The smallest difference in voltage that can be resolved is referred to as the resolution. The resolution can be calculated with the following formula:
resolution=voltage range / digital range
The voltage range in our case is from -500mV to 500mV, which gives 1000mV.
The digital range on the other hand is 2^(number of bits).
It depends on what type of bit board we are using. If the ADC we are using is a 16 bit board, then 2^16=65536.
So, the resolution is:
resolution=1000mV/65536=0.015 mV