I recently ordered in a set of four PCI-6251 cards for Analog data aquisition. Three of them tested consistently to each other, but one appeared to have issues right out of the box. I attempted to sample a 10Hz sine wave, and noticed some unusual glitching in the sampled waveform; in several spots, the sine wave appeared to be sampled correctly, but had a DC offset for a number of samples before returning to the expected value. The part of the sine wave on which the glitching occured, and the direction of the DC offset appeared fairly consistent (always occured in about the same spot with an offset in the same direction), though the duration of the glitch seemed random (the width of the glitch flickered around on the scope).
After placing an oscilloscope in parallel with the signal to verify the sine wave was being transmitted correctly, I noticed the glitching became worse, and remained worse until the oscilloscope was removed, at which point it went back to its initial behavior. My guess is the additional impedance in parallel with the channel somehow amplifies the problem. Additionally, changing the scan interval didn't seem to help the glitching one way or the other.
I'm assuming there is some kind of defect since none of the other cards exhibited this behavior... has anyone experienced similar behavior with these cards? If so, is there any workaround for them, or is the only option to send the card in for repairs?
Thanks!
