Reduction of electromagnetic interference in a capacitive touchscreen system

• US 8,508,493 B2
• Filed: 06/21/2010
• Issued: 08/13/2013
• Est. Priority Date: 06/21/2010
• Status: Active Grant

• Current Assignee: PixArt Imaging Incorporated
• Original Assignee: PixArt Imaging Incorporated
• Inventors: Souchkov, Vitali
• Primary Examiner(s): PARDO, THUY N
• Application Number:
  US12/819,453
• Publication Number:
  US 20110310051A1
• Time in Patent Office: 1,149 Days
• Field of Search: 345/173, 345/174, 345/520
• US Class Current: 345/173
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

Disclosed herein are various embodiments of circuits and methods for reducing electromagnetic interference in mutual capacitance measurement or sensing systems, devices, components and methods such as capacitive touchscreens. Charge integrator circuits and switched capacitance filtering circuits are disclosed that improve the signal-to-noise ratio (or the ratio of a desired sensed mutual capacitance signal to an undesired EMI signal) in a capacitive sensor readout circuit without the need to increase the amplitude of the drive signal. The various embodiments of the charge integrator and switched capacitance filtering circuits described herein permit an improvement in noise immunity without requiring the excessive power levels typically associated with high amplitude drive circuitry, and moreover result in boosting signal-to-noise ratios during early stages of signal processing.

Claims

1. A capacitive touchscreen system, comprising:

a touchscreen comprising a first plurality of electrically conductive drive electrodes arranged in rows or columns, and a second plurality of electrically conductive sense electrodes arranged in rows or columns arranged at an angle with respect to the rows or columns of the first plurality of electrodes, mutual capacitances existing between the first and second pluralities of electrodes at locations where the first and second pluralities of electrodes intersect, such mutual capacitances changing in the presence of one or more fingers or touch devices brought into proximity thereto;

at least one drive circuit operably connected to the first plurality of electrodes and configured to provide drive signals of high and low states thereto;

a plurality of sense circuits, one each of the sense circuits being operably connected to corresponding ones of the second plurality of electrodes;

a system ground associated with the capacitive touchscreen system;

an electric field ground associated with the one or more fingers or touch devices and the touchscreen;

wherein each of the sense circuits comprises a charge integrator circuit and a switched capacitance filtering circuit, the charge integrator circuit being configured to receive input signals provided by a corresponding one of the second plurality of electrodes and to provide integrated signals to the switched capacitance filtering circuit, the switched capacitance filtering circuit being configured to sample and store in first and second capacitors, respectively, first and second integrated signals delivered thereto by the charge integrator circuit where the first integrated signal corresponds to the high state drive signal, and the second integrated signal corresponds to the low state drive signal, the switched capacitance filtering circuit being configured to provide an output signal representative of the mutual capacitance of the one of the second plurality of electrodes corresponding to the sense circuit and further having an electromagnetic interference voltage between the system ground and the electric field ground substantially filtered therefrom by the switched capacitance filtering circuit.

2. The capacitive touchscreen system of claim 1, wherein the first and second capacitors are electrically connected in series.

3. The capacitive touchscreen system of claim 2, wherein the switched capacitance filtering circuit comprises first and second input switches corresponding to the first and second capacitors, respectively.

4. The capacitive touchscreen system of claim 3, wherein the first input switch is configured to close and charge the first capacitor when the first integrated signal corresponding to the high state drive signal is delivered thereto while the second switch is open.

5. The capacitive touchscreen system of claim 4, wherein the second input switch is configured to close and charge the second capacitor when the second integrated signal corresponding to the low state drive signal is delivered thereto while the first switch is open.

6. The capacitive touchscreen system of claim 5, wherein a reference voltage is switchably and operably connected between the first and second capacitors by a third switch.

7. The capacitive touchscreen system of claim 6, wherein the third switch is configured to close to provide the reference voltage to the switched capacitance filtering circuit when the first or second switch is closed.

