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Abstract

A capacitive sensing circuit may comprise an RC (resistive-capacitive) bridge circuit, with a switching signal simultaneously applied to a reference path, and a signal path comprising the capacitance to be detected. Small perturbations in the capacitance may be detected by mixing/correlating a difference signal representative of the difference between the reference path signal and the signal path signal, to the switching signal. The output of the mixer may be filtered to virtually eliminate all EMI signals. A narrowband approach may also allow filtering of unwanted signals, enabling operation in systems susceptible to high levels of noise. Frequency stepping of the switching signal may minimize inband signal interference, and allow operation in the presence of many signals that would otherwise result in failure of the sensing circuit. Pad calibration may be implemented to free the user from a need to characterize each button channel capacitance and tailor the operation for each channel.

First Claim