Methods and systems for detecting a capacitance using switched charge transfer techniques

• US 7,449,895 B2
• Filed: 06/03/2006
• Issued: 11/11/2008
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: Routley, Paul, Hargreaves, Kirk, Reynolds, Joseph Kurth, Ely, David, Haines, Julian
• Primary Examiner(s): Nguyen; Vincent Q
• Application Number:
  US11/446,323
• Publication Number:
  US 20070176609A1
• Time in Patent Office: 892 Days
• Field of Search: 324/660, 324/678, 324/679
• US Class Current: 324/678
• 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 3/044 by capacitive means
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

Methods, systems and devices are described for detecting a measurable capacitance using charge transfer techniques. According to various embodiments, a charge transfer process is performed for two or more times. During the charge transfer process, a pre-determined voltage is applied to the measurable capacitance, and the measurable capacitance is then allowed to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the charge transfer process. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. Such a detection scheme may be readily implemented using conventional components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to an input sensor.

Claims

1. A method for measuring a measurable capacitance, the method comprising the steps of:

performing a charge transfer process for a number of times equal to at least two, wherein the charge transfer process comprises the steps of applying a pre-determined voltage to the measurable capacitance and allowing the measurable capacitance to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the applying and allowing steps; and

determining a value of the measurable capacitance as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed.

2. The method of claim 1 further comprising the step of compensating the representation of the charge on the filter capacitance for a variation in a reference voltage.

3. The method of claim 2 wherein the compensating step comprises adding at least a portion of the reference voltage to a voltage on the filter capacitance.

4. The method of claim 3 wherein the reference voltage is coupled to the filter capacitance via an impedance network to add the at least a portion of the reference voltage to the voltage on the filter capacitance.

5. The method of claim 4 wherein the impedance network comprises a capacitive divider network.

6. The method of claim 4 wherein the impedance network comprises a resistive divider network.

7. The method of claim 1 further comprising the step of resetting the charge on the filter capacitance.

8. The method of claim 7 wherein the resetting step comprises applying a known voltage to the filter capacitance.

9. The method of claim 7 wherein the value of the measurable capacitance is determined from the number of times the charge transfer process is performed between the resetting step and when the representation of charge on the filter capacitance is detected to pass a threshold.

10. The method of claim 1 wherein the filter capacitance comprises an impedance divider coupled to two different power supply voltages.

11. The method of claim 10 wherein the impedance divider comprises a capacitive divider.

12. The method of claim 1 wherein the determining step comprises obtaining a multi-bit measurement of a voltage on the filter capacitance.

13. The method of claim 1 wherein the determining step comprises comparing the representation of charge on the filter capacitance with a threshold.

14. The method of claim 1 further comprising the step of performing a second charge transfer process for a second number of times equal to at least one, wherein the second charge transfer process comprises the steps of applying an opposing voltage to the measurable capacitance and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, wherein the pre-determined voltage transfers charge in a first direction, and the opposing voltage transfers charge in a second direction opposite the first direction.

15. The method of claim 14 further comprising the step of setting the representation of the charge on the filter capacitance to a first value after the performing of the charge transfer process for the number of times and setting the representation of the charge on the filter capacitance to a second value after the performing of the second charge transfer process for the second number of times.

16. The method of claim 14 wherein the determining step comprises ascertaining one of a number of performances of the first charge transfer process needed for the representation of the charge on the filter capacitance to cross a first threshold and a number of performances of the second charge transfer processes needed for the representation of the charge on the filter capacitance to cross a second threshold.

17. The method of claim 14 wherein the determining step comprises:

ascertaining a number of performances of the first charge transfer process needed for the representation of the charge on the filter capacitance to cross a first threshold;

ascertaining a number of performances of the second charge transfer processes needed for the representation of the charge on the filter capacitance to cross a second threshold; and

ascertaining a total number of performances of the charge and second charge transfer processes needed for the representation of the charge on the filter capacitance to cross both a first threshold and a second threshold.

18. The method of claim 14 wherein the charge transfer process is performed at least until the representation of the charge on the filter capacitance crosses a first threshold and the second charge transfer process is performed at least until the representation of the charge on the filter capacitance crosses a second threshold, and wherein the first threshold and the second threshold are the same.

19. The method of claim 18 wherein the charge transfer process is performed past when the representation of the charge on the filter capacitance crosses a first threshold and the second charge transfer process is performed past when the representation of the charge on the filter capacitance crosses a second threshold.

