If you look at the schematic for the ADC on Galileo :-
You'll see that each of the ADC Inputs is actually the centre of a voltage divider. (Two 4.99K Resistors in series).
And since full scale deflection of the ADC (12 Bits, 0x0fff, 4095 decimal) is achieved at 5V, and zero deflection (0x000) is at 0V then you must be careful if you are trying to bias an input voltage.
If you are trying to offset an input voltage (say one that can swing below 0) You will need to take the two 4.99K resistors into account when designing your input offsetting circuit.
As a simple test, connect two 10 K ohm resistors in series and connect the centre to the A0 pin and measure the voltage. It won't be 2.5V as you'd expect. It'll be more like 1.69 V
If you are feeding in a pure voltage then you'll get the results you expect, but with a biased voltage you better be careful with your biasing circuit!
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