CAPACITIVE TOUCH SENSING SYSTEM WITH INTERFERENCE REJECTION

• US 20140176482A1
• Filed: 01/30/2013
• Published: 06/26/2014
• Est. Priority Date: 12/20/2012
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Kwan, Tom W., Wei, Guo Wen
• Granted Patent: US 8,982,093 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

An interference tolerant capacitive touch sensor readout circuit having improved power and area efficiency is disclosed. Interference rejection for the capacitive touch sensing system is realized by transferring charge between a capacitive touch sensor and the readout circuit at frequencies outside bands where a level of interference is unacceptable. Improved power and area efficient come from the simplicity of the readout circuit which comprises a switched-capacitor integrator, a comparator, a digital accumulator and number of switches for driving a touch sensor and a capacitive feedback loop. The readout circuit is capable of interfacing with both self and mutual capacitance sensor to achieve compatibility with a larger collection of sensors and provides additional sensing and diagnostic functionalities.

Claims

1. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, the method comprising:

generating a driving signal, the driving signal driving the capacitive touch sensor during the conversion cycle;

alternately storing charge transferred from the capacitive touch sensor to a switched-capacitor integrator during the conversion cycle; and

recording total charge accumulated in the switched-capacitor integrator during the conversion cycle.

2. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 1, wherein the recording step further includes detecting an amount of charge stored in at least one capacitor of the switched-capacitor integrator by comparing the switched-capacitor integrator output voltage to a threshold level.

3. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 2, wherein the recording step further includes removing charge from the switched-capacitor integrator using a feedback loop after charge stored in the at least one capacitor of the switched-capacitor integrator exceeds a predetermined level.

4. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 3, wherein the recording step further includes accumulating the number of times charge is removed using the feedback loop.

5. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 1, wherein the driving signal switches in one fixed or multiple time-multiplexed patterns for modulating current flow from the capacitive touch sensor to the switched-capacitor integrator.

6. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 5, wherein the driving signal is tuned to avoid detected in-band interference by operating at one fixed or multiple time-multiplexed frequencies outside bands where the interference level is unacceptable.

7. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 6, wherein interference signals located outside of the pass-band of the tuned driving signal are attenuated.

8. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 1, wherein charge transferred from the capacitive touch sensor to the switched-capacitor integrator is stored in one of two capacitors of the switched-capacitor integrator depending on the positive/negative direction of current flow from the capacitive touch sensor to the switched-capacitor integrator

9. A method of sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 1, wherein the capacitive touch sensor is either a mutual capacitance sensor or a self-capacitance sensor.

10. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, the circuit comprising:

a driving signal generator,

a capacitive touch sensor, an output of the driving signal generator fed to an input of the capacitive touch sensor;

a switched-capacitor integrator connected to an output of the capacitive touch sensor;

a comparator connected to an output of the switched-capacitor integrator;

a feedback circuit operatively connected to an output of the comparator and an input of the switched-capacitor integrator; and

an accumulator operatively connected to an output of the comparator to record total charge collected from the capacitive touch sensor during the conversion cycle.

11. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the driving signal generator comprises alternate phase operations of a reference voltage through switch S1 and ground through switch S2.

12. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the driving signal comprises a square wave.

13. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the switched-capacitor integrator comprises a positive cycle branch having a first capacitor with two positive cycle switches (S4 and S5) set to close during a positive cycle of the driving signal to collect capacitive touch sensor charge in the first capacitor;

a negative cycle branch having a second capacitor and two switches (S6 and S7) set to close during a negative cycle of the driving signal to collect capacitive touch sensor charge in the second capacitor, and an operational amplifier to provide gain.

14. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the feedback circuit comprises a feedback loop, the feedback loop including a feedback DAC comprising at least a third capacitor and a set of driving signal controlled switches (S8 and S9).

15. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the feedback circuit includes removing charge from the switched-capacitor integrator using the feedback loop.

16. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the capacitive touch sensor is either a mutual capacitive touch sensor or a self-capacitive touch sensor.

17. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 16, wherein the circuit further comprises a switch (S 10) inserted between the capacitive touch sensor and the switched-capacitor integrator to enable use of either of the mutual capacitive touch sensor or the self-capacitive touch sensor.

18. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 10, wherein the driving signal generator switches at one fixed or multiple time-multiplexed frequencies outside bands where a level of the interference is unacceptable.

19. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, the circuit comprising:

a driving signal generator comprising multiple modes, a capacitive touch sensor, the capacitive touch sensor comprising either a mutual capacitive touch sensor or a self-capacitive touch sensor, an output of the driving signal generator operatively fed to an input of the capacitive touch sensor;

a switched-capacitor integrator connected to an output of the capacitive touch sensor;

a switch inserted between the capacitive touch sensor and the switched-capacitor integrator to sense either a mutual capacitive touch sensor in a first of the multiple modes or a self-capacitive touch sensor in a second of the multiple modes;

a comparator connected to an output of the switched-capacitor integrator;

a feedback circuit operatively connected to an output of the comparator and an input of the switched-capacitor integrator to remove charge from the switched-capacitor integrator; and

an accumulator operatively connected to an output of the comparator to record total charge removed from the capacitive sensor during the conversion cycle.

20. A circuit for sensing capacitance of a capacitive touch sensor and rejecting interference coupled-in through the touch sensor during a conversion cycle, as per claim 19, wherein the feedback circuit comprises a feedback loop, the feedback loop including a feedback DAC comprising at least a third capacitor and a set of driving signal controlled switches and wherein the feedback circuit includes removing the charge from the switched-capacitor integrator using the feedback loop when charge exceeds a predetermined level.