Resolution = Full Scale value / Number of Steps.
Number of Steps = (2^N)-1.
Resolution = 5 mV.
Full Scale value = 5.
5mV = 5 / No. of steps.
No. of steps = 1000.
No o bits required to represent 1000 levels = log(1000) (Note: log with base 2)
N = 9.96 ~ 10.