<p>A circuit is provided that includes a power source and a sensor circuit electrically coupled by a switch to the power source. The circuit further includes an A/D converter electrically coupled to the sensor circuit and adapted to read a voltage difference across a resistive element to determine an impedance of the sensor circuit. A method is provided that includes closing a switch electrically coupling a power source to a sensor circuit and measuring a voltage difference across a resistive element at an A/D converter electrically coupled to the sensor circuit. The method further includes determining an impedance of the sensor circuit based on the voltage difference.</p>
