The present invention relates to a scheme for simply implementing a demodulation circuit, which is a core circuit of a measuring instrument, using a lock-in detection method. The present invention provides a signal demodulation circuit includes an inverting amplifying circuit and a low pass filter circuit. The inverting amplifying circuit has an operational amplifier with a disable function. The low pass filter circuit is electrically connected to an output terminal of the inverting amplifying circuit to output a signal wave. Accordingly, the present invention provides a signal demodulation circuit, which has a simplified construction, thus reducing the size of the circuit, enabling a demodulation circuit to be implemented when a multi-channel signal is processed or when circuit space is limited, and greatly reducing production costs and power consumption.
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