An apparatus for sensing a change in capacitance of a sensing electrode to a system ground, such as that which may be used to form a touch sensor, with the sensing electrode forming touch sensitive surface is arranged to include a sleep mode of operation. The apparatus includes a sample capacitor having a first terminal coupled to the sensing electrode, and a second terminal coupled to a voltage measurement circuit. The voltage measurement circuit is arranged in operation to determine a voltage at the second terminal of the sample capacitor. A voltage biasing arrangement 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.