I noticed a strange behaviour, though, which I cannot explain.
- DSO is running on battery power (in order to rule out any influence from USB power supply) and rests on a table, test cable is not yet connected to the input
- Calibrate offset again (just to be sure)
- Timebase does not matter - it happens with slower (say 10ms/div) and faster timebases (10µs/div)
- Switch to 200mv/div or 100mV/div, AC coupled (AC vs DC coupling does not make a difference, though)
- Everything fine, the DSO shows a horizontal trace at 0V with a little bit of noise
- Connect the MCX-clip cable to the input and let the cable hang down.
- Still everything fine, the cables pick up several millivolts of AC hum and noise from the environment, but the average is still at 0V
- Now I short the two alligator clips of the MCX-clip cable (and still let the cable hang down)
- The noise almost disappears, however the horizontal trace moves down to about -100mV DC
Interestingly, if I pick-up the DSO and hold hold it in my hand, and touch the alligator clips with my other hand, then the unexpected DC offset disappears and the trace moves again to 0V. The DC offset also disappears if I don't short the alligator clips directly, but connect them to a low-Ohm resistor instead.
EDIT: Touching only the shorted alligator clips alone, without touching the case of the DSO as well, does not let the unexpected offset disappear.
Has anybody an idea what's going on here?