As I recall, the ATmega ADC circuit is optimised for impedances of 10k or less, It will work with the higher resistances but according the ATmega specs, the conversion will take longer because of increased charge time in the ADC sample and hold.
As I recall, the ATmega ADC circuit is optimised for impedances of 10k or less, It will work with the higher resistances but according the ATmega specs, the conversion will take longer because of increased charge time in the ADC sample and hold.