I have an AD2 that I use a lot and today the Channel 1 started showing an input voltage of -21 with nothing connected. Channel 2 and all other functions work great. I restarted the computer, updated the software, and power cycled the AD2. No change. I calibrated the waveform generator and then the scope, but scope calibration failed saying that Channel 1 is showing too high on the input when grounded. I tried reverting to the factory calibration, but the symptoms remained the same.