8. The capacitive touchscreen system of claim 7, wherein the reference voltage is switchably and operably connected to the second capacitor by a fourth switch, and the switched capacitance filtering circuit further comprises an output switch.

9. The capacitive touchscreen system of claim 8, wherein the fourth switch and the output switch are configured to close when the first and second switches are open, but after the first and second capacitors have been charged with the first and second integrated signals and the third switch has been closed and opened.

10. The capacitive touchscreen system of claim 1, wherein the switched capacitance filtering circuit comprises a plurality of pairs of first and second capacitors, each pair of first and second capacitors being electrically connected in series, the pairs being arranged in parallel with respect to one another.

11. The capacitive touchscreen system of claim 10, wherein only one pair of capacitors is charged during a single cycle of high and low drive states, and the pairs are charged with first and second integrated signals corresponding to sequential cycles of high and low drive states.

12. The capacitive touchscreen system of claim 11, wherein the output signal provided by the switched capacitance filtering circuit is representative of an averaged or enhanced signal-to-noise mutual capacitance.

13. A processor for a capacitive touchscreen system comprising a touchscreen comprising a first plurality of electrically conductive drive electrodes arranged in rows or columns, and a second plurality of electrically conductive, sense electrodes arranged in rows or columns arranged at an angle with respect to the rows or columns of the first plurality of electrodes, mutual capacitances existing between the first and second pluralities of electrodes at locations where the first and second pluralities of electrodes intersect, such mutual capacitances changing in the presence of one or more fingers or touch devices brought into proximity thereto, a system ground being associated with the capacitive touchscreen system, an electric field ground being associated with the one or more fingers or touch devices and the touchscreen, the processor comprising:

at least one drive circuit operably connected to the first plurality of electrodes and configured to provide drive signals of high and low states thereto;

a plurality of sense circuits, one each of the sense circuits being operably connected to corresponding ones of the second plurality of electrodes;

wherein each of the sense circuits comprises a charge integrator circuit and a switched capacitance filtering circuit, the charge integrator circuit being configured to receive input signals provided by a corresponding one of the second plurality of electrodes and to provide integrated signals to the switched capacitance filtering circuit, the switched capacitance filtering circuit being configured to sample and store in first and second capacitors, respectively, first and second integrated signals delivered thereto by the charge integrator circuit where the first integrated signal corresponds to the high state drive signal, and the second integrated signal corresponds to the low state drive signal, the switched capacitance filtering circuit being configured to provide an output signal representative of the mutual capacitance of the one of the second plurality of electrodes corresponding to the sense circuit and further having an electromagnetic interference voltage between the system ground and the electric field ground substantially filtered therefrom by the twitched capacitance filtering circuit.

14. The processor of claim 13, wherein the first and second capacitors are electrically connected in series.

15. The processor of claim 14, wherein the switched capacitance filtering circuit comprises first and second input switches, corresponding to the first and second capacitors, respectively.

16. The processor of claim 15, wherein the first input switch is configured to close and charge the first capacitor when the first integrated signal corresponding to the high state drive signal is delivered thereto while the second switch is open.

17. The processor of claim 16, wherein the second input switch is configured to close and charge the second capacitor when the second integrated signal corresponding to the low state drive signal is delivered thereto while the first switch is open.

18. The processor of claim 17, wherein a reference voltage is switchably and operably connected between the first and second capacitors by a third switch.

19. The processor of claim 18, wherein the third switch is configured to close to provide the reference voltage to the switched capacitance filtering circuit when the first or second switch is closed.

20. The processor of claim 19, wherein the reference voltage is switchably and operably connected to the second capacitor by a fourth switch, and the switched capacitance filtering circuit further comprises an output switch.

21. The processor of claim 20, wherein the fourth switch and the output switch are configured to close when the first and second switches are open, but after the first and second capacitors have been charged with the first and second integrated signals and the third switch has been closed and opened.

22. The processor of claim 13, wherein the switched capacitance filtering circuit comprises a plurality of pairs of first and second capacitors, each pair of first and second capacitors being electrically connected in series, the pairs being arranged in parallel with respect to one another.

