CONFIGURABLE ANALOG FRONT-END FOR MUTUAL CAPACITANCE SENSING AND SELF CAPACITANCE SENSING

• US 20140078096A1
• Filed: 12/18/2012
• Published: 03/20/2014
• Est. Priority Date: 09/14/2012
• Status: Active Grant

• Current Assignee: STMicroelectronics International N.V. (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV), STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Tan, Kien Beng, Lasalandra, Ernesto, Ungaretti, Tommaso, Guedon, Yannick, Guo, Dianbo, Angelini, Paolo, Tripoli, Giovanni Carlo
• Granted Patent: US 8,976,151 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 2203/04104 Multi-touch detection in digitiser, i.e. details about the simultaneous detection of a plurality of touching locations, e.g. multiple fingers or pen and finger
• CPC Class Codes: G06F 3/04166 Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184
• CPC Class Codes: G06F 3/041662 using alternate mutual and self-capacitive scanning
• CPC Class Codes: G06F 3/044 by capacitive means
• 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

Capacitance sensing circuits and methods are provided. A dual mode capacitance sensing circuit includes a capacitance-to-voltage converter having an amplifier and an integration capacitance coupled between an output and an inverting input of the amplifier, and a dual mode switching circuit responsive to mutual mode control signals for a controlling signal supplied from a capacitive touch matrix to the capacitance-to-voltage converter in a mutual capacitance sensing mode and responsive to self mode control signals for controlling signals supplied from the capacitive touch matrix to the capacitance-to-voltage converter in a self capacitance sensing mode, wherein the capacitance sensing circuit is configurable for operation in the mutual capacitance sensing mode or the self capacitance sensing mode.

Claims

1. A dual mode capacitance sensing circuit, comprising:

a capacitance-to-voltage converter including an amplifier and an integration capacitance coupled between an output and an inverting input or the amplifier; and

a dual mode switching circuit responsive to mutual mode control signals for controlling signals supplied from a capacitive touch matrix to the capacitance-to-voltage converter in a mutual capacitance sensing mode and responsive to self mode control signals for controlling signals supplied from the capacitive touch matrix to the capacitance-to-voltage converter in a self capacitance sensing mode, wherein the capacitance sensing circuit is configurable for operation in the mutual capacitance sensing mode or the self capacitance sensing mode.

2. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitance-to-voltage converter further comprises a switching element connected across the integration capacitance to reset the capacitance-to-voltage converter.

3. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitance-to-voltage converter further comprises a switching element connected in series with the inverting input of the amplifier and configured to connect a transmit node to the inverting input during portions of the mutual and self capacitance sensing modes.

4. A dual mode capacitance sensing circuit as defined in claim 3, wherein the switching circuit further comprises a switching element configured to connect the transmit node to a common mode voltage during at least a portion of the mutual capacitance sensing mode.

5. A dual mode capacitance sensing circuit as defined in claim 3, wherein the switching circuit further comprises a switching element configured to connect the transmit node to a sense node during at least a portion of the mutual capacitance sensing mode and at least a portion of the self capacitance sensing mode.

6. A dual mode capacitance sensing circuit as defined in claim 3, wherein the switching circuit further comprises a switching element configured to connect the transmit node to a force node during at least a portion of the self capacitance sensing mode.

7. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitance-to-voltage converter has a single-ended input and a differential output.

8. A dual mode capacitance sensing circuit as defined in claim 1, wherein the switching circuit comprises first chopper switching elements connected to force nodes of the capacitive touch matrix and second chopper switching elements connected to sense nodes of the capacitive touch matrix.

9. A dual mode capacitance sensing circuit as defined in claim 8, wherein the switching circuit further comprises third chopper switching elements connected to a floating node of the capacitive touch matrix.

10. A dual mode capacitance sensing circuit as defined in claim 9, wherein the third chopper switching elements are configured to connect the floating node to ground during portions of the mutual capacitance sensing mode.

11. A dual mode capacitance sensing circuit as defined in claim 1, wherein the switching circuit further comprises switching elements configured to connect the sense node, the force node and the floating node, respectively, to a common mode voltage during portions of the self capacitance sensing mode.

12. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitive touch matrix includes force nodes and sense nodes, further comprising a capacitance cancellation circuit coupled to the inverting input of the amplifier and operable in the mutual capacitance sensing mode, the capacitance cancellation circuit including a cancellation capacitance equal in value to a matrix element capacitance of the capacitive touch matrix, wherein an excitation signal is applied to a force node of the capacitive touch matrix and the complement of the excitation signal is applied to the cancellation capacitance.

13. A dual mode capacitance sensing circuit as defined in claim 1, further comprising a capacitance cancellation circuit coupled to the inverting input of the amplifier and operable in the self capacitance sensing mode, the capacitance cancellation circuit including a cancellation capacitance equal in value to parasitic capacitances of the capacitive touch matrix and an internal parasitic capacitance of the capacitance sensing circuit, wherein an excitation signal is applied to the capacitive touch matrix and the complement of the excitation signal is applied to the cancellation capacitance.

14. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitive touch matrix includes force nodes and sense nodes and wherein the sense nodes are alternately precharged to a common mode voltage and connected to a common mode voltage virtual ground during operation in the mutual capacitance sensing mode.

15. A dual mode capacitance sensing circuit as defined in claim 1, wherein the capacitive touch matrix includes force nodes and sense nodes and wherein the force nodes and the sense nodes are driven to the same voltage during operation in the self capacitance sensing mode so that parasitic capacitances have zero charge.

