HALF-BRIDGE FOR CAPACITIVE SENSING

• US 20070159184A1
• Filed: 12/22/2006
• Published: 07/12/2007
• Est. Priority Date: 12/22/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: HARGREAVES, Kirk, REYNOLDS, Joseph
• Granted Patent: US 7,924,029 B2
• Time in Patent Office: Days
• US Class Current: 324/662
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard

Abstract

Methods and apparatus are provided for measuring a ratio of capacitances. The apparatus include a comparator including a comparator input, a comparator output, and a reference voltage; a first capacitive element including a first capacitance, a first electrode coupled to the comparator input, and a second electrode; a first biasing element coupled to the first electrode; a digitally-controlled voltage waveform generator for generating a varying waveform, the voltage waveform generator including a voltage waveform generator output coupled to the second electrode; a second capacitive element including a second capacitance and a third electrode coupled to the comparator input; and a controller coupled to the comparator output, the controller configured to control the digitally-controlled voltage waveform generator, wherein an output signal of the comparator is dependent on a ratio comprising the first capacitance and the second capacitance, and the first capacitive element and/or the second capacitive element has a variable capacitance.

Claims

1. An apparatus for measuring a ratio of capacitances, comprising:

a first comparator including a first comparator input, a first comparator output, and a first reference voltage;

a first capacitive element including a first capacitance, a first electrode coupled to the first comparator input, and a second electrode;

a first biasing element coupled to the first electrode of the first capacitive element;

a digitally-controlled voltage waveform generator for generating a varying waveform, the voltage waveform generator including a voltage waveform generator output coupled to the second electrode of the first capacitive element;

a second capacitive element including a second capacitance and a third electrode coupled to the first comparator input; and

a controller coupled to the first comparator output, the controller configured to control the digitally-controlled voltage waveform generator, wherein;

a first output signal of the first comparator is dependent on a first ratio comprising the first capacitance and the second capacitance, and

at least one of the first capacitive element and the second capacitive element is a variable capacitive element.

2. The apparatus of claim 1, wherein the first electrode and the second electrode each comprise sensor electrodes configured to couple capacitively with each other to produce the first capacitance, and wherein when a conductive object is moved proximate to the sensor electrodes the first capacitance changes.

3. The apparatus of claim 1, wherein the third electrode comprises a sensor electrode configured to couple capacitively with an external object to change the second capacitance, and wherein when the external object is moved proximate to the sensor electrodes the second capacitance changes.

4. The apparatus of claim 1, further comprising:

a second comparator including a second comparator input, a second comparator output, and a second reference voltage;

a third capacitive element including a third capacitance, a fifth electrode coupled to the second comparator input, and a sixth electrode;

a second biasing element coupled to the fifth electrode of the third capacitive element; and

a fourth capacitive element including a fourth capacitance and a seventh electrode coupled to the second comparator input, wherein;

the digitally-controlled voltage waveform generator output is coupled to the sixth electrode of the third capacitive element, and

a second output signal of the comparator is dependent on a second ratio comprising the third capacitance and the fourth capacitance.

5. The apparatus of claim 4, wherein at least one of the first capacitive element, the second capacitive element, the third capacitive element, and the fourth capacitive element is a variable capacitive element.

6. The apparatus of claim 1, wherein at least one of the first capacitive element and the second capacitive element is a variable capacitive element.

7. The apparatus of claim 1, wherein the comparator is implemented utilizing a digital input and the reference voltage is a threshold voltage of the digital input.

8. The apparatus of claim 1, wherein the digitally-controlled voltage waveform generator comprises a digital-to-analog converter.

9. The apparatus of claim 1, wherein the digitally-controlled voltage waveform generator comprises a resistor-capacitor filter sensing circuit.

10. The apparatus of claim 9, wherein the digitally-controlled voltage waveform generator further comprises a pulse modulated signal generator coupled to the resistor-capacitor filter sensing circuit, wherein the pulse modulated signal generator is configured to drive one of a pulse code modulated signal and a pulse width modulated signal into the resistor-capacitor filter sensing circuit.

11. The apparatus of claim 1, wherein the digitally-controlled voltage waveform generator comprises a switched capacitor sensing circuit.

12. The apparatus of claim 1, wherein the first biasing element comprises a first switch.

13. The apparatus of claim 1, further comprising:

a guarding electrode disposed proximate to the first capacitive element and the second capacitive element.

14. The apparatus of claim 1, wherein the comparator, the first biasing element, and the controller form at least a portion of a microcontroller.

15. The apparatus of claim 14, wherein the microcontroller comprises at least one digital I/O having a digital input and a digital output, wherein the comparator is implemented utilizing the digital input and the first biasing element is implemented utilizing the digital output.

16. The apparatus of claim 1, wherein the digitally-controlled voltage waveform generator forms at least a portion of a microcontroller.

