Differential sensing for capacitive touch sensors
A first signal from a first sense line of a touch sensor is received. A second signal from a second of the touch sensor is received. The first signal is inverted. The inverted first signal and the second signal are summed to produce a differential signal. The differential signal is output to a touch sensor controller.
1. A touch sensor operable to sense a differential measurement between two sense lines, comprising:
a summing circuit connected to a touch sensor controller;
a first buffer that includes a first input connected to a first sense line of the touch sensor and a first output connected to the summing circuit;
a second buffer that includes a second input connected to a second sense line of the touch sensor and a second output connected to the first summing circuit; and
wherein the summing circuit is operable to:
receive a first capacitive measurement of the first sense line from the first buffer;
receive a second capacitive measurement of the second sense line from the second buffer; and
output a difference between the first capacitive measurement and the second capacitive measurement to the touch sensor controller.
2. The touch sensor of claim 1 , further comprising:
an inverter operable to invert one of the first output and the second output.
3. The touch sensor of claim 1 , wherein the summing circuit comprises a first summing circuit and the touch sensor further comprises:
a second summing circuit connected to the touch sensor controller, the second summing circuit operable to:
receive the first capacitive measurement of the first sense line from the first buffer;
receive the second capacitive measurement of the second sense line from the second buffer; and
output a difference between the second capacitive measurement and the first capacitive measurement.
4. The touch sensor of claim 1 , wherein the first sense line is separated from the second sense line by a plurality of sense lines.
5. The touch sensor of claim 1 , wherein a distance between the first sense line and the second sense line is sufficient such that a touch object does not simultaneously touch both the first sense line and the second sense line.
6. The touch sensor of claim 1 , wherein a signal conditioning circuit is connected between the summing circuit and the touch sensor controller, the signal conditioning circuit operable to:
apply a gain to the difference between the first capacitive measurement and the second capacitive measurement to produce a scaled difference; and
integrate the scaled difference and output the scaled difference to the touch screen controller.
7. The touch sensor of claim 1 , further comprising:
a plurality of amplifiers operable to apply a gain to the difference between the first capacitive measurement and the second capacitive measurement;
a plurality of switches operable to selectively apply the difference to one or more of the plurality of amplifiers.
8. A method, comprising:
receiving a first signal from a first sense line of a touch sensor;
receiving a second signal from a second sense line of the touch sensor;
inverting the first signal;
summing the inverted first signal and the second signal to produce a differential signal; and
outputting the differential signal to a touch sensor controller.
9. The method of claim 8 , further comprising
applying a scaling factor to the differential signal to produce a scaled differential signal;
wherein outputting the differential signal to the touch sensor controller comprises outputting the scaled differential signal to the touch sensor controller.
10. The method of claim 8 , wherein the touch sensor controller is operable to determine a location of a touch object proximate to the touch sensor based on the differential signal.
11. The method of claim 8 , wherein the first sense line is separated from the second sense line by a plurality of sense lines.
12. The method of claim 8 , wherein the first sense line and the second sense line are configured such that summing the inverted first signal and the second signal cancels noise on the first sense line and the second sense line.
13. The method of claim 8 , wherein the first sense line and the second sense line are configured such that summing the inverted first signal and the second signal cancels a common mode charge on the first sense line and the second sense line.
14. The method of claim 8 , the method further comprising:
selectively summing the inverted first signal with an output of one of a plurality of amplifiers, the amplifiers operable to scale the second signal; wherein the selection is made based at least in part on a characteristic of the touch sensor.
15. The method of claim 8 , the method further operable to:
in response to the touch sensor controller determining to measure a noise level on one of the first sense line and the second sense line, outputting the first signal and the second signal to the touch sensor controller.
16. A system comprising:
a touch sensor, operable to measure a differential signal that represents a difference between a first measurement of a first sense line and a second measurement of a second sense line, the touch sensor further comprising:
an inverting buffer operable to receive the first measurement from the first sense line and output an inverted first measurement; and
a summing circuit operable to receive the inverted first measurement and the second measurement and output the differential signal to the touch sensor controller; and
a touch sensor controller coupled to the touch sensor and operable to:
receive the differential signal from the touch sensor; and
determine a location of a touch object based on the differential signal.
17. The system of claim 16 , wherein the difference between the first measurement and the second measurement cancels a common mode charge on the first sense line and the second sense line.
18. The system of claim 16 , further comprising:
a signal conditioning circuit operable to apply a gain to the differential signal to produce a scaled differential signal; and
an analog-to-digital converter operable to digitize the scaled differential signal and output the digitized scaled differential signal to the touch sensor controller.
19. The system of claim 16 , wherein the touch sensor comprises a drive line and the touch sensor controller is further operable to:
transmit a pulse on the drive line; and measure the differential signal in response to transmitting the pulse.