Floating Ground analog input (Differential input) Daq 6251

[LSUgrad85]

My problem is that I have 8 differential channels that all seem to be at different levels, far from 0v, even while nothing is on. They all float in the negative between -3 and -8, although they tend to float in the similar places with a little bit of give. My understanding is that the ground for each channel is the same - so with no inputs on the signal wires, why are they all over the place?

My project measures 8 accelerometers, which are amplified by an op amp circuit and fed into an audio amplifier. Some floating is tolerable, in the range of a few volts, though these channels are floating too close to the bounds of the +/- 10v limits for me to be able to see my data. Any advice?

I apologize that I'm asking what seems to be a fairly common problem, although I didn't find an answer for differential input.

[LSUgrad85]

An update I wrote was eaten, but the gist was:

After playing around with the audio amplifier, turning it off and on while unplugged to discharge it, my problem returned to a single channel's (ch06) ground being wayyy off all the others. I've checked the cables and can't find a good reason for this failure.

In differential input, is it necessary for the ground, say from my audio amplifer's output, to be tied to the DAQ's input?

[P.Sanders]

Your differential pair should be e.g ch0 and ch8...ch1 and ch9 and so on. If you have u are using differential signal inputs, your signals should not be floating all over the place. Try supplying a constant voltage to ch6 e.g from a battery source (which is a good floating signal input) and be sure to use the correct differential channel pair on the DAQ for the negative terminal of the battery. Check your input readings in measurement and automation explorer. If you obtain correct results, your signal wiring to the DAQ is off. See the following document that explains different signal sources and how to connect them to your DAQ.