PURPOSE: A differential column ADC(Analog to Digital Converter) architecture for a CMOS image sensor application is provided to eliminate a low-wise noise source or a column-wise noise source. CONSTITUTION: A crossover detection sub system is electrically connected to a differential sampling sub system. If a ramp signal level is changed in a second differential input node while a ramp reset level of a first differential input node is maintained, at least one output of a gain stage detects crossover time during a third operation state. The crossover detection sub system outputs a digital code corresponding to the crossover time.