... well strange to me.
I built an external precision 5V reference (temperature-compensated) using a LM336-5. This reference was running off its own power source, because of the system design.
I noticed that when Vcc was removed from the micro using this reference, that the reference voltage was pulled down to about 1.6V. Found a thread here, which suggested that it was not a good idea to have a voltage on a pin, when there was no supply!
So, I thought why not put a 1K resistor in series to "limit current". Now the ADC readout increases about 4%. A close look at the datasheet suggests that Rref is 32K. And 1K in series with 32K would make about this much difference. However, the voltage on the Aref pin did NOT change. Even going up to a 10K series resistor and the ADC reading increasing by 13% only dropped the 5.00V reference down to 4.995V on Aref. I do not understand what sort of load the Aref pin is applying to my reference circuitry to give this sort of behaviour.
I will change to a 2.56Volt reference (LM336-2.5) fed from Vcc on micro to get around the initial problem, but can anyone explain what is happening inside the micro "looking" into the Aref pin?