Methods and systems for detecting a capacitance using sigma-delta measurement techniques

• US 20070046299A1
• Filed: 06/03/2006
• Published: 03/01/2007
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: Hargreaves, Kirk, Ely, David, Haines, Julian, Reynolds, Joseph
• Granted Patent: US 7,301,350 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

Methods, systems and devices are described for detecting a measurable capacitance using sigma-delta measurement techniques. According to various embodiments, a voltage is applied to the measurable capacitance using a first switch. The measurable capacitance is allowed to share charge with a passive network. If the charge on the passive network is past a threshold value, then the charge on the passive network is changed by a known amount for a sufficient number of repetitions until the measurable capacitance can be detected. 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 a button, slider, touchpad or other input sensor.

Claims

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

applying a voltage to the measurable capacitance using a first switch;

allowing the measurable capacitance to share charge with a passive network;

changing a charge on the passive network by an amount of charge if the charge on the passive network is past a threshold; and

repeating each of the applying step, the allowing step, and the changing step at least once.

2. The method of claim 1 wherein the voltage is a pre-determined voltage and the amount of charge is based on the pre-determined voltage.

3. The method of claim 1 further comprising the step of determining a value for the measurable capacitance based at least in part upon a number of repetitions of the changing step.

4. The method of claim 1 wherein a number of repetitions of the applying step is not equal to a number of repetitions of the changing step.

5. The method of claim 1 wherein the changing step comprises altering the charge on the passive network multiple times.

6. The method of claim 1 wherein the applying step is performed multiple times for each repetition of the changing step, and the allowing step is performed multiple times for each repetition of the changing step.

7. The method of claim 1 wherein the applying step comprises actuating an output pin of a digital control circuit.

8. The method of claim 1 wherein the amount of the charge changed on the passive network corresponds to a charge on a feedback capacitance.

9. The method of claim 1 wherein the amount of charge changed on the passive network corresponds to a current applied through a resistance for a known period of time.

10. The method of claim 9 wherein the current is applied by applying a second voltage across the resistance.

11. The method of claim 1 further comprising the step of multiplexing the applying, allowing and changing steps between a plurality of measurable capacitances, wherein the measurable capacitance is one of the plurality of measurable capacitances.

12. The method of claim 1 wherein the applying and allowing steps are performed using only switches and passive elements.

13. The method of claim 12 wherein the switches comprise digital outputs of a controller.

14. The method of claim 1 wherein the applying, allowing and changing steps are performed using digital inputs and outputs of a controller.

15. The method of claim 1 wherein the threshold is determined by a threshold voltage of a digital input of a controller.

16. The method of claim 1 wherein the threshold is determined by an analog-to-digital converter.

17. The method of claim 1 wherein the applying step comprises charging the measurable capacitance and the changing step comprises removing charge from the passive network.

18. The method of claim 1 wherein the applying step comprises discharging the measurable capacitance and the changing step comprises adding charge to the passive network.

19. The method of claim 1 wherein the allowing step comprises passively permitting the measurable capacitance to share charge with the passive network using a sharing circuit, and the applying step occurs substantially faster than a time constant of the sharing circuit.

20. The method of claim 1 wherein the allowing step comprises controlling a second switch.

21. The method of claim 1 further comprising the steps of:

storing a plurality of quantized values of the charge on the passive network in memory; and

applying a digital filter to the plurality of values.

22. The method of claim 1 further comprising the step of applying a guarding voltage to a guarding electrode distinct from the measurable capacitance.

23. The method of claim 22 wherein the guarding voltage applied to the guarding electrode during the applying step is substantially equal to the voltage applied to the measurable capacitance during the applying step.

24. The method of claim 22 wherein the guarding voltage applied to the guarding electrode is substantially equal to a voltage on the measurable capacitance after the allowing step.

25. The method of claim 22 wherein a change in the guarding voltage applied to the guarding electrode between the applying and allowing steps approximates a change in voltage on the measurable capacitance between the applying and allowing steps.