17. An apparatus for measuring a ratio of capacitances, comprising:

a first comparator including a first comparator input, a first comparator output, and a first reference voltage;

a first capacitive element including a first capacitance, a first electrode, and a second electrode;

a first node coupling the first comparator input to the second electrode;

a digitally-controlled voltage waveform generator for generating a varying waveform, the digitally-controlled voltage waveform generator including a digitally-controlled voltage waveform generator output;

a first switch configured to selectively couple the digitally-controlled voltage waveform generator output to the first node;

a first biasing element coupled to the first electrode;

a second capacitive element including a second capacitance and a third electrode coupled to the first electrode; and

a controller coupled to the first comparator output, the controller configured to control the digitally-controlled voltage waveform generator, wherein;

a first output signal of the comparator is dependent on a first ratio comprising the first capacitance and the second capacitance, and

at least one of the first capacitive element and the second capacitive element is a variable capacitive element.

18. The apparatus of claim 17, further comprising:

a second comparator including a second comparator input, a second comparator output, and a second reference voltage;

a third capacitive element including a third capacitance, a fifth electrode, and a sixth electrode;

a second node coupling the second comparator input to the sixth electrode;

a second switch configured to selectively couple the digitally-controlled voltage waveform generator output to the second node;

a second biasing element coupled to the fifth electrode; and

a fourth capacitive element including a fourth capacitance and a seventh electrode coupled to the fifth electrode, wherein;

the controller is coupled to the second comparator output, and

a second output signal of the second comparator is dependent on a second ratio comprising the third capacitance and the fourth capacitance.

19. The apparatus of claim 18, further comprising a multiplexer comprising the first switch and the second switch.

20. The apparatus of claim 17, wherein at least one the first capacitive element and the second capacitive element are variable capacitive elements.

21. The apparatus of claim 17, wherein the digitally-controlled voltage waveform generator comprises a digital-to-analog converter.

22. The apparatus of claim 17, wherein the first comparator and the first biasing element form at least a portion of a microcontroller comprising at least one digital I/O including a digital input and a digital output, wherein the first comparator is implemented utilizing the digital input and the first biasing element comprises a second switch implemented utilizing the digital output.

23. A method for generating a first comparator output signal dependent on a ratio including a first capacitance (C₁) of a first capacitive element and a second capacitance (C₂) of a second capacitive element, the method comprising the steps of:

setting an initial voltage across the first capacitive element;

generating a varying voltage waveform (V_w);

applying V_w to at least one of the first capacitive element and the second capacitive element to generate a first voltage (V_n1) such that V_n1 exhibits values above and below a first reference voltage (V_ref1) at different times, wherein a change in V_n1 is substantially proportional to the ratio including the first capacitance and the second capacitance;

comparing V_n1 to V_ref1; and

generating the first comparator output signal, wherein the first comparator output signal changes state in response to V_n1 crossing from one of being below V_ref1 to being above V_ref1 and from being above V_ref1 to being below V_ref1, and wherein a change in V_w is representative of a first ratio including the first capacitance and the second capacitance when V_n1 is substantially equal to V_ref1.

24. The method of claim 23, wherein the first ratio comprises one of C₁ divided by (C₁+C₂) and C₂ divided by (C₁+C₂).

25. The method of claim 23, wherein the generating V_w step comprises the step of generating V_w with a predefined voltage ramp.

26. The method of claim 23, wherein the generating V_w step comprises the step of generating V_w with a digital-to-analog converter.

27. The method of claim 23, wherein the generating V_w step is variable dependent on the comparator output signal.

28. The method of claim 23, further comprising the steps of:

generating a second comparator output signal dependent on a second ratio including a third capacitance of a third capacitive element and a fourth capacitance of a fourth capacitive element;

setting an initial voltage across the third capacitive element;

applying V_w to at least one of the third capacitive element and the fourth capacitive element to generate a second voltage (V_n2) such that V_n2 exhibits values above and below a second reference voltage (V_ref2) at different times, wherein a change in V_n2 is substantially proportional to the second ratio including the third capacitance and the fourth capacitance;

comparing V_n2 to V_ref2; and

generating the second comparator output signal, wherein the second comparator output signal changes state in response to V_n2 crossing from one of being below V_ref2 to being above V_ref2 and from being above V_ref2 to being below V_ref2, and wherein a change in V_w is representative of a second ratio including the second capacitance and the third capacitance when V_n2 is substantially equal to V_ref2.

29. The method of claim 23, further comprising the step of repeating the applying V_w step, the comparing V_n1 step, and the generating the first comparator output signal step at least one additional time.

30. The method of claim 23, wherein the generating V_w step comprises the step of generating a continuously varying voltage signal.

31. The method of claim 23, wherein the generating V_w step comprises the step of generating a discretely varying voltage signal.