Capacitive sensing

Abstract

A multi-channel capacitive sensor for measuring the capacitances of a plurality of sense electrodes to a system reference potential. The sensor comprises a sample capacitor having a first terminal and a second terminal, a first diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a first sense electrode, and a second diode having a first terminal coupled to the second terminal of the sample capacitor and a second terminal coupled to a second sense electrode. The sample capacitor and diodes are coupled to a control circuit, e.g. implemented in a microcontroller. The control circuit is operable to apply a drive signal, e.g. a series of voltage pulses, to the first terminal of the sample capacitor while simultaneously applying a bias signal to the second terminal of one or other of the diodes to prevent the diode from conducting the drive signal. Thus charge transfer techniques can be used to measure multiple capacitances while sharing a common sample capacitor. This helps reduce inter-channel drift. Further measurement channels may be added by providing further diodes and corresponding sense electrodes. With three or more channels the scheme requires only one additional control circuit connection per additional channel.