An image sensor for sampling a pixel signal output by a pixel array wherein the pixel signal is sampled a plurality of times during a readout time, the image sensor comprising an analog-to-digital conversion circuit comprising: an analog comparator configured to compare a signal level of the pixel signal with a signal level of a target ramp signal (RAMP) that is any one of a plurality of ramp signals; a counter configured to output counting data based on a comparison result of the analog comparator; and a digital comparing circuit configured to compare a binary value of a target reference code corresponding to the target ramp signal with a binary value of the counting data and determine whether to output a digital signal corresponding to the counting data to a data output circuit based on a result of the comparison between the binary value of the counting data and the binary value of the target reference code, wherein the binary value of the target reference code comprises a number of clock signals corresponding to a time period length of the target ramp signal; the image sensor further comprising a ramp signal generator configured to generate the plurality of ramp signals during the readout time and transmit the plurality of ramp signals to the analog comparator, wherein the plurality of ramp signals are a plurality of successive target ramp signals, and each of the plurality of ramp signals has a maximum signal level that is less than a maximum signal level of a previous ramp signal generated previously during the readout time, and each of the plurality of ramp signals has a time period length that is less than the time period length of the previous ramp signal generated previously during the readout time. an analog comparator configured to compare a signal level of the pixel signal with a signal level of a target ramp signal (RAMP) that is any one of a plurality of ramp signals; a counter configured to output counting data based on a comparison result of the analog comparator; and a digital comparing circuit configured to compare a binary value of a target reference code corresponding to the target ramp signal with a binary value of the counting data and determine whether to output a digital signal corresponding to the counting data to a data output circuit based on a result of the comparison between the binary value of the counting data and the binary value of the target reference code, wherein the binary value of the target reference code comprises a number of clock signals corresponding to a time period length of the target ramp signal; the image sensor further comprising a ramp signal generator configured to generate the plurality of ramp signals during the readout time and transmit the plurality of ramp signals to the analog comparator, wherein the plurality of ramp signals are a plurality of successive target ramp signals, and each of the plurality of ramp signals has a maximum signal level that is less than a maximum signal level of a previous ramp signal generated previously during the readout time, and each of the plurality of ramp signals has a time period length that is less than the time period length of the previous ramp signal generated previously during the readout time.