Capacitive loading mode measurement circuit with compensation of measurement errors due to parasitic sensor impedances

An impedance measurement circuit for determining a sense current of a guard-sense capacitive sensor operated in loading mode including an electrically conductive sense electrode and an electrically conductive guard electrode proximally arranged and mutually galvanically separated from each other, the impedance measurement circuit including

• a periodic signal voltage source that is configured to provide a periodic measurement voltage to the guard electrode,
• a sense current measurement circuit that is configured to determine a current flowing through the sense electrode that is indicative of an unknown impedance and represents a position of an object relative to the sense electrode,
• a differential amplifier that is configured to sense a complex voltage difference between the sense electrode and the guard electrode,
• demodulation means that are configured for demodulating an output signal of the differential amplifier and for obtaining, with reference to the periodic measurement voltage, an in-phase component and a quadrature component,
• a first control loop that is configured to receive the in-phase component as an input signal, and a second control loop that is configured to receive the quadrature component as an input signal,
• wherein an output signal of the first control loop and an output signal of the second control loop are used to form a complex voltage that serves as a complex reference voltage for the sense current measurement circuit.