Hello AEC
I have found it difficult to get the stability I need from the 10 bit ADCs in the AVRs. I have found that a 3 digit reading is the best you can hope for with the last digit varying, to get 3 stable digits I have to go to a 12 bit ADC (see blog section of forum for my work) and to get to a 3.5 digit 14 bit ADC is needed. Your readings are a bit more unstable so try running the AVR off batteries and measure the voltage on a 1.5v AA or AAA cell this gives a noise free power source, taking 8 to 16 readings then averaging them will help a little. Note most multimeters are 14 bit or more and 3 digit panel meter ICs are 14 bit.
Regards Paul
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