Low-cost complex impedance measurement circuit

A method of measuring impedance of a sense-only capacitive sensor operated in loading mode, the method comprising the following steps:

• generating a plurality of synchronized output signals having a test signal frequency by at least one pulse width modulation unit, said plurality of synchronized output signals exhibiting a defined relative phase shift between each other;
• combining a said plurality of synchronized output signals having a test signal frequency by summing the synchronized output signals,
• applying low-pass filtering to the summed output signals to generate a sinusoidal test signal of test signal frequency;
• applying the sinusoidal test signal to a sensor signal voltage divider circuit comprising at least one impedance member and at least one capacitor connected in series, and a capacitive sensor having a complex impedance which at the test signal frequency is electrically connected in parallel to the at least one capacitor;
• determining amplitude and relative phase, with respect to the sinusoidal test signal, of a sensor voltage signal that is indicative of the complex impedance of the capacitive sensor;
• applying the sinusoidal test signal to a reference voltage divider circuit that is substantially identical to the sensor signal voltage divider circuit except for the absence of a capacitive sensor;
• determining amplitude and relative phase, with respect to the sinusoidal test signal, of a reference voltage signal that is indicative of a complex impedance of the reference voltage divider circuit;
• obtaining a difference voltage signal by subtracting the reference voltage signal from the sensor voltage signal;
• and
• feeding the difference voltage signal to a vector demodulator unit for obtaining a real part and an imaginary part of the complex impedance of the capacitive sensor.