Method and circuit for parasitic capacitance cancellation for self capacitance sensing
First Claim
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1. An electronic device comprising:
- a touch-sensitive capacitive matrix having a plurality of force channels and a plurality of sense channels that are used for capacitive measurements to detect touch of the touch-sensitive capacitive matrix;
an output amplifier configured to be coupled to and decoupled from at least one sense channel of the plurality of sense channels;
wherein said at least one sense channel includes a parasitic capacitance; and
a charge removal circuit including a discharge circuit configured to sink a constant magnitude current from the parasitic capacitance at said at least one sense channel for a duration of time so as to cancel a parasitic charge stored by said parasitic capacitance at said at least one sense channel.
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Abstract
Apparatus and methods to measure capacitance changes for a touch-sensitive capacitive matrix are described. Charge-removal circuits and measurement techniques may be employed to cancel deleterious effects of parasitic capacitances in the touch-sensitive capacitive matrix. Capacitively switching a supply during timed charge removal may be used to cancel unwanted effects due to clock jitter. The apparatus and methods can improve signal-to-noise characteristics, sensitivity, and/or dynamic range for capacitive measurements relating to touch-sensitive capacitive devices.
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Citations
30 Claims
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1. An electronic device comprising:
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a touch-sensitive capacitive matrix having a plurality of force channels and a plurality of sense channels that are used for capacitive measurements to detect touch of the touch-sensitive capacitive matrix; an output amplifier configured to be coupled to and decoupled from at least one sense channel of the plurality of sense channels; wherein said at least one sense channel includes a parasitic capacitance; and a charge removal circuit including a discharge circuit configured to sink a constant magnitude current from the parasitic capacitance at said at least one sense channel for a duration of time so as to cancel a parasitic charge stored by said parasitic capacitance at said at least one sense channel. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A method for measuring capacitance at a sense channel of an electronic device wherein a parasitic capacitance is associated with the sense channel, the method comprising:
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applying a first voltage to the sense channel; executing a calibration measurement to determine a value of the parasitic capacitance; determining a value of a compensating charge based at least in part on the determined value of the parasitic capacitance and the applied first voltage; and connecting a discharge circuit to the sense channel for a duration of time to sink a constant magnitude current from the parasitic capacitance and generate said compensating charge which cancels a parasitic charge stored by the parasitic capacitance at said sense channel. - View Dependent Claims (15, 16, 17, 18, 19, 20)
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21. An electronic device comprising:
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a sense channel configured to be coupled to a touch-sensitive capacitive matrix, wherein the sense channel has a parasitic capacitance; an output amplifier having an input selectively connected to the sense channel and configured to convert a sensed capacitance to an output voltage; and a discharge circuit selectively connected to the sense channel and configured to sink a constant magnitude current from the parasitic capacitance at the sense channel for a time period having a selected length so as to discharge a parasitic charge stored by said parasitic capacitance. - View Dependent Claims (22, 23, 24, 25, 26, 27, 28, 29, 30)
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Specification