fofro:
I believe that the A0 pin's impedance is too small, giving a different voltage value on the wiper terminal. It needs to be a very high value in order to read voltage like a voltmeter does...Anyone knows the A0-A5 pin's impedance? And if it's tunable?
It is nothing to do with the impedance. It is very high and connecting a low impedance to it will not affect the reading.
Why do you use the a.analogueRead and not just analogueRead?