Well, it has to be less than 1/2 LSB, hopefully you can make it 1/10 LSB and let the quantization noise dominate.

Hi, The above two comments are very useful. Generally you would start with the required resolution of you analog input to the ADC. By that I mean what is the amplitude difference between two adjacent sampled inputs which you want your ADC to allot two different adjacent digital codes. Obviously this determines the ADC resolution or the required LSB. Say this required resolution or LSB is around 2mV. The ADC required resolution is then obviously 9 bits. Generally the resolution of an ADC should be such that it makes quantization error (plus minus half an LSB) of ADC insignificant so that we can accommodate the inaccuracies introduced by other circuit elements in the ADC like the comparator. So unless I know what is your required analog resolution, I can't say anything about comparator resolution. Pick an ADC with LSB 10 times lower than the required analog resolution; resolution of comparator will then be LSB of the picked ADC.