Active canceling of display noise in simultaneous display and touch sensing using an impulse response
Techniques for removing display-based corrupting components from a capacitive sensing signal when display and capacitive sensing is performed at or nearly at the same time. A routing carrying display related signals (e.g., a source signal for sub-pixel updating) may induce a corrupting current into a routing for carrying capacitive sensing signals. This corrupting current would reduce the ability to determine presence of an input object via the sensing signal. Therefore, the corrupting signal is effectively removed from a signal received from a sensor electrode when driven for sensing. The corrupting component may be removed either in analog or digitally. In analog, the corrupting component is removed via a tunable capacitance. Digitally, a corrupting component is calculated as an impulse response convolved with the source driver voltage. This corrupting component is then subtracted from a digital value output by the sensing circuitry to obtain a “clean” value.
1. A processing system configured for simultaneously updating a display and performing capacitive sensing, the processing system comprising:
a source driver configured to:
drive a source line with a reference voltage during a first time period in which a sensor electrode is not driven for capacitive sensing to produce first resulting signals on a routing trace coupled to the sensor electrode; and
drive the source line with a source driver signal during a second time period to produce second resulting signals on the routing trace during a third time period that at least partially overlaps with the second time period, the routing trace and the source line being routed proximate each other;
an analog-to-digital converter configured to:
receive the first resulting signals and generate a first digital signal representing the first resulting signals; and
receive the second resulting signals and generate a second digital signal representing the second resulting signals; and
an interference remover configured to remove source driver effects in the second resulting signals that exist due to driving the source line with the source driver signal by:
determining at least one impulse response, associated with receiver circuitry configured to process the first resulting signals, based on the first digital signal;
receiving the second digital signal;
calculating an interference value based on the source driver signal and the determined at least one impulse response; and
subtracting the calculated interference value from the second digital signal.
2. The processing system of claim 1 , wherein the interference remover is configured to calculate the interference value by:
convolving the determined at least one impulse response with the source driver signal.
3. The processing system of claim 1 , wherein:
the interference remover is configured to:
determine a second impulse response associated with receiver circuitry configured to process fourth resulting signals produced by a second source driver signal applied on a second source line, proximate the routing trace; and
calculate the interference value further based on the second source driver signal and the determined second impulse response.
4. The processing system of claim 1 , wherein:
the source line and the routing trace are disposed in a common layer.
5. The processing system of claim 1 , wherein:
the source line and the routing trace are disposed in separate layers; and
the source line is parallel to the routing trace.
6. The processing system of claim 1 , wherein the interference remover includes:
a capacitor.
7. The processing system of claim 6 , wherein the interference remover further comprises:
a digital-to-analog converter configured to: scale and negate a voltage associated with the source driver signal to generate a scaled signal; and apply the scaled signal to the capacitor to generate the interference value.
8. An input device configured for simultaneously updating a display and performing capacitive sensing, the input device comprising:
a sensor electrode coupled to a routing trace;
a display element coupled to a source line, the source line and the routing trace being routed proximate each other; and
a processing system coupled to the routing trace and the source line, the processing system comprising:
a source driver configured to:
drive the source line with a reference voltage during a first time period in which the sensor electrode is not driven for capacitive sensing to produce first resulting signals on the routing trace; and
drive the source line with a source driver signal during a second time period to produce second resulting signals on the routing trace during a third time period that at least partially overlaps with the second time period,
an analog-to-digital converter configured to:
receive the first resulting signals and generate a first digital signal representing the resulting signals; and
receive the second resulting signals and generate a second digital signal representing the second resulting signals; and
an interference remover configured to remove source driver effects in the second resulting signals that exist due to driving the source line with the source driver signal by:
determining at least one impulse response, associated with receiver circuitry configured to process the first resulting signals, based on the first digital signal;
receiving the second digital signal;
calculating an interference value based on the source driver signal and the determined at least one impulse response; and
subtracting the calculated interference value from the second digital signal.
9. The input device of claim 8 , wherein the interference remover is configured to calculate the interference value by:
convolving the determined at least one impulse response with the source driver signal.
10. The input device of claim 8 , wherein the interference remover is configured to:
determine a second impulse response associated with receiver circuitry configured to process fourth resulting signals produced by a second source driver signal applied on a second source line, proximate the routing trace; and
calculate the interference value further based on the second source driver signal and the determined second impulse response.
11. The input device of claim 8 , wherein:
the source line and the routing trace are disposed in a common layer.
12. The input device of claim 8 , wherein:
the source line and the routing trace are disposed in separate layers; and
the source line is parallel to the routing trace.
13. The input device of claim 8 , wherein the interference remover includes:
a capacitor.
14. The input device of claim 13 , wherein the interference remover further comprises:
a digital-to-analog converter configured to: scale and negate a voltage associated with the source driver signal to generate a scaled signal; and apply the scaled signal to the capacitor to generate the interference value.
15. A method for simultaneously updating a display and performing capacitive sensing, the method comprising:
driving a source line with a reference voltage during a first time period in which a sensor electrode is not driven for capacitive sensing to produce first resulting signals on a routing trace coupled to the sensor electrode, the routing trace and the source line being routed proximate each other;
driving the source line with a source driver signal during a second time period to produce second resulting signals on the routing trace during a third time period that at least partially overlaps with the second time period;
receiving the first resulting signals;
generating a first digital signal representing the first resulting signals;
determining at least one impulse response, associated with receiver circuitry configured to process the first resulting signals, based on the first digital signal;
receiving the second resulting signals;
generating a second digital signal representing the second resulting signals; and
removing source driver effects in the second resulting signals that exist due to driving the source line with the source driver signal by:
receiving the second digital signal;
calculating an interference value based on the source driver signal and the determined at least one impulse response; and
subtracting the calculated interference value from the second digital signal.
16. The processing system of claim 1 , wherein:
the reference voltage comprises a step signal, and
determining the at least one impulse response comprises calculating a derivative of the first digital signal.