I don't remember offhand if the M32 has the option to turn off the digital functions on analog pins, but that may be another source of error.
Message
Re: [AVR-Chat] Apparent non-linear output from ADC
2012-10-26 by David VanHorn
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Message
2012-10-26 by David VanHorn
I don't remember offhand if the M32 has the option to turn off the digital functions on analog pins, but that may be another source of error.