Low-cost, protected capacitive sensing circuit for loading mode operation of capacitive sensors employing heater members

An impedance measurement circuit for determining a complex impedance of a capacitive sensor that is configured for employing at least one electric heater member as an electrode to be operated in loading mode, the impedance measurement circuit comprising:

• a signal generating unit for providing an electric measurement signal,
• a signal sensing circuit including current measuring means for sensing a current flowing through the at least one electric heater member in response to the signal generating unit measurement signal,
• an electric heater member connection circuit comprising a plurality of MOSFETs for selectively electrically connecting the at least one electric heater member either to the signal sensing circuit for complex impedance measurement or to a DC electric power unit for heating purposes, and at least two DC bias units, each DC bias unit including a DC voltage source, wherein:
• at least a first and a second MOSFET are electrically connectable to one output port each of the DC electric power unit and in series with the at least one electric heater member, which is connectable between the first and the second MOSFETs, and
• at least a first protection MOSFET, which is electrically connected in series to the first MOSFET such that the intrinsic diode of the first protection MOSFET is connected in reverse to the intrinsic diode of the first MOSFET, and is connectable in series to the at least one electric heater member,
• the at least two DC bias units are configured for providing a predefined DC voltage across the drain and source terminals of the at least first protection MOSFET, and
• a control and evaluation unit that is at least configured for:
• controlling a switching status of the plurality of MOSFETs according to a predefined sequence, and
• determining a complex impedance from the determined currents with reference to a complex reference potential during predefined stages of the predefined sequence.