So I just found out why my Ultimaker temp measurements are off. Turns out some idiot thought referencing the ADC to USB +5V was a good idea.
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Yes, I am sure. I read the code, and confirmed that bumping up VBUS to 5V fixes the temperature issue. By "built-in accurate voltage reference" I meant that internal bandgap. ACBG is for the comparator, BTW. You want REFSn in ADMUX for the ADC.
There is no easy firmware only fix because the reference is 2.56V, which means the maximum temperature you can measure is 256°C with the 10mV/°C AD597. They should've divided down the input with e.g. a 2:1 resistor divider first, or used an external accurate vref.
