An image sensor includes a delta-sigma analog-to-digital converter (ADC) including a delta-sigma modulator (DSM) and a voltage adjusting circuit. The DSM is configured to perform delta-sigma modulation on an analog signal from a unit pixel. The delta-sigma ADC is configured to convert the analog signal to a digital signal. The voltage adjusting circuit includes a replica inverter having a same configuration as at least one inverter included in the DSM. The voltage adjusting circuit is configured to adjust a power supply voltage and an input voltage provided to the at least one inverter based on a current flowing in the replica inverter.