We have two 8-channel waveform generators that were shipped 4 months ago, and came back because the customer ordered too many or something. We routinely test anything that comes back, before returning them or returning to stock.
What's interesting is that all 16 channels have a negative DC offset. Each channel is a diff-current-output cmos DAC, an opamp diffamp, a passive LC filter, and an output amp; the opamps are fast bipolars. We apply a software cal factor to the DAC data (saved in a cal table) to get the offsets way below 1 mV when we ship. After 4 months, we're seeing offsets from -5 to -10 mV. These are not actual failures, but I don't like or understand the trend.
We'll be doing some tests to try to isolate the drift to dac, diffamp, or output amp. I figure we could measure things on one board, bake to accelerate aging, and re-measure.
My general question, to people who understand semi physics: what are the physical mechanisms that could make the DAC, or the opamps, have this ensemble negative drift vs time?
Parts are DAC2904, LMH6642, and THS3062.
THS3062 is known to be buggy, latching up if slewed hard at high frequency, but this board doesn't stress them up there.