The present invention provides a circuit applied to a bio-information acquisition system, wherein the circuit includes a terminal, an output circuit, a feedback circuit and a calibration circuit. In the operations of the circuit, the terminal is arranged to receive an input signal, the output circuit is configured to generate an output signal according to the input signal, the feedback circuit is configured to receive the output signal to generate a current signal to the terminal, and the calibration circuit is configured to generate a control signal to control the feedback circuit to determine a level of the current signal according to the output signal.