Patent ID: 8395395

Claim:
A capacitive sensing circuit comprising: a sense plate for reflecting a capacitance to be measured; and a capacitance measurement circuit including: a charge transfer circuit that, during a charge transfer cycle, introduces a charge on said sense plate which reflects a capacitance associated with said sense plate, said charge being introduced by applying a charging voltage of a fixed value to said sense plate, and transfers an amount of charge from said sense plate to said capacitance measurement circuit, said charge transfer cycle being repeated a number of times to form a complete capacitance measurement cycle; and a charge subtracting circuit that effectively reduces the measured capacitance by removing an amount of charge from said capacitance measurement circuit during each charge transfer cycle, wherein, during each charge transfer cycle, the amount of said charge removed from said capacitance measurement circuit by the charge subtracting circuit is in a proportion to the amount of said charge transferred from said sense plate to said capacitance measurement circuit, which proportion remains the same for each charge transfer cycle in the complete capacitance measurement cycle, irrespective of an amount of charge accumulated in the measurement circuit throughout the complete capacitance measurement cycle.