M series USB-6225 Common ground question

[ChrissyC]

Hi,

I'm trying to collect voltage data from 75 sources, which all share a common ground. I've looked at the pin-out diagram for the USB-6225 and can see that it contains 10 analog input grounds

Is there anyway that these grounds can measure voltage data and the whole set-up simply use one AI ground to serve as the common ground for all?

Cheers

[MarcoMauri]

Yes, there is.

You can read votages in NRSE configuration, increasing the number of available AI channels and reducing the need of common cabling.

In this case you have simply to connect the common ground to AISENSE pins.

You have to ground the COM pins ass well (if i'm not mistaken you don't need to ground all of them).

Give a look here.

Marco

[ChrissyC]

Thanks Marco,

I've read the article and that all makes sense, however the pin configuration of the USB-6225 is a little confusing.

I've attached a copy of the pin diagram, would i simply need to connect the common ground from my circuit to one of the AISENSE ports or to both?

There are also no COM ports on the 6225 (as there are with other DAQs) so would anything else need to be grounded?

Christian

[MarcoMauri · Accepted solution]

I wuold suggest to give a look to the manual

Figure 4-11 can help (and also chapter 4 in general)

To answer you questions, quoting the manual:

"On NI 6225 devices, the reference for each AI <16..63> signal is AI SENSE 2, and each AI <64..79> signal is AI SENSE
in NRSE mode."

"Analog Input Ground—These terminals are the reference
point for single-ended AI measurements in RSE mode and the
bias current return point for DIFF measurements. All three
ground references—AI GND, AO GND, and D GND—are
connected on the device."

Marco

[ChrissyC]

---

Got it! I was just being stupid and not counting the number of Analog inputs correctly!

Thanks for all your help Marco, much appreciated.

Christian

[ajcyr]

I understand how old this post is, but I'm hopping for some clarification nonetheless. I've inherited the 6225 and am playing catch up.

As Marco transcribed from the manual, AI SENSE is the common ground for AI64-79, while AI Sense 2 is that for AI16-63. This is quite clear from the manual (attached), but it still seems odd given the physical location of the AI SENSE1/2 terminals wrt their associated AI's (see attached pin diagram). Nonetheless - the text does not mention AI<0...15> for NRSE mode - are these terminals excluded from referencing AI SENSE terminals? Do they reference something else (AI GND perhaps?) in NRSE, or do they just simply not function in NRSE?

Marco's comment was very helpful. I see this text in the manual and have a better understanding about how to reference my signals. I have two different devices (a force plate, and EMG) to wire in. The system was in RSE mode, with the force plate connected to AI<1...5>, and grounded to AI GND. The EMG is sensitive to the cross-talk from the force plate, as I realized when I was in RSE and connected the EMG signals to other AI's. It would appear that NRSE mode using the AI SENSE terminals is the correct course, but some clarification on the AI<0...15> terminals in this mode is much appreciated. Per Marco, the manual specifically calls out the 6225 wrt how AI SENSE is used, but Table 3-1 in reference to the AI channels mentions that AI<16...64> is AI SENSE 2, while AI<0...15> is AI SENSE(1).