I have just got a Uno board and been playing with the ADC A0 input.
If I connect A0 to 0 gnd via a 10k resistor I get a reading of 0 as expected. If I connect A0 to the board 5V via a 10k resistor I get 1023 as expected. If I connect A0 to the 3.3V via a 10k resistor I get 940 which calculates out to about 4.5v not 3.3v What I expect to get was (1023/5)*3.3 = 675 I have also tried without the 10k resistor and get the same result. I do a loop of 100 readings each time and the readings are stable just 940 when I expected 675 for 3.3Volts.
Please what am I doing wrong or not understanding? thanks.