The effect of redundancy on mismatch-induced offset and random noise in a dynamic comparator

We present an analysis of offset voltage and noise in a dynamic comparator. To limit the offset and noise to acceptable levels, a single comparator must be sized quite large. We show that better use can be made of this die area by dividing it into an array of redundant comparators from which the lowest-offset device is chosen. Monte Carlo simulations with a 45nm CMOS process confirm that the input-offset standard deviation can be reduced arbitrarily in the absence of noise. As the area is divided into a greater number of smaller comparators, random noise overtakes offset as the factor limiting the sensitivity. The competing effects of offset and noise combine to give an optimum number of comparators that maximizes sensitivity for a given total area.