Impedance measuring circuits are known. In the impedance measuring circuits, input signals such as AC signals are input to a target such as a living body, and an impedance in the target is measured based on a magnitude of the input signal reflected or transmitted by the target.
When the target is a human body, the Pharmaceutical Affairs Law limits a current that can be supplied to a human body.
Therefore, impedance measuring circuits of the related art include a protective resistor such that a current larger than or equal to a limited value is prevented from flowing in a human body.
However, a protective resistor results in a smaller voltage amplitude of the input signal reflected or transmitted by the target, and thus it is desirable that a gain of an amplifier for amplifying the input signal is high. Since the input signal includes a noise component, a high gain of the amplifier may cause a signal component of the original target to be buried in the noise component and thus accurately measuring an impedance in the target may become difficult.
Therefore, instead of the protective resistor, a capacitor may be connected in series to the impedance in the target, thereby removing a DC voltage component that includes a noise component and detecting an AC signal component. However, the capacitor connected in series to the impedance in the target may hinder determination of a DC point of the aforementioned amplifier, thereby hindering correct measurement of the impedance in the target.