Mitigating interference in a capacitive sensing device
An example method of capacitive sensing includes: transmitting a first waveform over a first time period; transmitting a second waveform over a second time period, wherein the second waveform is independent of the first waveform, and wherein at least a portion of the second time period does not overlap the first time period; receiving, from sensor electrodes, a first resulting signal in response to capacitive coupling of the first waveform and a second resulting signal in response to capacitive coupling of the second waveform; and processing the second resulting signal over at least a portion of the first time period, and the second time period, to obtain independent capacitive measurements.
1. A method of capacitive sensing comprising:
transmitting a first waveform over a first transmitter period;
transmitting a second waveform over a second transmitter period, wherein the second waveform is independent of the first waveform, and wherein at least a portion of the second transmitter period does not overlap the first transmitter period;
receiving, from sensor electrodes, a first resulting signal in response to capacitive coupling of the first waveform and a second resulting signal in response to capacitive coupling of the second waveform; and
filtering the second resulting signal using a first finite impulse response (FIR) filter having an impulse response that spans at least a portion of the first transmitter period and the second transmitter period to obtain independent capacitive measurements.
2. The method of claim 1 , wherein no portion of the second transmitter period overlaps the first transmitter period.
3. The method of claim 2 , wherein the first and second waveforms are transmitted in sequence using a single transmitter.
4. The method of claim 1 , wherein the first waveform is transmitted using a first transmitter and the second waveform is transmitted using a second transmitter.
5. The method of claim 1 , wherein the first waveform comprises an in-phase waveform and the second waveform comprises a quadrature waveform having a phase that is substantially orthogonal to a phase of the in-phase waveform.
6. The method of claim 1 , further comprising:
filtering the first resulting signal using a second FIR filter having an impulse response that spans the first transmitter period and at least a portion of the second transmitter period, to obtain another capacitive measurement.
7. The method of claim 1 , further comprising:
transmitting a third waveform over a third transmitter period, wherein the second waveform is independent of the third waveform, and wherein at least another portion of the first transmitter period does not overlap the third transmitter period; and
receiving, from an additional sensor electrode, a third resulting signal in response to capacitive coupling of the third waveform, wherein the impulse response of the first FIR filter spans the at least a portion of the first transmitter period, the second transmitter period, and at least a portion of the third transmitter period.
8. The method of claim 1 , wherein the first FIR filter comprises a triangle FIR filter.
9. The method of claim 8 , wherein a weight of the first FIR filter is greatest during the portion of the second transmitter period that does not overlap the first transmitter period.
10. An input device comprising:
a plurality of sensor electrodes; and
a processing system coupled to the plurality of sensor electrodes, the processing system comprising:
at least one transmitter configured to transmit a first waveform over a first transmitter period and a second waveform over a second transmitter period, wherein the second waveform is independent of the first waveform, and wherein at least a portion of the second transmitter period does not overlap the first transmitter period;
a first receiver configured to receive a first resulting signal from a first sensor electrode of the plurality of sensor electrodes in response to capacitive coupling of the first waveform to the first sensor electrode;
a second receiver configured to receive a second resulting signal from a second sensor electrode of the plurality of sensor electrodes in response to a capacitive coupling of the second waveform to the second sensor electrode; and
a first finite impulse response (FIR) filter configured to filter the second resulting signal, wherein an impulse response of the first FIR filter spans at least a portion of the first transmitter period and the second transmitter period, to obtain independent capacitive measurements.
11. The input device of claim 10 , wherein no portion of the second transmitter period overlaps the first transmitter period.
12. The input device of claim 11 , wherein the at least one transmitter comprises a single transmitter that transmits the first and second waveforms in sequence.
13. The input device of claim 10 , further comprising a second FIR filter configured to filter the first resulting signal, wherein an impulse response of the second FIR filter spans the first transmitter period and at least a portion of the second transmitter period, to obtain another capacitive measurement.
14. The input device of claim 10 , wherein the first FIR filter comprises a triangle FIR filter.
15. The input device of claim 14 , wherein a weight of the first FIR filter is greatest during the portion of the second transmitter period that does not overlap the first transmitter period.
16. A processing system for a capacitive sensing device, comprising:
at least one transmitter configured to transmit a first waveform over a first transmitter period and a second waveform over a second transmitter period, wherein the second waveform is independent of the first waveform, and wherein at least a portion of the second transmitter period does not overlap the first transmitter period;
a first receiver configured to receive a first resulting signal from a first sensor electrode in response to capacitive coupling of the first waveform to the first sensor electrode;
a second receiver configured to receive a second resulting signal from a second sensor electrode in response to a capacitive coupling of the second waveform to the second sensor electrode; and
a finite impulse response (FIR) filter configured to filter the second resulting signal, wherein an impulse response of the FIR filter spans at least a portion of the first transmitter period and the second transmitter period, to obtain independent capacitive measurements.
17. The processing system of claim 16 , wherein no portion of the second transmitter period overlaps the first transmitter period.
18. The processing system of claim 17 , wherein the at least one transmitter comprises a single transmitter that transmits the first and second waveforms in sequence.
19. The processing system of claim 16 , wherein the FIR filter comprises a triangle FIR filter.
20. The processing system of claim 19 , wherein a weight of the FIR filter is greatest during the portion of the second transmitter period that does not overlap the first transmitter period.