I've made some progress in debugging my ADC errors. The symptom is easy to reproduce: whenever a heater is on, the ADC readings go up (temp readings go down), permanently, until the heater is turned off. More heaters, and more heater current, produces a higher error.
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I've been measuring voltage between the ground connection of the thermistors and Due's GND pins next to Vin (presumably these have the best connection to GNDANA on the SAM3X8E). I find that the errors correspond to an increased and positive voltage here. In the worst case, with all heaters on, the voltage is about 0.03V. This must be due to some currents related to heaters being on. So I added a wire connection to the bottom of the board.
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The wire decreased the voltage to 0.005V worst case, and the temp errors have decreased with that. I suspected there's extra current coming into my wire from the on-board connection of thermistor ground, so I cut that trace (the one at the bottom ground pin on my picture). Surprisingly, it made things worse, voltage has risen to 0.01V.
So I'm not sure what the unwanted current is due to, but I believe the thermistors should have a more direct connection to the Due, both ground and 3.3V. And ground should probably go to the two GND pins next to Vin, since I think these have the best connection to GNDANA.
P.S. credit goes to Triffid_hunter on IRC for suggesting a voltage may be present between GNDANA and thermistor grounds.
I've been measuring voltage between the ground connection of the thermistors and Due's GND pins next to Vin (presumably these have the best connection to GNDANA on the SAM3X8E). I find that the errors correspond to an increased and positive voltage here. In the worst case, with all heaters on, the voltage is about 0.03V. This must be due to some currents related to heaters being on. So I added a wire connection to the bottom of the board.
The wire decreased the voltage to 0.005V worst case, and the temp errors have decreased with that. I suspected there's extra current coming into my wire from the on-board connection of thermistor ground, so I cut that trace (the one at the bottom ground pin on my picture). Surprisingly, it made things worse, voltage has risen to 0.01V.
So I'm not sure what the unwanted current is due to, but I believe the thermistors should have a more direct connection to the Due, both ground and 3.3V. And ground should probably go to the two GND pins next to Vin, since I think these have the best connection to GNDANA.
P.S. credit goes to Triffid_hunter on IRC for suggesting a voltage may be present between GNDANA and thermistor grounds.