That's the effective impedance, not exactly when sampling.the Pico ADC input impedance is >100k,
Also look for noise/dropout on 3.3V rail.
I've done some tests here: viewtopic.php?t=379699&start=25#p2270726
Use a scope, not just a multimeter.
Try lower resistor values to see if it improves.
The ADC input is capacitive, and when sampling, it places about 1pF across the input (there will be additional
capacitance from outside the ADC, such as packaging and PCB routing, to add to this). The effective impedance, even
when sampling at 500ksps, is over 100kΩ, and for DC measurements there should be no need to buffer.
Statistics: Posted by gmx — Tue Jan 07, 2025 12:02 am