1. A comparator outputting offset calibration, comprising: a MOS current mode logic (MCML) circuit, receiving input signals and generating differential logic signals on output terminals thereof, and comprising a calibration unit coupled to the output terminals, calibrating output offsets at the output terminals according to digital calibration codes; and an output stage coupled to the differential logic signals at the output terminals of the MCML circuit, amplifying the differential logic signals and generating a comparison resulting signal.