big_jose:
Sorry. Yes you are right what I meant was V not mV. But do you know how to fix this problem?
I'm not even sure you have a problem, maybe just expectation of ADC performance that don't account for real world electrical noise both internal and external to the AVR chip. What are you using to generate the test voltages? What is the source impedance of the device that is generating the test voltages? At what analog input voltage do you see a one count returned? If you put a steady 2.5 Volt test signal to the analog input pin what count do you see returned and how steady is the reading? You are now in the analog electronics domain and there are many variables to account for rather then the more simple ON and OFF states of the digital domain.
Lefty