A readout circuit that includes an amplifier circuitry, an analog-to-digital converter, a feedback circuit and a control logic is introduced. The amplifier circuitry may receive and amplify a differential signal that is obtained according to an input signal and a feedback signal to generate an amplified signal. The analog-to-digital converter is configured to convert the amplified signal to generate a n-bit digital code, wherein n is a positive integer. The feedback circuit is configured to search and generate a m-bit digital code based on a value of the n-bit digital code and convert the m-bit digital code to generate the feedback signal, wherein m is a positive integer. The control logic is coupled to the analog-to-digital converter and the feedback circuit, and configured to control the analog-to-digital converter and the feedback circuit. A multi-bit digital output of the readout circuit is generated according to the n-bit digital code and the m-bit digital code.