Hi James
Voltage measurement can be a problem when using resistive voltage dividers as if the divider resistors are too high the 10MΩ resistance of most meters can have a significant effect on the actual voltage present. The meter reading will show the voltage WHILE THE METER IS CONNECTED. Not the real life value.
The input resistance on the pin on the micro of choice will behave the same. If low enough the lower resistance of the divider will be modified by this input resistance. One reason why expected numbers are better with lower divider resistance values.
Back in Sept '21 I attempted with limited success to actually measure the input resistance of an Arduino analog input. I had a bit of bother as I do not have the luxury of an Electrostatic Voltmeter at home but I got what I think is a reasonable approximation by using the actual ADC readings as a volt meter. I emphasise here the word “approximation”.
It ended up being in the GΩ range.
If you are interested the method and problems are described in this post.
Arduino Analog input resistance
Cheers Bob