16. A dual mode capacitance sensing circuit as defined in claim 1, wherein an excitation voltage greater than two times the common mode voltage of the capacitance-to-voltage converter is applied to the capacitive touch matrix during operation in the self capacitance sensing mode.

17. A dual mode capacitance sensing circuit as defined in claim 1, wherein the self capacitance sensing mode includes a first phase wherein a force node of the capacitive touch matrix is sensed while all sense nodes are bootstrapped and a second phase wherein a sense node is sensed while all force nodes are bootstrapped.

18. A touch screen system comprising a capacitive touch matrix, the dual mode capacitance sensing circuit as defined in claim 1, and a controller to provide mutual mode control signals and self mode signals to the dual mode capacitance sensing circuit.

19. A method for sensing capacitance, comprising:

providing a capacitance sensing circuit including capacitance-to-voltage converter and a dual mode switching circuit coupled between a capacitive touch matrix and the capacitance-to-voltage converter;

controlling operation of the capacitance sensing circuit for mutual capacitance sensing in response to selection of a mutual capacitance sensing mode; and

controlling operation of the capacitance sensing circuit for self capacitance sensing in response to selection of a self capacitance sensing mode.

20. A method as defined in claim 19, wherein the capacitive touch matrix includes force nodes and sense nodes, and wherein controlling operation of the capacitance sensing circuit for mutual capacitance sensing comprises alternately precharging the sense nodes to a common node voltage and connecting the sense nodes to a common node voltage virtual ground.

21. A method as defined in claim 19, wherein the capacitive touch matrix includes force nodes and sense nodes, and wherein controlling operation of the capacitance sensing circuit for self capacitance sensing comprises driving the force nodes and the sense nodes to the same voltage during operation so that parasitic capacitances have zero charge.

22. A method as defined in claim 19, wherein the capacitive touch matrix includes force nodes and sense nodes, and wherein controlling operation of the capacitance sensing circuit for mutual capacitance sensing comprises coupling to the capacitance-to-voltage converter a cancellation capacitance equal in value to a matrix element capacitance of the capacitive touch matrix, applying an excitation signal to a force node of the capacitive touch matrix and applying the complement of the excitation signal to the cancellation capacitance.

23. A method as defined in claim 19, wherein controlling operation of the capacitance sensing circuit for self capacitance sensing comprises coupling to the capacitance-to-voltage converter a cancellation capacitance equal in value to the parasitic capacitances of the capacitive touch matrix and the internal parasitic capacitance of the capacitance sensing circuit, applying an excitation signal to the capacitive touch matrix and applying the complement of the excitation signal to the cancellation capacitance.

24. A method as defined in claim 19, wherein the capacitance-to-voltage converter has an associated common mode voltage and wherein controlling operation of the capacitance sensing circuit for self capacitance sensing comprises applying an excitation voltage to the capacitive touch matrix that is greater than two times the common mode voltage of the capacitance-to-voltage converter.

25. A method as defined in claim 19, wherein the capacitive touch matrix includes force nodes and sense nodes and wherein controlling operation of the capacitance sensing circuit for self capacitance sensing comprises operating in a first phase wherein a force node is sensed while all sense nodes are bootstrapped and operating in a second phase wherein a sense node is sensed while all force nodes are bootstrapped.

26. A capacitance sensing circuit coupled to a capacitive touch matrix and operable in a mutual capacitance sensing mode or in a self capacitance sensing mode, comprising:

a capacitance-to-voltage converter comprising an amplifier and an integration capacitance coupled between an output and an inverting input of the amplifier;

first, second and third sense node switching elements coupled between a sense node of the capacitive touch matrix and an input voltage, a common mode voltage and ground, respectively;

first, second and third force node switching elements coupled between a force node of the capacitive touch matrix and the input voltage, the common mode voltage and ground, respectively;

first, second and third floating node switching elements coupled between a floating node and the input voltage, the common mode voltage and ground, respectively;

a first switching element coupled between the sense node and a transmit node;

a second switching element coupled between the force node and the transmit node; and

an input switching element coupled between the transmit node and the inverting input of the amplifier.

27. A capacitance sensing circuit as defined in claim 26, wherein the capacitance-to-voltage converter has a single ended input and a differential output.

28. A capacitance sensing circuit as defined in claim 26, further comprising a capacitance cancellation circuit coupled to the inverting input of the amplifier and operable in the mutual capacitance sensing mode, the capacitance cancellation circuit including a cancellation capacitance equal in value to a matrix element capacitance of the capacitive touch matrix, wherein an excitation signal is applied to a force node of the capacitive touch matrix and the complement of the excitation signal is applied to the cancellation capacitance.

29. A capacitance sensing circuit as defined in claim 26, further comprising a capacitance cancellation circuit coupled to the inverting input of the amplifier and operable in the self capacitance sensing mode, the capacitance cancellation circuit including a cancellation capacitance equal in value to parasitic capacitances of the capacitive touch matrix and an internal parasitic capacitance of the capacitance sensing circuit, wherein an excitation signal is applied to the capacitive touch matrix and the complement of the excitation signal is applied to cancellation capacitance.

30. A touch screen system comprising a capacitive touch matrix, the capacitance sensing circuit of claim 26 and a controller configured to provide control signals to the switching elements of the capacitance sensing circuit.