weird sawtooth wave even when grounded probe

I just upgraded to

HW V 2.6
APP V2.32 and APP V2.33
SYS V1.34

Err, what’s the deal with this sawtooth like wave I’m seeing?

It happens on 2 us per div and shorter, the probe is grounded. I don’t understand why it would do this, especially at every voltage/div

If I measure the output of a 200KHz square wave using 2 us per div, I see the sawtooth pattern mixed in with the square wave

Also I can’t save screenshots, it sort of “works” but it turned out like this

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.

if both channels are on, then it doesn’t seem to do it, but if only one of either two are on, then it shows the triangular wave

giving some more thought, it wouldn’t make sense that it’s the ADC ramp. That would mean the ADC is sampling itself faster than the ramp used to sample.

Maybe it’s from the display or something, but it’s strange that it’s only when one channel is on.

It’s a beta, hopefully someone can figure out what this is. It’d be useful to have a second scope to troubleshoot, but I don’t have one available (at home).

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.