noisy DAQ output

[NIquist]

Hmmm, I have several of those 2090s as well and it's my favorite BNC interface because of the differential inputs.  They have always been very quiet.  Can you post a screenshot from your o-scope so we can see the noise level?  I agree that a ground problem is a likely cause but that often manifests as a 60Hz sin wave or spikes at some multiple of the line frequency.  Use a power strip to plug everything into the same building ground and see if that helps.  Also use another channel of your scope to check the grounds and make sure they're clean of noise.

Another slight possibility is that you have a bad PCI to 2090 cable.  Are you using the one supplied by NI?

[Soran]

Thanks for suggestions. I will work on them.

By the way, the screenshot of the scopes have been uploaded in my first post for this question. The file DAQ.JPG is the output of the DAQ card. As you see the Mean signal for both (Function generator and the DAQ) is ~50mV but the Pk-Pk value for DAQ is ~15mV while this value for function generator is only ~3mV.

[NIquist]

I assume you are setting your generator and DAQ wafeform to produce that 50mV DC signal right?  What happens if you set the outputs to zero?  To get a good baseline it's best to set your DAQ output to 0V.  Further,  I would shut down LabVIEW, open the DAQ test panel in NI-MAX and work from there to ensure there's nothing funky in your code or the waveform you're using.  

The spikes you see in your DAQ output could indicate a problem with the card (unlikely) or that there's still a grounding issue.  The next step would be to collect a big sampling of the noisy signal and analyze the resulting waveform with a FFT to determine if there is a distict frequency causing the noise.  There may be a problem in your PC that is radiating into the 6351.  This is rare but faulty power supplies, fans and PC wiring issues can affect your PCIe cards.