I just purchased a USB-6215. The claim is 16 bits analog output resolution, but I see the Analog Output ground itself have noise of about 60-100 mV.
If I clip together BOTH the signal and ground of a scope probe, and then clip those to the Analog Output ground, I see 60 and 120 Hz noise (AC outlet noise, seen on the FFT). All three of signals (scope gnd, scope sig, NI AN OUT gnd) should be grounded together, so there shouldn't be any noise.
If I disconnect the scope from AN OUT ground but keep the scope sig/gnd clipped together as before, the noise drops to 15-20 mV, so it's not scope noise or noise induced through the scope leads.
This instrument is isolated, what mechanism causes this coupled noise, and how can it be eliminated?
