I’m seeing the same thing. It’s only visible at the smallest timebases, so my guess would be it’s the ramp in the ADC. Maybe that waveform is somehow coupled to the input. I didn’t check, does it happen on both channels?
I could be dead wrong. I just can’t imagine what else would cause that.
Another thread sees similar results viewtopic.php?f=22&t=1956 and it appears to be inconsistent ground reference and amplifier gain issues when both ADCs are being used for one channel. With the grounded channel active by itself, you are most likely seeing the ADC level switches that are appearing while in single channel mode and a grounded input.
This is a guess on my part but it seems to link these two threads with common symptoms. BenF had found a way to auto calibrate the ADC ground level on the DSO Nano. I guess this needs to be incorporated for the dual ADCs to work properly in the DSO Quad, along with an auto gain compensation between the two ADCs for each range setting. Without this auto gain compensation, then manual compensation would be required in the calibration procedure.
Just maybe a proper manual calibration procedure would help to solve this problem, although this would be lengthy because all V/Div scales would have to be calibrated on both analog channels. Another problem with manual shared ADC compensation would be the inherent thermal drift of all electronic DC circuits.
It would be interesting to see what happens with both channels in AC input mode.