AI / DI with isolated grounds connected to DAQ

[-Raul-]

Hello,

In a new application we are planning to use a PXI system connected to a test fixture, where the test points from 2 PCBs will be routed to the PXI-6229 DAQ, as follows:

The boards are powered up using the isolated +/-20V output from the PXI-4130 PSU. The grounds from each board under test are independent from each other; therefore if I am right we shouldn’t use the same AI GND input from the DAQ as main reference to measure the signals from both boards. My plan is to use a Single-Ended Non-referenced (NRSE) measurements for Board 1 and Single-Ended Ground Referenced (RSE) for Board 2. Note that additional measurements will be taken using Differential input configuration, but those signals are not included in the above drawing.

My doubts are:

A)    Is the attached configuration valid for the purpose of our application?

B)    Since the Digital Inputs from Board 1 and Board 2 are referenced to different grounds (GND / GNDC), is there any conflict connecting those grounds to the D GND inputs of the DAQ?

Thank you!

[taper]

I personally don't like using RSE for anything.  Differential is best is you can afford the pins, NRSE is good enough for almost everything.  Your card does have 2 AISENSE lines, you don't have to use the AIGND to isolate the 2 modules.  I believe all DGNDs are tied together.  Optoisolators are pretty cheap and easy to add in if needed.  The M-series user manual explains this pretty well and shows various connection options.

[-Raul-]

Thank you for your reply. Differential is not possible; I am running out of AI for my design already! 🙂 I didn't realize about that additional AISENSE input, thanks. I will consider using it for NRSE measurements in board 2. 

Regarding the DI, are the D GND tied together to the AI GND and/or AI SENSE in the DAQ? If they are not, I would try to connect the DI from both boards to the same D GND reference, since a bit of noise in the digital signals would be acceptable.

Thanks,

[taper]

The DGND, AIGND, and AISENSE are all separate.  You'll want to look into biasing resistors on the AI to keep the measurements in the common mode range.  Grounding on floating/isolated measurements is always a little tricky.  The field wiring guide is a good read, lots of other info on this site if you search.

http://zone.ni.com/devzone/cda/tut/p/id/3344