Capacitive measurement circuit for a touch sensor drive
In one embodiment, a method for determining the location of a touch on a touch sensor includes receiving input signals in response to a touch proximate to a location on the touch sensor. Each input signal may have a total capacitance that includes a first capacitance associated with the touch and a second capacitance that is parasitic capacitance. The parasitic capacitance of one or more of the input signals may be adjusted to result in the second capacitance of each of the input signals being substantially equal. An average capacitance of the adjusted input signals may be calculated. The location of the touch on the touch sensor may then be determined based on a comparison of the total capacitance of each of the input signals and the average capacitance of the input signals.
1. A capacitance measurement circuit, comprising:
a plurality of compensation buffers operable to receive a plurality of partial capacitances, respectively, from a plurality of sense lines, respectively, of a touch sensor and to scale the plurality of partial capacitances, respectively, to create a plurality of scaled capacitances;
a current summer/divider operable to receive the plurality of scaled capacitances from the plurality of compensation buffers and determine an average capacitance of the plurality of scaled capacitances from the plurality of compensation buffers; and
at least one processor operable to:
receive, from the current summer/divider, the average capacitance of the plurality of scaled capacitances from the plurality of compensation buffers;
subtract the average capacitance of the plurality of scaled capacitances of the plurality of compensation buffers from a total capacitance associated with one of the plurality of sense lines to determine a capacitance associated with a touch input on the touch sensor; and
determine a location on the touch sensor associated with the touch input based on the determined capacitance associated with the touch input.
2. The capacitance measurement circuit of claim 1 , wherein each of the plurality of sense lines have a total capacitance comprising:
a first capacitance associated with the touch input; and
a second capacitance comprising a parasitic capacitance.
3. The capacitance measurement circuit of claim 2 , wherein scaling the plurality of partial capacitances comprises adding or subtracting an amount of capacitance to result in the second capacitance of each of the plurality of scaled capacitances being substantially equal.
4. The capacitance measurement circuit of claim 1 , further comprising an input buffer operable to generate, based on the plurality of scaled capacitances, a first output signal and a second output signal, and wherein:
the current summer/divider uses the first output signal to determine the average capacitance of the plurality of scaled capacitances; and
the at least one processor is operable to use the second output signal when subtracting the average capacitance from the total capacitance associated with one of the plurality of sense lines.
5. The capacitance measurement circuit of claim 1 , further comprising a proximity circuit operable to detect an inadvertent touch on the touch sensor based on the average capacitance.
6. An apparatus comprising:
a touch sensor comprising a plurality of sense electrodes;
a plurality of compensation buffers operable to receive a plurality of partial capacitances, respectively, from the plurality of sense electrodes, respectively, and to scale the plurality of partial capacitances, respectively, to create a plurality of scaled capacitances;
a current summer/divider operable to receive the plurality of scaled capacitances from the plurality of compensation buffers and determine an average capacitance of the plurality of scaled capacitances from the plurality of compensation buffers; and
at least one processor operable to:
receive, from the current summer/divider, the average capacitance of the plurality of scaled capacitances from the plurality of compensation buffers;
subtract the average capacitance of the plurality of scaled capacitances of the plurality of compensation buffers from a total capacitance associated with one of the plurality of sense electrodes to determine a capacitance associated with a touch input on the touch sensor; and
determine a location on the touch sensor associated with the touch input based on the determined capacitance associated with the touch input.
7. The apparatus of claim 6 , wherein each of the plurality of sense lines have a total capacitance comprising:
a first capacitance associated with the touch; and
a second capacitance comprising a parasitic capacitance.
8. The apparatus of claim 7 , wherein the plurality of compensation buffers are operable to scale the plurality of partial capacitances, respectively, by adding or subtracting an amount of capacitance to result in the second capacitance of each of the plurality of respective scaled capacitances being substantially equal.
9. The apparatus of claim 6 , further comprising an input buffer operable to generate, based on the plurality of scaled capacitances, a first output signal and a second output signal, and wherein:
the current summer/divider is operable to use the first output signal when determining the average capacitance of the plurality of scaled capacitances; and
the at least one processor is operable to use the second output signal when subtracting the average capacitance from the total capacitance associated with one of the plurality of sense lines.
10. The apparatus of claim 6 , further comprising a proximity circuit operable to detect an inadvertent touch on the touch sensor based on the average capacitance.
11. A method comprising:
scaling a plurality of partial capacitances received from a plurality of sense lines, respectively, of a touch sensor to create a plurality of scaled capacitances;
determining an average capacitance of the plurality of respective scaled capacitances from the plurality of compensation buffers;
subtracting the average capacitance of the plurality of scaled capacitances from the plurality of compensation buffers from a total capacitance associated with one of the plurality of sense lines to determine a capacitance associated with a touch input on the touch sensor; and
determining a location on the touch sensor associated with the touch input based on the determined capacitance associated with the touch input.
12. The method of 11 , wherein each of the plurality of sense lines have a total capacitance comprising:
a first capacitance associated with the touch input; and
a second capacitance comprising a parasitic capacitance.
13. The method of claim 12 , wherein scaling the plurality of partial capacitances, respectively, comprises:
adding or subtracting an amount of capacitance to result in the second capacitance of each of the plurality of scaled capacitances being substantially equal.
14. The method of claim 11 , further comprising:
generating, based on the plurality of scaled capacitances, a first output signal and a second output signal, and
wherein the first output signal is used when determining the average capacitance of the plurality of scaled capacitances; and
wherein the second output signal is used when subtracting the average capacitance from the total capacitance associated with one of the plurality of sense lines.
15. The method of claim 11 , further comprising detecting an inadvertent touch on the touch sensor based on the average capacitance.