An integrated circuit includes a ramp signal generator circuit, a comparator, a counter and a control circuit. The ramp signal generator circuit is configured to generate a ramp reference signal. The comparator configured to compare a pixel output signal and the ramp reference signal thereby generating a comparator output signal. The counter is coupled to the comparator, and configured to be enabled or disabled in response to the comparator output signal. The control circuit coupled to the comparator, and configured to enable or disable the comparator by a first enable signal, the first enable signal generated in response to at least the comparator output signal.