Hello, I experience the following problem : I have an AC signal which I want to measure (volts) with Uno. Of course, since 328's ADCs are single polarity, I "cut " the negative part using a schotky diode (clamp to the ADC input), that means the ADC input goes maximum 0,15V negative (absolute maximum spec is -0.3V).
In practice I have 2 AC signals to measure. These signals are same frequency, but may be in phase or opposit phase (0 or 180 deg) in respect to a ref signal.
The strange part is that if the signals are in same phase between them (ie both 0 or both 180 deg to ref), measurements are ok, if they are opposite between them (one 0 the other 180 deg to ref), measurements are not ok. There comes an error which is related to their amplitude.
Any comments on possible input behavior of ADC because one input is positive and the neighbour (a little) negative?