On the Nano RP2040 Connect, I would like to use the the AREF pin as an external voltage reference - in conjuction with a LM4040. I intend to provide a 3v reference for 10K thermistor circuits. The Arduino IDE library for the RP2040 doesn't have AREF support.
How is the AREF pin specified, intialized, etc. in MicroPython?
All my other programming has been in MicroPython, including code for the thermistor circuits without an external voltage reference. So far it has worked adequately using the +3V3 pin as the reference. It looses accuracy when <10C or >40C, so its been suggested to use an external reference, along with Steinhart-Hart calculations in the code.
The (dummy) AREF pin is only there to have a "nano" compatible pinout.
How it is supposed to be :
The thermistor should be powered with the same voltage as the ADC of the processor. On your board that is a filtered 3.3V. Power the thermistor with the 3.3V pin of the board and you get maximum accuracy because it is "ratiometric".
The average of a few samples can get rid of electronic noise.
The Steinhart-Hart gives average result, use a table and interpolation for maximum accuracy.
The most accurate range is when the resistor to 3.3V has the same value as the thermistor. Suppose that you need the best accurate around 90°C and the thermistor is 450Ω at that themperature, then select 450Ω (or 470Ω) for the resistor to 3.3V.
What powers the thermistor ?
What powers your thermistors ? Is than an external voltage ? That is against everything. You can try whatever you want, but it might get worse. Can you measure that 3V voltage with your RP2040 board ? Maybe you can measure the 3V and use that to correct the values from the thermistors. Perhaps you can make a external voltage reference, and use that for the calculations. Then you have multiple things that makes it less accurate. The "ratiometric" way is so much better.
Can you look into the datasheet of the voltage regulator to see how accurate it is ?