We are having trouble in getting a ground signal (0V) when we connect to the ground on the scope. For some reason when we connect the scope to the internally generated square wave signal and to the scope ground, the ground seems to be floating at about 20mV.
In addition the ground level doesn't change even if the scope ground is connected to an external power-supply ground.
Even if I connected the probe to the ground i.e.connect both the probe wires to the Scope ground, I still see the ground at about 20mV. In addition when I change the scale on the oscilloscope i.e. go from 10mV to say 100mV, the amount of offset on the ground signal varies.
I am never able to see the ground level at 0V.
I also connected the probe to the ground of an external power supply and it doesn't seem to make any difference. The offset was still there.