Low-cost, high measurement speed capacitive sensing circuit for loading mode operation of capacitive sensors

An impedance measurement circuit for determining a complex impedance of a capacitive sensor having at least one sense electrode operable in loading mode and at least one guard electrode, comprising:

• a pulse generator unit configured to provide a periodic electric measurement signal that comprises a plurality of different fundamental measurement frequencies, and for providing a periodic electric guard signal,
• a signal sensing circuit configured to sense a sense current flowing through the at least one sense electrode in response to the pulse generator unit measurement signal,
• a control and evaluation unit configured to determine a complex impedance from the determined sense currents with reference to a complex reference potential, and
• one or more remotely controllable electric switches for at least one out of selectively connecting one out of at least two provided reference potentials to a reference input port of the current measuring means, and selectively electrically operatively connecting at least one reference impedance of an a priori known impedance either in parallel to the at least one sense electrode or to the provided guard signal, wherein the control and evaluation unit is further configured for controlling the one or more remotely controllable electric switches according to a predefined sequence, and for determining a complex impedance from the determined sense currents with reference to a complex reference potential during predefined stages of the predefined sequence.