Calibrating a continuous-time receiver for capacitive sensing
Embodiments herein describe input devices that include receivers for sampling capacitive sensing signals. In one embodiment, the receivers perform continuous demodulation where the sampling of the capacitive sensing signal is not synchronized with the modulated signal applied to the sensor. To calibrate, the receiver generates first and second measurements of the capacitive sensing signal when driving a modulated signal onto one or more sensor electrodes during two respective time periods. However, the phase of at least one signal is controlled during the time periods so that the first and second measurements have a phase difference of ninety degrees. Using the first and second measurements, the receiver can determine a phase offset between the capacitive sensing signal and the modulated signal which can be used to alter future measurements so that at least some of these measurements are captured when the capacitive sensing signal is at a peak amplitude.
1. An input device, comprising:
a plurality of sensor electrodes in a sensing region of the input device; and
a processing system coupled to the plurality of sensor electrodes, the processing system configured to:
generate a first measurement of a capacitive sensing signal acquired using a first sensor electrode of the plurality of sensor electrodes during a first time period, wherein the capacitive sensing signal comprises effects of a first modulated signal driven onto at least one of the plurality of sensor electrodes;
generate a second measurement of the capacitive sensing signal acquired using the first sensor electrode during a second time period, wherein a phase difference between the first and second measurements is ninety degrees; and
determine a phase offset between the capacitive sensing signal and the first modulated signal based on the first and second measurements; and
phase shift a sampling signal used to generate a third measurement of the capacitive sensing signal acquired using the first sensor electrode by the phase offset,
wherein phase shifting the sampling signal by the phase offset aligns the third measurement with a peak amplitude of the capacitive sensing signal, and
wherein the capacitive sensing signal has the same shape as the first modulated signal but with different corresponding amplitudes and phase delays.
2. The input device of claim 1 , wherein the processing system is configured to, during the second time period:
phase shift a second sampling signal used to generate the second measurement by ninety degrees relative to a phase of a first sampling signal used to generate the first measurement during the first time period.
3. The input device of claim 1 , wherein the processing system is configured to, during the second time period:
phase shift a second modulated signal by ninety degrees relative to a phase of the first modulated signal used during the first time period.
4. The input device of claim 1 , wherein the first and second time periods correspond to one of: consecutive sensing bursts in a same capacitive sensing frame and consecutive capacitive sensing frames.
5. The input device of claim 1 , wherein the processing system is configured to:
phase shift one of the sampling signal and the first modulated signal to identify an extrapolation error when generating a fourth measurement of the capacitive sensing signal acquired using the first sensor electrode during a third time period, wherein the third time period occurs after the first and second time periods; and
update the phase offset based on the extrapolation error.
6. The input device of claim 1 , wherein the processing system includes continuous-time receiver for generating the first and second measurement, wherein the first modulated signal is not synchronized with a sampling signal used to determine when the receiver generates the first and second measurements.
7. A processing system, comprising:
sensor circuitry configured to drive a first modulated signal onto at least one of a plurality of sensor electrodes for capacitive sensing;
a receiver configured to:
generate a first measurement of a capacitive sensing signal acquired using a first sensor electrode of the plurality of sensor electrodes during a first time period, wherein the capacitive sensing signal comprises effects of the first modulated signal being driven onto the at least one sensor electrode;
generate a second measurement of the capacitive sensing signal acquired using the first sensor electrode during a second time period, wherein a phase difference between the first and second measurements is ninety degrees; and
determine a phase offset between the capacitive sensing signal and the first modulated signal based on the first and second measurements; and
phase shift a sampling signal used to generate a third measurement of the capacitive sensing signal acquired using the first sensor electrode by the phase offset,
wherein phase shifting the sampling signal by the phase offset aligns the third measurement with a peak amplitude of the capacitive sensing signal, and
wherein the capacitive sensing signal has the same shape as the first modulated signal but with different corresponding amplitudes and phase delays.
8. The processing system of claim 7 , wherein the receiver is configured to, during the second time period:
phase shift a second sampling signal used to generate the second measurement by ninety degrees relative to a phase of a first sampling signal used to generate the first measurement during the first time period.
9. The processing system of claim 7 , wherein the sensor circuitry is configured to, during the second time period:
phase shift a second modulated signal by ninety degrees relative to a phase of the first modulated signal used during the first time period.
10. The processing system of claim 7 , wherein the first and second time periods correspond to one of: consecutive sensing bursts in a same capacitive sensing frame and consecutive capacitive sensing frames.
11. The processing system of claim 7 , wherein the receiver is configured to:
phase shift the sampling signal to identify an extrapolation error when generating a fourth measurement of the capacitive sensing signal acquired using the first sensor electrode during a third time period, wherein the third time period occurs after the first and second time periods; and
update the phase offset based on the extrapolation error.
12. The processing system of claim 7 , wherein the receiver is a continuous-time receiver, wherein the first modulated signal is not synchronized with a sampling signal used to determine when the receiver generates the first and second measurements.
13. The processing system of claim 7 , wherein the receiver is configured to:
generate a plurality of first measurements for each of the plurality of sensor electrodes when driving a plurality of modulated signals onto the plurality of sensor electrodes during the first time period;
generate a plurality of second measurements for each of the plurality of sensor electrodes during the second time period, wherein respective phase differences between the plurality of first and second measurements for each of the plurality of sensor electrodes is ninety degrees; and
determine respective phase offsets between a plurality of sensing signals acquired using the plurality of sensor electrodes and the plurality of modulated signals based on the plurality of first and second measurements.
14. A method for performing capacitive sensing, the method comprising:
generating a first measurement of a capacitive sensing signal acquired using a first sensor electrode of a plurality of sensor electrodes during a first time period, wherein the capacitive sensing signal comprises effects of a first modulated signal driven onto at least one of the plurality of sensor electrodes;
generating a second measurement of the capacitive sensing signal acquired using the first sensor electrode during a second time period, wherein a phase difference between the first and second measurements is ninety degrees;
determining a phase offset between the capacitive sensing signal and the first modulated signal based on the first and second measurements; and
phase shifting a sampling signal used to generate a third measurement of the capacitive sensing signal acquired using the first sensor electrode by the phase offset,
wherein phase shifting the sampling signal by the phase offset aligns the third measurement with a peak amplitude of the capacitive sensing signal, and
wherein the capacitive sensing signal has the same shape as the first modulated signal but with different corresponding amplitudes and phase delays.
15. The method of claim 14 , further comprising:
phase shifting during the second time period a second sampling signal used to generate the second measurement by ninety degrees relative to a phase of a first sampling signal used to generate the first measurement during the first time period.
16. The method of claim 14 , further comprising:
phase shifting during the second time period a second modulated signal by ninety degrees relative to a phase of the first modulated signal used during the first time period.
17. The method of claim 14 , wherein the first and second time periods correspond to one of: consecutive sensing bursts in a same capacitive sensing frame and consecutive capacitive sensing frames.