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.
Typical arduino coder: why use the built-in accurate voltage reference when you can use USB VBUS? Yeah, let's do that.
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I can't believe I'm the first one to notice this. Way to put a fancy thermocouple and preamp IC and then completely *destroy* accuracy.
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Yea, the +5V from Usb is super noisy! I once neeed it and had to use big inductors/caps to filter it.
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That's hilarious... and really no excuse for it. It's not like the internal voltage references are locked away behind obscure documentation.
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I've found most built-in references on embedded systems to be terrible. You're often better off supplying your own somehow.
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You're definitely not better off with *VBUS* though.
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Well, it does make sense when you just use an NTC and a resistor as voltage divider on the VUSB rail.
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Although I always would use a regulated rail.
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30ct voltage reference IC's
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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.
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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.
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