Voltage driven self-capacitance measurement
In one embodiment, a method includes substantially simultaneously applying a pre-determined voltage to a sense electrode and a corresponding drive electrode of a touch sensor. The application of the pre-determined voltage providing a measurement current at a capacitance including the sense electrode. The method also includes determining a difference between the measurement current at the capacitance and a reference value; and determining whether a proximity input to the touch sensor has occurred based at least in part on the difference.
1. A method comprising:
coupling a first electrode of a touch sensor and a corresponding second electrode of the touch sensor to a voltage source, wherein the first electrode is capacitively coupled to the second electrode;
applying, through the voltage source, a pre-determined voltage to the first electrode and also substantially simultaneously applying the pre-determined voltage to the corresponding second electrode, the application of the pre-determined voltage providing a measurement current at a capacitance comprising the first electrode, wherein the measurement current results in a voltage at a negative input to a differential amplifier, and wherein the first electrode is oriented in a first orientation and the second electrode is oriented in a second orientation;
determining a difference between the measurement current at the capacitance and a reference value;
adjusting in real time a voltage of a reference path coupled to a positive input of the differential amplifier such that a voltage at the positive input of the differential amplifier is substantially equal to the voltage at the negative input of the differential amplifier resulting from the measurement current;
toggling an input of a lock-in demodulation circuit between an output voltage of the differential amplifier and an inverse of the output voltage from the differential amplifier every half cycle of the pre-determined voltage, wherein the output voltage of the differential amplifier is a function of the difference between the measurement current at the capacitance and the reference value, and wherein the pre-determined voltage comprises an alternating-current (AC) voltage;
generating an output voltage of the lock-in demodulation circuit by multiplying the voltage at the negative input to the differential amplifier by the pre-determined voltage, wherein the multiplication comprises applying lock-in demodulation to the voltage at the negative input to the differential amplifier; and
determining whether a proximity input to the touch sensor has occurred based at least in part on the output voltage.
2. The method of claim 1 , wherein the application of the pre-determined voltage comprises substantially simultaneously applying the pre-determined voltage to a plurality of second electrodes and a plurality of first electrodes, the plurality of second and first electrodes comprising all electrodes of the touch sensor.
3. The method of claim 1 , wherein applying lock-in demodulation comprises attenuating one or more components of the voltage at the capacitance out of phase with the pre-determined voltage.
4. The method of claim 1 , further comprising applying low-pass filtering to the output voltage.
5. A non-transitory computer-readable storage medium embodying logic configured when executed to:
couple a first electrode of a touch sensor and a corresponding second electrode of the touch sensor to a voltage source, wherein the first electrode is capacitively coupled to the second electrode;
apply, through the voltage source, a pre-determined voltage to the first electrode and also substantially simultaneously apply the pre-determined voltage to the corresponding second electrode, the application of the pre-determined voltage providing a measurement current at a capacitance comprising the first electrode wherein the measurement current results in a voltage at a negative input to a differential amplifier, and wherein the first electrode is oriented in a first orientation and the second electrode is oriented in a second orientation;
determine a difference between the measurement current at the capacitance and a reference value;
adjust in real time a voltage of a reference path coupled to a positive input of the differential amplifier such that a voltage at the positive input of the differential amplifier is substantially equal to the voltage at the negative input of the differential amplifier resulting from the measurement current;
toggle an input of a lock-in demodulation circuit between an output voltage of the differential amplifier and an inverse of the output voltage from the differential amplifier every half cycle of the pre-determined voltage, wherein the output voltage of the differential amplifier is a function of the difference between the measurement current at the capacitance and the reference value, and wherein the pre-determined voltage comprises an alternating-current (AC) voltage;
generate an output voltage of the lock-in demodulation circuit by multiplying the voltage at the negative input to the differential amplifier by the pre-determined voltage, wherein the multiplication comprises applying lock-in demodulation to the voltage at the negative input to the differential amplifier; and
determine whether a touch input to the touch sensor has occurred based at least in part on the output voltage.
6. The medium of claim 5 , wherein the software is further configured to substantially simultaneously apply the pre-determined voltage to a plurality of second electrodes and a plurality of first electrodes, the plurality of second and first electrodes comprising all electrodes of the touch sensor.
7. The medium of claim 5 , wherein the software is further configured to attenuate one or more components of the voltage at the capacitance out of phase with the pre-determined voltage.
8. The medium of claim 5 , wherein the software is further configured to apply low-pass filtering to the output voltage.
9. A device comprising:
a measurement circuit; and
a non-transitory computer-readable storage medium coupled to the measurement circuit and embodying logic configured when executed to:
couple a first electrode of a touch sensor and a corresponding second electrode of the touch sensor to a voltage source, wherein the first electrode is capacitively coupled to the second electrode;
apply, through the voltage source, a pre-determined voltage to the first electrode and also substantially simultaneously apply the pre-determined voltage to the corresponding second electrode, the application of the pre-determined voltage providing a measurement current at a capacitance comprising the first electrode, wherein the measurement current results in a voltage at a negative input to a differential amplifier, and wherein the first electrode is oriented in a first orientation and the second electrode is oriented in a second orientation;
determine a difference between the measurement current at the capacitance and a reference value;
adjust in real time a voltage of a reference path coupled to a positive input of the differential amplifier such that a voltage at the positive input of the differential amplifier is substantially equal to the voltage at the negative input of the differential amplifier resulting from the measurement current; and
toggle an input of a lock-in demodulation circuit between an output voltage of the differential amplifier and an inverse of the output voltage from the differential amplifier every half cycle of the pre-determined voltage, wherein the output voltage of the differential amplifier is a function of the difference between the measurement current at the capacitance and the reference value, and wherein the pre-determined voltage comprises an alternating-current (AC) voltage;
generate an output voltage of the lock-in demodulation circuit by multiplying the voltage at the negative input to the differential amplifier by the pre-determined voltage, wherein the multiplication comprises applying lock-in demodulation to the voltage at the negative input to the differential amplifier; and
determine whether a touch input to the touch sensor has occurred based at least in part on the output voltage.
10. The device of claim 9 , wherein the software is further configured to substantially simultaneously apply the pre-determined voltage to a plurality of second electrodes and a plurality of first electrodes, the plurality of second and first electrodes comprising all electrodes of the touch sensor.