Typical arduino coder: why use the built-in accurate voltage reference when you can use USB VBUS? Yeah, let's do that.
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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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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Eeexactly. Which happens to be 4.4V for me (because there's a hub and two long cables along the way).
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(Unless you're measuring a resistor divider, then you *want* vref to just be whatever you're dividing regardless of the absolute voltage)
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Note to myself: have a look to the controller board of my Prusa...
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That would explain the 3~4ºC temperature jumps I see sometimes.
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Aren't those just relative values, because the resistor networks are referenced to the same voltage?
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Nope. It's a thermocouple with a preamp IC that spits out an absolute voltage.
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I sometimes wonder why MCU designs allow selection of other reference voltages for the ADC, unless measuring VBUS of course :)
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