20. The method of claim 14 wherein the charge transfer process is performed at least until the representation of the charge on the filter capacitance crosses a first threshold and the second charge transfer process is performed at least until the representation of the charge on the filter capacitance crosses a second threshold, and wherein the first threshold differs from the second threshold.

21. The method of claim 20 further comprising the steps of

resetting the charge on the filter capacitance to a reset value after performing the charge transfer process for the number of times;

and

resetting the charge on the filter capacitance to the reset value after performing the second charge transfer process for the second number of times.

22. The method of claim 1 wherein the determining step comprises measuring a voltage using a digital input threshold of a controller.

23. The method of claim 22 wherein the digital input has hysteresis.

24. The method of claim 1 wherein the determining step comprises measuring a voltage using an analog-to-digital converter of a controller.

25. The method of claim 1 wherein the charge transfer process further comprises:

varying a rate of execution of the charge transfer process such that the rate of execution is different during a first portion of the number of times of the performing of the charge transfer process than during a second portion of the number of times of the performing of the charge transfer process; and

comparing the representation of charge on the filter capacitance with a threshold for the second portion and not for the first portion.

26. The method of claim 25 wherein the rate of execution is faster for the first portion than for the second portion.

27. The method of claim 1 wherein the representation of the charge on the filter capacitance is a voltage on the filter capacitance.

28. The method of claim 1 wherein the applying step comprises coupling the measurable capacitance to the pre-determined voltage using a switch.

29. The method of claim 28 wherein the switch comprises a digital output of a controller.

30. The method of claim 28 wherein the switch comprises one of a buffer integrated circuit and a transistor.

31. The method of claim 1 wherein the applying of the predetermined voltage lasts for a period shorter than a response time of a network comprising the passive impedance.

32. The method of claim 1 further comprising the step of providing a guard signal configured to guard the measurable capacitance during at least a portion of the charge transfer process.

33. The method of claim 32 wherein a voltage of the guard signal changes during the charge transfer process.

34. The method of claim 32 wherein a voltage of the guard signal changes substantially contemporaneously with at least one of the applying and allowing steps.

35. A digital storage medium having computer-executable instructions stored thereon configured to execute the method of claim 1.

36. A method for measuring a measurable capacitance, the method comprising the steps of:

applying a pre-determined voltage to the measurable capacitance; and

allowing the measurable capacitance to share charge through a passive impedance with a filter capacitance, wherein the passive impedance remains coupled to both the measurable capacitance and the filter capacitance throughout the applying and allowing steps; and

determining a value of the measurable capacitance as a function of a voltage on the filter capacitance.

37. The method of claim 36 further comprising the step of resetting the charge on the filter capacitance.

38. The method of claim 37 wherein the step of determining a value of the measurable capacitance comprises deriving the value of the measurable capacitance from the number of times the charge transfer process is performed between the resetting step and when the representation of charge on the filter capacitance is detected to pass a threshold.

39. A device for measuring a measurable capacitance, the device comprising:

a filter capacitance means for storing charge;

means for applying a pre-determined voltage to the measurable capacitance;

passive impedance means statically coupled to both the measurable capacitance and the filter capacitance means for allowing the measurable capacitance to share charge with the filter capacitance means; and

means for determining a value of the measurable capacitance as a function of a representation of a charge on the filter capacitance means.

40. A circuit for measuring a measurable capacitance at a first electrical node, the circuit comprising:

a passive impedance statically coupled between the first electrical node and a second electrical node;

a filter capacitance coupled between the second electrical node and a third electrical node; and

a controller coupled to the first electrical node, wherein the controller is configured to perform a charge transfer process for a number of times greater than one, wherein the charge transfer process comprises applying a pre-determined voltage to the first electrical node for a period of time and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the measurable capacitance as a function of a voltage on the filter capacitance and the number of times.

41. The circuit of claim 40 further comprising a second filter capacitance coupled between the second electrical node and a fourth electrical node.

42. The circuit of claim 41 wherein the third electrical node is coupled to a first power supply voltage, and wherein the fourth electrical node is coupled to a second power supply voltage difference from the first power supply voltage.

43. The circuit of claim 40 wherein the third electrical node is connected to a reference voltage.

44. The circuit of claim 40 wherein the third electrical node is coupled to the controller.

45. The circuit of claim 44 further comprising a second measurable capacitance coupled to the third electrical node.

46. The circuit of claim 40 wherein the second electrical node is coupled to the controller and the third electrical node is coupled to the controller.

47. The circuit of claim 46 further comprising:

a fourth electrical node coupled to the controller;

a second passive impedance coupled to the third electrical node and to the fourth electrical node; and

a second measurable capacitance coupled to the fourth electrical node.

