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

• US 20070075710A1
• Filed: 11/30/2006
• Published: 04/05/2007
• 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 7,288,946 B2
• Time in Patent Office: Days
• US Class Current: 324/658
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/0383 Signal control means within the pointing device
• 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 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03J 9/00 Remote-control of tuned circuits; Combined remote-control of tuning and other functions, e.g. brightness, amplification mechanical remote-control arrangements H03J1/00 ; using a microprocessor H03J1/0025; constructional details of remote control switching devices H01H9/0235
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/960775 Emitter-receiver or "fringe" type detection, i.e. one or more field emitting electrodes and corresponding one or more receiving electrodes

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

a first switch coupled to a first capacitance;

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

a charge changing circuit coupled to the passive network; and

a controller configured to measure the capacitance value by repeatedly applying a voltage to the first capacitance using the first switch, repeatedly storing charge received from the first 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 a voltage on the passive network being past a threshold of a quantizer.

2. The electrical circuit of claim 1 wherein the controller is further configured to measure the capacitance value by repeatedly allowing the first capacitance to share charge with the passive network such that charge from at least two applications of the voltage to the first capacitance is accumulated on the passive network.

3. The electrical circuit of claim 1 further comprising a voltage conditioning circuit coupled to the passive network, the voltage conditioning circuit adapted to provide a conditioned voltage to the passive network that varies with the threshold of the quantizer.

4. The electrical circuit of claim 3 wherein the voltage conditioning circuit includes a voltage divider having a first divider impedance and a second divider impedance, the first divider impedance and the second divider impedance coupled at a divider node, and wherein the voltage conditioning circuit is configured to couple a power supply voltage to the passive network through the voltage divider node.

5. The electrical circuit of claim 4 wherein the first divider impedance comprises a first divider resistance and wherein the second divider impedance comprises a second divider resistance.

6. The electrical circuit of claim 4 wherein the first divider impedance comprises a first divider capacitance and wherein the second divider impedance comprises a second divider capacitance.

7. The electrical circuit of claim 6 further comprising a resistance coupled to the power supply voltage and to the divider node, in parallel with the first divider capacitance.

8. The electrical circuit of claim 3 wherein the electrical circuit is further configured for measuring a second capacitance value, the electrical circuit further comprising:

a second switch coupled to a second capacitance; and

a second passive network coupled to the second capacitance and to the voltage conditioning circuit, wherein the second passive network is configured to store charge received from the second capacitance;

wherein the controller is further configured to measure the second capacitance value by repeatedly applying a second voltage to the second capacitance using the second switch, repeatedly storing charge received from the second capacitance on the second passive network, and repeatedly changing a charge on the second passive network by a second amount of charge in response to a voltage on the second passive network being past a threshold of a second quantizer.

9. The electrical circuit of claim 8 wherein the voltage and the second voltage are substantially similar, and wherein the threshold of the quantizer and the threshold of the second quantizer are substantially similar.

10. The electrical circuit of claim 1 wherein the quantizer is implemented using a digital input of the controller.

11. The electrical circuit of claim 1 wherein the passive network is further configured to provide a conditioned voltage that varies with the threshold of the quantizer.

12. The electrical circuit of claim 11 wherein the passive network includes a voltage divider having a first divider capacitance and a second divider capacitance coupled at a divider node, wherein the first divider capacitance and the second divider capacitance comprise integrating capacitances configured to store charge received from the first capacitance.

13. The electrical circuit of claim 12 wherein the divider node is coupled to the first capacitance through a resistance.

14. The electrical circuit of claim 1 wherein the first capacitance is a variable capacitance, and the capacitance value comprises a measure of the first capacitance.

15. The electrical circuit of claim 1 wherein the charge changing circuit includes a second capacitance, wherein the second capacitance is a variable capacitance, and wherein the capacitance value comprises a measure of the second capacitance.

16. The electrical circuit of claim 1 wherein the charge changing circuit includes a second capacitance, and wherein the capacitance value comprises a measure of a ratio of the first capacitance and the second capacitance.

17. The electrical circuit of claim 1 wherein the capacitance value comprises a measure of a capacitance measured between a sensor electrode and proximate object, wherein the capacitance measured between the sensor electrode and the proximate object is variable.

18. The electrical circuit of claim 1 wherein the capacitance value comprises a transcapacitance, wherein the transcapacitance is a variable capacitance measured between sensor electrodes.

19. The electrical circuit of claim 1 wherein the capacitance value comprises a measure of a transcapacitance measured between sensor electrodes, and wherein applying the voltage to the first capacitance using the first switch comprises applying the voltage to a first one of the sensor electrodes.

20. The electrical circuit of claim 1 wherein the capacitance value comprises a measure of a transcapacitance measured between a first sensor electrode and a second sensor electrode, the first sensor electrode directly coupled to the passive network, and the electrical circuit further comprising a reference voltage source, the reference voltage source configured to couple to an input of the quantizer using a second switch, and wherein the controller is further configured to apply the reference voltage to the input of the quantizer.

21. The electrical circuit of claim 20 wherein the reference voltage source is coupled to the first sensor electrode using the second switch, and wherein the controller is configured to apply the reference voltage to the first sensor electrode.

22. The electrical circuit of claim 20 wherein reference voltage is substantially equal to the threshold of the quantizer.

23. A method for measuring a capacitance value, the method comprising the steps of:

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

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

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

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

24. The method of claim 23 wherein the capacitance value comprises a measure of a variable capacitance measured between a sensor electrode and an object proximate the sensor electrode.

25. The method of claim 23 wherein the capacitance value comprises a measure of a variable transcapacitance measured between sensor electrodes.

26. The method of claim 25 wherein the first capacitance comprises the variable transcapacitance, and wherein the applying a voltage to a first capacitance step comprises applying a voltage across the transcapacitance by applying a first voltage to a first one of the sensor electrodes.

27. The method of claim 26 wherein the allowing the first capacitance to share charge with a passive network comprises disconnecting the first voltage to the first one of the sensor electrodes, the first one of the sensor electrodes coupled to the passive network and changing the voltage on a second sensor electrode.

28. The method of claim 23 wherein the capacitance value comprises a transcapacitance measured between sensor electrodes, and wherein the method further comprises the step of selectively driving voltages on the sensor electrodes.

29. The method of claim 28 wherein the step of selectively driving voltages on the sensor electrodes capacitance value comprises selectively driving one of the sensor electrodes to a voltage substantially equal to a reference voltage during at least a portion of applying step to reduce voltage swings on the one of the sensor electrodes during the allowing step.