NI 9205 ground loop issues & differential mode

[awc66]

I'm having some trouble getting two diferent measurement signals to behave on the same NI 9502 module. The 9502 is runing on a cDAQ, using LabView 8.6 and NI-MAX 9.4 on a Windows XP system. 

There are two sources of measurement signals into the module:

1) An output monitor for a large power supply. This generates 0-10 VDC, and the ground comes from the power supply which in turn gets it's power from a dedicated 3-phase 220VAC line.

2) A group of pressure transducers. These generate 0-5 VDC, and their ground comes from a 24VDC source in our instrumentation box, which itself is converted from a standard 110VAC line.

If I plug only #1 above into the NI 9502 with nothing in the COM terminal, I can read the correct ouput voltages in NI-MAX using differential mode. If I plug in only the signals from #2 above, I can also read those just fine in NI-MAX, again in diferential mode (though they work better if the ground from their 24VDC source is connected to COM). But if they're both connected to the NI 9502 simultaneously, they influence each other and cause faulty/near-zero readings. I used a simple multimeter to compare the two signal sources, and their individual grounds are roughly 38V apart, which appears to be the source of the issue. This NI resource suggests that none of the input signals can deviate more than 10V from common ground 9and therefore each other?).

Is this resolvable with clever use of bias resistors as the white paper I linked suggests, or is the diference in floating ground between the two signal sources too much to overcome on a single module? We're a small outfit and I'm a mech E not an EE so this is not my area of expertise. I am nervious to simply connect both grounds to the COM port as the large diference may cause unexpected damage to one of the systems.

[Daniel.Parrott]

Bias resistors wouldn't help in this situation. The 9205 won't be able to handle the big difference in the individual grounds as it does not have channel to channel isolation which means they all need to share a somewhat similar ground signal. One of our isolated channel modules that can withstand this voltage difference would be your best bet (something along the lines of the NI 9229). 

The other option is to somehow get the high ground source down before entering the module, but for legal and practical reasons I can't give you any good suggestions on how to do that. 

[Daniel.Parrott]

Oh and definitely don't try connecting both of those grounds into COM on the 9205, as it will most likely damage something. 

[awc66]

Thanks for the response, that is roughly what I suspected but it's good to have confirmation. Would it work if the two signal sources are on seperate modules plugged into the same cDAQ, or could that still cause problems for the cDAQ? We do have a second 9205 unit we could use in that case. (Doing this would require some software re-working, which is why I originally hoped to get them all on the same module).

[Daniel.Parrott]

We did a bit more research to make sure the different module's COMs were isolated from each other, and we found that they are. 

So yes, measuring the two signals separately, 1 on each 9205 module, will be just fine. Make sure the voltages don't exceed the safety values though. http://www.ni.com/pdf/manuals/374188d.pdf#page=27