An analog to digital convertor (ADC) includes a plurality of comparators one of which is referred to as an auxiliary comparator (e.g., comparator “Aux”). This comparator Aux is calibrated in the background while other comparators function as usual. Once having been calibrated, the comparator Aux replaces a first comparator, which becomes a new comparator Aux, is calibrated, and replaces the second comparator. This second comparator becomes the new comparator Aux, is calibrated, and replaces the third comparator, etc., until all comparators are calibrated. In effect, at any one point in time, a comparator may be calibrated as desire while other comparators and thus the ADC are operating as usual.