Arduino's Resolution with Multiple Analog Inputs
simulate this circuit &ndash; Schematic created using CircuitLab

Typical 8-channel micro ADC input. Channel selector logic not shown.

The ADC (analog-digital converter) is preceded by a multiplexed input selector. There is only one ADC in the chip.


All analog inputs share the one ADC.
Only one input pin can be connected to the ADC at any time.
Since all channels use the same ADC any comparison between the various inputs will share the same input offset errors and non-linearity. This will help minimise comparison errors.
Note that there is generally some input resistance and capacitance and the data sheet will normally recommend that you select a channel, wait long enough for the capacitor value to stabilise (5 x RC would give > 99%) and then trigger the ADC. This is one of the factors which limits the maximum ADC scan rate.
Since you can't sample two inputs simultaneously it is not possible to compare two inputs at exactly the same instant. There will be a small delay between the readings.