How do I measure a signal source that is floating?

[cj 25]

I've been trying to measure a source signal that is floating (a voltage around 1V). I'm using the Sc 2345 and I've got the signal coming through a low pass filter module (scc-LP02). I've set up the DAQ board to read in NRSE. When my data aquisition board and laptop are plugged into a battery power supply I have no problem reading the signal correctly. The problem arises when I connect my laptop OR data aquisition board to the AC outlet. When I do this I get a reading of -5V. I believe this is because I'm actually grounding everything to building ground when I use the AC outlet. I've tried plugging my DAQ board into the AC outlet without grounding it, and I get a signal close to what I'm looking for but with a lot
of noise. The SCC-LP02 manual says that the internal circuitry does have bias resistors that are required for floating points, so I'm not sure if I'm still forgetting about something. I've read other liturature suggesting that I need to connect the ground reference of the floating point to the DAQ device analog input ground, but I'm not sure how to physically do this (is there a screw terminal on the board for AIGND?). I have limited electrical wiring knowledge, but I would think that if I referenced the floating ground to the board, it would essentially ground everything to building ground.
Hopefully I've overlooked something relatively simple. Thanks,

Chris

[Jeremy_C]

First of all, what type of power supply are you using for the SC-2345 (e.g. PW01)? When you say "data acquisition board" what exactly are you referring to? The SC-2345? Or do you have an external data acquisition board (e.g. DAQPad 60xx)? It sounds like if nothing is plugged into an AC outlet, then you are getting correct readings. However if one or both of your components are plugged in, then you get bad readings. You mentioned plugging your DAQ board into an AC outlet without grounding it. What exactly are you doing here? How are you accomplishing this? Below is a extensive tutorial on wiring up different types of signals in the field. Take a look at that and see if that helps explain what may be going on here. Another suggestion would be to tie the -
ch of you signal to the chassis ground of your SC-2345 carrier. This is simply a screw mounted to the board in the corner. Refer to the SC-2345 manual for more information. I hope this helps.

Field Wiring