Filter 50Hz hum picked up by the oscilloscope
This smells strongly of not having the probe ground connected, or perhaps there is a break in that wire or connection somewhere.  Try putting the probe on the ground of the circuit that contains this A/D.  If you still get a lot of noise, then the probe ground is not connected.

To see if something is broken inside the probe, hook the probe onto the ground clip of the same probe, then touch that connection to the ground of the circuit.  If the probe ground connection is working, then you should seen nothing or a few mV of noise maybe.

Depending on what you find, diagnose from there.