I am measuring huge 50Hz oscillations on my oscilloscope, but I am not entirely sure the probe is generating those by behaving like an antenna (an ADC at the same point gives stable values but it's not sampling fast enough to give a definitive answer).
Would adding a capacitor in parallel to the probe (tip and gnd) filter out that noise (22nF in parallel to 1Mohm in my case)?
I will answer that question as soon as I test it (for the sake of others too, it appears there are not a lot of answers to that question online, and no good one), but I would like an intuitive or step-by-step description of what is or would be happening.
Best Answer
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.