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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Are you sure your used firmware doesn't switch to the internal bandgap reference (ACBG bit in ACSR)? Of course you should also decouple AVCC witch a LC lowpass from VCC for better preformance but a fixed reference for AVCC is definitely not mandatory.
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.
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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.
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