48. The circuit of claim 40 further comprising an impedance divider selected from the group consisting of a capacitive divider network and a resistive divider network, wherein the impedance divider is coupled to the third electrical node and a reference voltage, wherein the impedance divider is configured to add at least a portion of the reference voltage to the voltage on the filter capacitance.

49. The circuit of claim 40 wherein the passive impedance comprises a resistor.

50. The circuit of claim 40 wherein the controller is further configured to compare the voltage at the first electrical node to a reference voltage to ascertain the voltage on the filter capacitance.

51. The circuit of claim 40 wherein the controller is further configured to measure the voltage at the first electrical node using a multi-bit analog-to-digital converter to ascertain the voltage on the filter capacitance.

52. The circuit of claim 51 wherein the controller is further configured to compare the voltage at the first electrical node with a second reference voltage to ascertain the voltage on the filter capacitance.

53. The circuit of claim 51 wherein the controller is further configured to set the voltage on the filter capacitance to a known value to reset the filter capacitance.

54. The circuit of claim 52 wherein the first reference voltage and the second reference voltage are provided by a digital input of the controller.

55. The circuit of claim 40, wherein the controller is further connected to the second electrical node.

56. The circuit of claim 55 wherein the controller is further configured to compare the voltage at the second electrical node with a reference voltage to ascertain the voltage on the filter capacitance.

57. The circuit of claim 56 wherein the controller is further configured to compare the voltage at the second electrical node with a second reference voltage to ascertain the filter capacitance charge.

58. The circuit of claim 57 wherein the first reference voltage and the second reference voltage are provided by a digital input of the controller.

59. The circuit of claim 40 wherein the filter capacitance is coupled to a reset voltage through an impedance for a pre-determined length of time to reset the filter capacitance.

60. A proximity sensor comprising:

a sensor electrode having a measurable capacitance;

a switch coupled to the measurable capacitance;

a passive network coupled to the measurable capacitance and the switch, the passive network comprising a passive impedance and a filter capacitance, wherein the passive impedance statically couples the measurable capacitance to the filter capacitance; and

a controller coupled to the switch, wherein the controller is configured to perform a charge transfer process for a number of times greater than one, wherein the charge transfer process comprises applying a pre-determined voltage to the measurable capacitance using the switch and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the measurable capacitance as a function of a representation of a charge of the filter capacitance and the number of times.

61. The proximity sensor of claim 60 further comprising:

a second sensor electrode having a second measurable capacitance; and

a second switch coupled to the second measurable capacitance,

a second passive network,

wherein the second passive network is coupled to the second measurable capacitance and the second switch, wherein the second passive network further comprises a second passive impedance and a second filter capacitance, and wherein the second passive impedance statically couples the second measurable capacitance to the second filter capacitance; and

wherein the controller is coupled to the second switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance using the second switch and allowing the second measurable capacitance to share charge with the second filter capacitance through the second passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a representation of a charge of the second filter capacitance and the second number of times.

62. The proximity sensor of claim 60 further comprising:

a second sensor electrode having a second measurable capacitance; and

a second switch coupled to the second measurable capacitance,

wherein the passive network is coupled to the second measurable capacitance and the second switch, and wherein the passive impedance statically couples the second measurable capacitance to the filter capacitance, and

wherein the controller is coupled to the second switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance using the second switch and allowing the second measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a second representation of a second charge of the filter capacitance and the second number of times.

63. The proximity sensor of claim 60 further comprising:

a second sensor electrode having a second measurable capacitance;

which is not connected directly to the passive network,

a multiplexer coupled to the first measurable capacitance and the second measurable capacitance and the passive network,

wherein the controller is coupled to the multiplexer and is further configured to multiplex the passive network at least between the first measurable capacitance and the second measurable capacitance using the multiplexer, and

wherein the controller is coupled to the switch and is further configured to perform a second charge transfer process for a second number of times greater than one, wherein the second charge transfer process comprises applying a second pre-determined voltage to the second measurable capacitance through the multiplexer and allowing the second measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the second measurable capacitance as a function of a second representation of a second charge on the filter capacitance and the second number of.

64. The proximity sensor of claim 60 wherein the measurable capacitance represents a capacitance between the sensor electrode and another electrode.

65. The proximity sensor of claim 60 wherein the measurable capacitance represents a capacitance between the sensor electrode and an external object.

66. The proximity sensor of claim 65 wherein the external object is part of a human body.

67. The proximity sensor of claim 60 wherein the controller is further configured to provide a guard signal configured to guard the measurable capacitance during at least a portion of the charge transfer process.