CAPACITIVE SENSING ANALOG FRONT END

• US 20110242048A1
• Filed: 12/17/2010
• Published: 10/06/2011
• Est. Priority Date: 03/31/2010
• Status: Active Grant

• Current Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Incorporated (STMicroelectronics NV)
• Inventors: GUEDON, Yannick, Tan, Kien Beng, Guo, Dianbo, Ningrat, Kusuma Adi
• Granted Patent: US 9,244,569 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes

Abstract

A capacitive sensing analog front end for a touchscreen system having an improved signal-to-noise ratio includes a capacitance-to-voltage converter having an input for coupling to an external sampling capacitor, a summer having a first input coupled to an output of the capacitance-to-voltage converter, a low pass filter having an input coupled to an output of the summer and an output for providing an output signal; and a sample-and-hold circuit having an input coupled to the output of the low pass filter and an output coupled to a second input of the summer. The signal-to-noise ratio of the touchscreen system is improved by extracting the DC shift of a touch signal during a monitoring period and then subtracting the DC shift before integrating the touch signal.

Claims

1. A capacitive sensing analog front end for a touchscreen system having an improved signal-to-noise ratio, comprising:

a capacitance-to-voltage converter having an input for coupling to an external sampling capacitor;

a summer having a first input coupled to an output of the capacitance-to-voltage converter;

a low pass filter having an input coupled to an output of the summer and an output for providing an output signal; and

a sample-and-hold circuit having an input coupled to the output of the low pass filter and an output coupled to a second input of the summer.

2. The front end of claim 1 wherein the first input of the summer comprises a positive input.

3. The front end of claim 1 wherein the second input of the summer comprises a negative input.

4. The front end of claim 1 wherein the low pass filter comprises an RC filter.

5. The front end of claim 1 wherein the low pass filter comprises an RC integrator.

6. A method of operating a capacitive touchscreen system comprising extracting the DC shift of a touch signal during a monitoring period and then subtracting the DC shift before integrating the touch signal.

7. A capacitive sensing analog front end for a touchscreen system having an improved signal-to-noise ratio, comprising:

a switch matrix having inputs for coupling to an external sampling capacitor and first and second outputs;

first and second switches coupled to the first output of the switch matrix;

third and fourth switches coupled to the second output of the switch matrix;

a charge amplifier having an input coupled to the third switch;

a low pass filter having an input coupled to an output of the charge amplifier; and

a sample-and-hold circuit having an input selectively coupled to the output of the low pass filter and an output for selectively providing an output signal.

8. The front end of claim 7 wherein the first switch is coupled between the first output of the switch matrix and ground.

9. The front end of claim 7 wherein the second switch is coupled between the first output of the switch matrix and VDD.

10. The front end of claim 7 wherein the third switch is coupled between the second output of the switch matrix and the input of the charge amplifier.

11. The front end of claim 7 wherein the fourth switch is coupled between the second output of the switch matrix and a reference voltage.

12. The front end of claim 7 further comprising a fifth switch for switching a hold capacitor associated with the charge amplifier.

13. The front end of claim 7 further comprising a sixth switch coupled between the output of the low pass filter and the input of the sample-and-hold circuit.

14. The front end of claim 7 wherein the low pass filter comprises an RC filter.

15. The front end of claim 7 wherein the low pass filter comprises an RC integrator.

16. The front end of claim 7 comprising first, second, and third operational modes.

17. The front end of claim 7 wherein the second and fourth switches are closed during the first operational mode.

18. The front end of claim 7 wherein the third switch is closed during a second operational mode.

19. The front end of claim 7 wherein the first switch and the third switch are closed during a third operational mode.

20. The front end of claim 7 wherein the switches are operated such that the DC shift of a touch signal associated with the external sampling capacitor is extracted during a monitoring period and then subtracting the DC shift before integrating the touch signal.