23. The processor of claim 22, wherein only one pair of capacitors is charged during a single cycle of high and low drive states, and the pairs are charged with first and second integrated signals corresponding to sequential cycles of high and low drive states.

24. The processor of claim 23, wherein the output signal provided by the switched capacitance filtering circuit is representative of an averaged or enhanced signal-to-noise mutual capacitance.

25. The processor of claim 13, further comprising a reset circuit configured to drain charge remaining on each of the second plurality of electrodes after a sense cycle has been completed.

26. The processor of claim 13, wherein the processor is an integrated circuit.

27. The processor of claim 26, wherein the integrated circuit is one of a microprocessor, a controller or an application specific integrated circuit (ASIC).

28. The processor of claim 26, wherein the integrated circuit is formed using a CMOS or BiCMOS process.

29. A method of reducing electromagnetic interference in a capacitive touchscreen system comprising a touchscreen comprising a first plurality of electrically conductive drive electrodes arranged in rows or columns, and a second plurality of electrically conductive sense electrodes arranged in rows or columns arranged at an angle with respect to the rows or columns of the first plurality of electrodes, mutual capacitances existing between the first and second pluralities of electrodes at locations where the first and second pluralities of electrodes intersect, such mutual capacitances changing in the presence of one or more fingers or touch devices brought into proximity thereto, a system ground being associated with the capacitive touchscreen system, an electric field ground being associated with the one or more fingers or touch devices and the touchscreen, the method comprising:

sequentially driving the first plurality of electrodes with alternating high and low state drive signals;

sensing the mutual capacitances associated with each of the second plurality of electrodes during the high and low state drive signals with a charge integrator circuit for each sense circuit;

for each charge integrator circuit, providing first and second integrated signals corresponding to the high and low state drive signals, respectively, to a switched capacitance filtering circuit;

sampling and storing in first and second capacitors, respectively, of the switched capacitance filtering circuit the first and second integrated signals;

providing, with the switched capacitance filtering circuit, an output signal representative of the mutual capacitance of the one of the second plurality of electrodes corresponding to the sense circuit and further having an electromagnetic interference voltage between the system ground and the electric field ground substantially filtered therefrom by the switched capacitance filtering circuit.

30. The method of claim 29, wherein the first and second capacitors are electrically connected in series.

31. The method of claim 30, wherein the switched capacitance filtering circuit comprises first and second input switches corresponding to the first and second capacitors, respectively.

32. The method of claim 31, wherein the first input switch closes and charges the first capacitor when the first integrated signal corresponding to the high state drive signal is delivered thereto while the second switch is open.

33. The method of claim 32, wherein the second input switch closes and charges the second capacitor when the second integrated signal corresponding to the low state drive signal is delivered thereto while the first switch is open.

34. The method of claim 33, wherein a reference voltage is switchably and operably connected between the first and second capacitors by a third switch.

35. The method of claim 34, wherein the third switch closes to provide the reference voltage to the switched capacitance filtering circuit when the first or second switch is closed.

36. The method of claim 35, wherein the reference voltage is switchably and operably connected to the second capacitor by a fourth switch, and the switched capacitance filtering circuit further comprises an output switch.

37. The method of claim 36, wherein the fourth switch and the output switch close when the first and second switches are open, but after the first and second capacitors have been charged with the first and second integrated signals and the third switch has been closed and opened.

38. The method of claim 29, wherein the switched capacitance filtering circuit comprises a plurality of pairs of first and second capacitors, each pair of first and second capacitors being electrically connected in series, the pairs being arranged in parallel with respect to one another.

39. The method of claim 38, wherein only one pair of capacitors is charged during a single cycle of high and low drive states, and the pairs are charged with first and second integrated signals corresponding to sequential cycles of high and low drive states.

40. The method of claim 39, wherein the output signal provided by the switched capacitance filtering circuit is representative of an averaged or enhanced signal-to-noise mutual capacitance.