Referring to FIG. 3, a flowchart diagram illustrating steps of a readout method in accordance with some embodiments. In step S310, a differential signal that is obtained according to an input signal and a feedback signal is amplified to generate an amplified signal. In some embodiments, the differential signal is generated based on a difference between the input signal and the feedback signal. In step S320, the amplified signal is converted to a n-bit digital code, wherein n is a positive integer. In some embodiments, the n-bit digital code is a digital representation of the amplified signal. In step S330, a m-bit digital code is iteratively searched, for example, through binary-search method or direct-counting method, and generated based on the value of n-bit digital code and the m-bit digital code is converted to generate the feedback signal, wherein m is a positive integer. In some embodiments, the feedback signal is an analog representation of the m-bit digital code. In step S340, a multi-bit digital output is generated according to the n-bit digital code and the m-bit digital code. In some embodiments, the n-bit digital code is combined with the m-bit digital code to generate the multi-bit digital output, where the n-bit digital code includes least significant bits of the multi-bit digital output and m-bit digital code includes most significant bits of the multi-bit digital output.