Example embodiments are directed to an analog-to-digital converter (ADC) that controls a gain by changing a system parameter, an image sensor including the ADC and a method of operating the ADC. The ADC includes a sigma-delta modulator which receives an input signal and a clock signal and sigma-delta modulates the input signal into a digital output signal based on the clock signal and an accumulation unit which accumulates the digital output signal at each cycle of the clock signal according to an analog-to-digital conversion time and outputs an accumulation result. A system parameter is varied during the analog-to-digital conversion time to control a gain of the ADC. The method of operating the analog-to-digital converter includes sigma-delta modulating an input signal into a digital output signal in response to a clock signal input to the ADC; and accumulating the digital output signal at each cycle of the input clock signal according to an analog-to-digital conversion time and outputting an accumulation result.