System and method of calibrating the DC offsets of alternate comparators in an ADC in the background based on the digital outputs of the ADC. In parallel with AID conversion of a plurality of samples,the calibration logic uses two counters to count the occurrences of the ADC outputs that represent samples falling in a first analog range and a second analog range, respectively. The two ranges aresymmetric about the MSB reference voltage and in combination cover the nominal voltage range of the bit. The DC offset is derived based on a ratio of the difference between the two counts and a sum ofthe two counts. The calibration logic may alternately calibrate the comparators. Each comparator may be calibrated successively based on various bits associated therewith.