn't the hardwares' fault. Whoops

[WillPwr]

Thanks for following up, cswieson! I'm glad you were able to find the root of the issue.