Patent ID: 8633714

Claim:
An impedance detection circuit, comprising: an AC signal generator that generates an AC signal; a detection circuit that includes a first operational amplifier having an inverting input terminal coupled to one end of a signal line to whose other end a measured capacitor is coupled, a non-inverting input terminal coupled with a shielding wire covering at least a part of the signal line and an output terminal of the AC signal generator, and an output terminal, and a first feedback resistor coupled between the output terminal and the inverting input terminal of the first operational amplifier; a correction circuit that includes a first resistor whose one end is coupled with the output terminal of the first operational amplifier and whose other end is coupled with a first node and a variable capacitor whose one end is electrically coupled with the first node, and that corrects a detection signal outputted from the first operational amplifier by adjusting a capacitance of the variable capacitor and outputs the detection signal after the correction from the first node; and a subtraction circuit that subtracts a signal inputted into the inverting input terminal of the first operational amplifier from the detection signal after the correction outputted from the correction circuit to output an output voltage based on a capacitance of the measured capacitor.