Patent ID: 7986153
Filing Date: 2011-07-26
Classification: G01R,H03K

Abstract:
1. An apparatus for sensing a change in capacitance of a sensing electrode to a system ground, the apparatus comprising a sample capacitor having two terminals, a first of the two terminals being coupled to the sensing electrode, a voltage measurement circuit being coupled to a second of the terminals of the sample capacitor, the voltage measurement circuit being arranged in operation to determine a voltage at the second terminal of the sample capacitor, and a voltage biasing arrangement which is arranged under the control of a controller to apply a biasing voltage or the system ground to the first terminal or the second terminal of the sample capacitor in accordance with a measurement cycle, the measurement cycle comprising grounding the first and second terminals of the sample capacitor during an initialisation part of the measurement cycle by connecting the first and second terminals to the system ground, charging the sensing electrode via the sample capacitor, during a charging part of the measurement cycle by applying a drive voltage to the second terminal of the sample capacitor, whilst floating the first terminal by not applying the system ground or a biasing voltage, and during a measurement part of the measurement cycle grounding the first terminal whilst floating the second terminal by not applying the system ground or a biasing voltage, and measuring the voltage at the second terminal, and determining whether there has been a change in capacitance of the sensing electrode in accordance with the measured voltage, wherein before or after the measurement cycle the controller is arranged to perform a sleep mode in which the sensing electrode is biased at a voltage which is greater than the system ground.