Hi Nicholas
So you are having trouble with your voltage dividers.
100kΩ is too high for this application. Apparently the Pico ADC has a fairly low input resistance and will effectively be in parallel with the lower 100kΩ of your divider. Suggested maximum resistance total is 10kΩ which is 5kΩ each. It is not good practice to measure this sort of thing with a multimeter either as the multimeter resistance will do the same thing, especially with 100kΩ as the lower resistor.
Recently Trevor277988 had a voltage divider problem with Pico ADC and the eventual solution (successful I believe) was to interface an OP Amp configured as a unity gain voltage follower between voltage divider and ADC input to present a very high (>10 TeraΩ) resistance to the divider and low resistance source to the ADC The suggested OpAmp was LMC6482 (dual) or LMC6484 (quad) as these are "rail to rail (essential) devices and quite happy at 3.3V.
I don’t feel inclined to go into all the detail behind my thoughts on this again but I would recommend reading this thread.
PICO w ADC strangeness
Just below this. last activity 7hrs ago.
I have not got a Pico and have not used RPi bits so have not been able to analyse just what was happening with this ADC but isolating his divider with the voltage follower fixed everything.
Cheers Bob
