jremington:
The term is "aliasing". See Aliasing - WikipediaYou MUST sample the input at a frequency at least twice the highest frequency in the input to avoid aliasing, so for 50 Hz signals, sample at 100 Hz or higher.
First of all, thank you.
I know aliasing phenomena, to be honest I don't check it deep enough...but I tried more than one sample frequency and the result was the same. As a matter of fact my next step will be try to acquire at the nyquist frequency.
I'll update you very soon.