A circuit for a bio-information acquisition system, comprising: a terminal, for receiving an input signal; an output circuit, configured to generate an output signal according to the input signal; a feedback circuit, configured to receive the output signal to generate a current signal to the terminal; a calibration circuit, coupled to the output circuit and the feedback circuit, configured to generate a control signal to control the feedback circuit to determine a level of the current signal according to the output signal; a calibration signal generator, configured to generate a plurality of test signals to the terminal to serve as a plurality of input signals; and wherein the output circuit is configured to receive the plurality of input signals to generate a plurality of output signals, and the calibration circuit is configured to generate the control signal to control the feedback circuit to determine the level of the current signal according to the plurality of output signal; wherein the plurality of test signals comprises a first test signal and a second test signal, and the calibration signal generator is configured to generate the first test signal to the terminal without passing through any resistor, and the output circuit is configured to receive the first test signal to generate a first output signal; and the calibration signal generator is configured to generate the second test signal to the terminal through a resistor, and the output circuit is configured to receive the second test signal to generate a second output signal; and the calibration circuit is configured to generate the control signal to control the feedback circuit to determine the level of the current signal according to the first output signal and the second output signal. a terminal, for receiving an input signal; an output circuit, configured to generate an output signal according to the input signal; a feedback circuit, configured to receive the output signal to generate a current signal to the terminal; a calibration circuit, coupled to the output circuit and the feedback circuit, configured to generate a control signal to control the feedback circuit to determine a level of the current signal according to the output signal; a calibration signal generator, configured to generate a plurality of test signals to the terminal to serve as a plurality of input signals; and wherein the output circuit is configured to receive the plurality of input signals to generate a plurality of output signals, and the calibration circuit is configured to generate the control signal to control the feedback circuit to determine the level of the current signal according to the plurality of output signal; wherein the plurality of test signals comprises a first test signal and a second test signal, and the calibration signal generator is configured to generate the first test signal to the terminal without passing through any resistor, and the output circuit is configured to receive the first test signal to generate a first output signal; and the calibration signal generator is configured to generate the second test signal to the terminal through a resistor, and the output circuit is configured to receive the second test signal to generate a second output signal; and the calibration circuit is configured to generate the control signal to control the feedback circuit to determine the level of the current signal according to the first output signal and the second output signal.