Pressure sensing apparatus and method

1. An apparatus for processing signals from a touch panel, the touch panel comprising a layer of piezoelectric material disposed between a plurality of sensing electrodes and at least one common electrode, the apparatus comprising:

a capacitive touch controller for connection to the plurality of sensing electrodes;

a switch network comprising a plurality of inputs for connection to some or all of the plurality of sensing electrodes, and an output connected to system ground or a common mode voltage;

a second circuit for connection to the at least one common electrode and configured to generate, based on signals received from the at least one common electrode, a second pressure signal indicative of a total pressure applied to the touch panel;

a controller configured to:

control the switch network to couple any connected sensing electrodes to the system ground or the common mode voltage during a pressure measurement period;

determine an estimate of the total pressure based on the second pressure signal during the pressure measurement period.

2. The apparatus according to claim 1 , wherein the second circuit is an internal charge amplifier of the capacitive touch controller.

3. The Apparatus according to claim 2 , wherein the apparatus comprises a third switch network configured to connect the internal charge amplifier to a sensing electrode during a capacitance measurement period, and to connect the internal charge amplifier to the at least one common electrode during the pressure measurement period.

4. The apparatus according to claim 3 , wherein the third switch network is further configured to connect the at least one common electrode to the system ground or the common mode voltage during the capacitance measurement period, and to connect the sensing electrode to the system ground or the common mode voltage during the pressure measurement period.

5. The apparatus according to claim 1 , further comprising a common electrode switch configured to connect the at least one common electrode to the system ground or the common mode voltage during a capacitance measurement period, and to connect the at least one common electrode to the second circuit during the pressure measurement period.

6. The apparatus according to claim 1 , wherein the switch network is configured for connection to a subset of the sensing electrodes, the apparatus further comprising:

a first circuit for connection to sensing electrodes not included in the subset, the first circuit configured to generate a first pressure signal indicative of a pressure acting on the touch panel proximate to the connected sensing electrodes;

wherein the controller is configured to determine the estimate of the total pressure based on a weighted difference of the second pressure signal and the first pressure signal.

7. The apparatus according to claim 1 , wherein the capacitive touch controller is configured for connection to each sensing electrode via a corresponding first switch;

wherein each input of the switch network is connected to the system ground or the common mode voltage by a corresponding second switch;

wherein the second circuit comprises a common electrode charge amplifier configured to generate the second pressure signal;

wherein the controller is configured to control the first and second switches to:

close the first switches and open the second switches during a capacitance measurement period;

open the first switches and close the second switches during the pressure measurement period.

8. The apparatus according to claim 7 , wherein the common electrode charge amplifier comprises an operational amplifier having a feedback network connected in parallel with a third switch;

wherein the controller is further configured to control the third switch to close during one or more reset periods during the pressure measurement period.

9. The apparatus according to claim 1 , wherein the capacitive touch controller is configured to generate, for each sensing electrode, a capacitance signal indicative of a capacitance of the sensing electrode;

wherein the controller or the capacitive touch controller is configured to determine a location at which pressure is applied to the touch panel based on the capacitance signals.

10. A touch panel system comprising:

the apparatus according to claim 1 ; and

the touch panel comprising the layer of piezoelectric material disposed between the plurality of sensing electrodes and the at least one common electrode.

11. An electronic device comprising the touch panel system according to claim 10 .

12. A method of processing signals from a touch panel, the touch panel comprising a layer of piezoelectric material disposed between a plurality of sensing electrodes and at least one common electrode, wherein the plurality of sensing electrodes are connected to a capacitive touch controller and the at least one common electrode is connected to a second circuit, the method comprising:

coupling, using a switch network, some or all of the plurality of sensing electrodes to system ground or a common mode voltage during a pressure measurement period;

generating, using the second circuit, a second pressure signal indicative of a total pressure applied to the touch panel, the second pressure signal based on signals received from the at least one common electrode;

determining, using a controller, an estimate of the total pressure based on the second pressure signal during the pressure measurement period.

13. The method according to claim 12 , wherein the second circuit is an internal charge amplifier of the capacitive touch controller.

14. The method according to claim 13 , further comprising:

connecting, using a third switch network, the internal charge amplifier to a sensing electrode during a capacitance measurement period;

connecting, using the third switch network, the internal charge amplifier to the at least one common electrode during the pressure measurement period.

15. The method according to claim 14 , further comprising:

connecting, using the third switch network, the at least one common electrode to the system ground or the common mode voltage during the capacitance measurement period;

connecting, using the third switch network, the sensing electrode to the system ground or the common mode voltage during the pressure measurement period.

16. The method according to claim 12 , further comprising:

connecting, using a common electrode switch, the at least one common electrode to the system ground or the common mode voltage during a capacitance measurement period;

connecting, using the common electrode switch, the at least one common electrode to the second circuit during the pressure measurement period.

17. The method according to claim 12 , wherein the switch network is connected to a subset of the plurality of sensing electrodes, the method further comprising:

generating, using a first circuit connected to sensing electrodes not included in the subset, a first pressure signal indicative of a pressure acting on the touch panel proximate to the connected sensing electrodes;

wherein determining the estimate of the total pressure comprises determining the estimate of the total pressure based on a weighted difference of the second pressure signal and the first pressure signal.

18. The method according to claim 12 , wherein each sensing electrode is connected to the capacitive touch controller by a corresponding first switch, wherein the switch network comprises a plurality of second switches, each second switch connecting a sensing electrode to the system ground or the common mode voltage, and wherein the second circuit comprises a common electrode charge amplifier configured to generate the second pressure signal, the method further comprising:

using the controller to control the first and second switches to:

close the first switches and open the second switches during a capacitance measurement period;

open the first switches and close the second switches during the pressure measurement period.

19. The method according to claim 18 , wherein the common electrode charge amplifier comprises an operational amplifier having a feedback network connected in parallel with a third switch, the method further comprising:

using the controller to control the third switch to close during one or more reset periods during the pressure measurement period.

20. The method according to claim 12 , further comprising:

generating for each sensing electrode, using the capacitive touch controller, a capacitance signal indicative of a capacitance of the sensing electrode;

determining, using the controller or the capacitive touch controller, a location at which pressure is applied to the touch panel based on the capacitance signals.