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

27. A system for measuring a measurable capacitance, the system comprising:

means for repeatedly applying a voltage to the measurable capacitance using a first switch;

means for repeatedly allowing the measurable capacitance to share charge with a passive network; and

means for repeatedly changing a charge on the passive network by an amount of charge if the charge on the passive network is past a threshold.

28. An electrical circuit for measuring a measurable capacitance, the electrical circuit comprising:

a first switch coupled to the measurable capacitance;

a passive network coupled to the measurable capacitance, wherein the passive network is configured to store charge received from the measurable capacitance; and

a charge changing circuit coupled to the passive network; and

a controller configured to determine a value for the measurable capacitance by repeatedly applying a voltage to the measurable capacitance using the first switch, repeatedly storing charge received from the measurable capacitance on the passive network, and repeatedly changing a charge on the passive network by an amount of charge using the charge changing circuit in response to the charge on the passive network being past a threshold level.

29. The electrical circuit of claim 28 wherein the voltage is a pre-determined voltage and the amount of charge is based on the pre-determined voltage.

30. The electrical circuit of claim 28 wherein the voltage is a power supply voltage.

31. The electrical circuit of claim 28 wherein the passive network comprises a capacitor.

32. The electrical circuit of claim 28 wherein the first switch is a digital output of a controller.

33. The electrical circuit of claim 32 further comprising a second switch coupled to the charge changing circuit and to the passive network, wherein the second switch is operable to change the charge on the passive network by the amount of charge.

34. The electrical circuit of claim 32 further comprising a third switch coupled to the measurable capacitance and the passive network, wherein the third switch is operable to allow the measurable capacitance to share charge with the passive network.

35. The circuit of claim 28 further comprising a second switch coupled to the measurable capacitance and the passive network, wherein the second switch is operable to allow the measurable capacitance to share charge with the passive network.

36. The electrical circuit of claim 31 wherein the passive network comprises a resistor.

37. The electrical circuit of claim 28 further comprising a voltage measuring circuit coupled to the passive network.

38. The electrical circuit of claim 37 wherein the voltage measuring circuit comprises a comparator.

39. The electrical circuit of claim 37 wherein the voltage measuring circuit comprises an analog-to-digital converter.

40. The electrical circuit of claim 37 wherein the voltage measuring circuit comprises an input threshold of a digital input of a controller.

41. The electrical circuit of claim 40 wherein the input threshold has hysteresis.

42. The electrical circuit of claim 28 further comprising a signal pin of a controller, the signal pin coupled to the measurable capacitance, wherein the controller is configured to apply the voltage to the measurable capacitance via the signal pin.

43. The electrical circuit of claim 28 further comprising a second switch and a third switch, and wherein the passive network comprises:

a first capacitor coupled between the third switch and the measurable capacitance; and

a second capacitor coupled between the first switch and the second switch.

44. The electrical circuit of claim 28 further comprising a second switch, and wherein the passive network comprises:

a first capacitor coupled between the first switch and the second switch; and

a resistor coupled to the first switch and to a second voltage; and

a resistor coupled to the second voltage.

45. The electrical circuit of claim 28 wherein the passive network comprises a capacitor and a resistor coupled to the first switch in series with each other and to a second voltage.

46. The electrical circuit of claim 28 further comprising a second switch and a third switch, and wherein the passive network comprises:

a capacitor coupled between the third switch and the measurable capacitance; and

a resistor coupled between the first switch and the second switch.

47. The electrical circuit of claim 46 further configured to change the charge on the passive network by activating the second switch for a period of time, thereby allowing charge on the passive network to pass through the resistor.

48. The electrical circuit of claim 28 further comprising a guarding electrode, wherein the controller is further configured to apply a guarding voltage to the guarding electrode.

49. The electrical circuit of claim 48 further comprising an impedance divider coupled to a second switch and the guarding electrode.

50. The electrical circuit of claim 48 wherein the guarding electrode is driven by a digital to analog converter.