An image sensing system includes a pixel array, an analog-to-digital converter circuit, and an average calculator. The analog-to-digital converter circuit converts a first pixel signal to first pixel data and converts a second pixel signal to second pixel data. The average calculator generates a first average bit based on a first bit of the first pixel data and a first bit of the second pixel data during a first time and generates a second average bit based on a second bit of the first pixel data and a second bit of the second pixel data during a second time.