Analog Output Interactions PXI-6723

[asaunder]

Hello,

I have a test setup using the PXI-6723 which utilizes a few of its single-ended outputs to send analog voltages to a separate device which performs differential analog input measurements. Here are the basic symptoms of the problem I'm trying to troubleshoot.

• When the 3 outputs are at the same voltage (constant or programmed varying signal), the readings are perfect on all 3 of my input channels.
• When the 3 outputs are at different voltages (example AO_8 = 7V, AO_9 = 0V, AO_10 = -7V), one reading will generally be accurate (7V) and the others will be shifted slightly (0V reads >0.5V, -7V reads nearly -6V).

This leads me to infer that the signals are interfering with one another. I do not think it is an issue on the input side (tried lowering sample rate already with no change), but am not ruling out anything for certain yet. I am using a cable to an SCB-68 terminal block, which then connects to the device with the analog inputs. My cabling is not shielded, which I didn't originally think was necessary for this setup. Perhaps that was an incorrect assumption. Unfortunately due to the nature of my project, I cannot share any LabVIEW code or diagrams.

Here were a couple of my thoughts as potential upcoming troubleshooting steps.

• Modify the connections so the outputs are differential, essentially using two analog outputs for every one analog input.
• Buy shielded cabling and re-wire.

If anyone has encountered issues like this previously, I'd love to hear your thoughts. Thanks!

[asaunder]

Following up after further testing. The voltages were monitored at the SCB-68 terminal block and at the end of the cable which mates to my PC104 analog input module. No anomalies with voltage levels or noise were detected. Additionally, when the input measurement program was initiated, it appeared to add noise to the signals. Changing the signal outputs to a differential configuration did not solve the issue.

Bottom line, the signs are pointing to the issue being on the input side rather than with the PXI-6723 or the wiring. Still troubleshooting to resolve the problem. 

[asaunder · Accepted solution]

Follow-up #2: The issue has shown itself to be ghosting on the multiplexed input side. No issues with the PXI-6723. The easiest solution has been to do something like what is described in this linked article on ghosting, where you take a "dummy" measurement of each signal prior to taking the actual channel measurement. 

How Do I Eliminate Ghosting from My Measurements? - NI