We are using an Xmega32a4u for a project and when reading some of the inputs which are set to ground, the ADC returns a value of 203. All the ADC inputs seem to have this offset for some reason.
We are using the ADC with an external ref on AREFB of 2.5V. It measure around 2.466V in practice.
Also, is it possible to cal the reference against the interval 1.1V bandgap reference - we tried but the value comes out at 2.55V instead of 2.46V (this may also be related to the ADC offsat)