Capacitive field sensor with sigma-delta modulator

• US 8,570,053 B1
• Filed: 02/23/2009
• Issued: 10/29/2013
• Est. Priority Date: 07/03/2007
• Status: Active Grant

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Ryshtun, Andriy, Kremin, Victor
• Primary Examiner(s): Nguyen, Vincent Q
• Application Number:
  US12/380,141
• Time in Patent Office: 1,709 Days
• Field of Search: 324/658, 324/678, 324/681, 324/683
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 2203/04101 2.5D-digitiser, i.e. digitiser detecting the X/Y position of the input means, finger or stylus, also when it does not touch, but is proximate to the digitiser's interaction surface and also measures the distance of the input means within a short range in the Z direction, possibly with a separate measurement setup
• CPC Class Codes: G06F 2203/04107 Shielding in digitiser, i.e. guard or shielding arrangements, mostly for capacitive touchscreens, e.g. driven shields, driven grounds
• CPC Class Codes: G06F 2203/04108 Touchless 2D- digitiser, i.e. digitiser detecting the X/Y position of the input means, finger or stylus, also when it does not touch, but is proximate to the digitiser's interaction surface without distance measurement in the Z direction
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers
• 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 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2017/9613 using two electrodes per touch switch
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/96074 Switched capacitor
• CPC Class Codes: H03K 2217/960745 Capacitive differential; e.g. comparison with reference capacitance

Abstract

A capacitive sensor includes a switching capacitor circuit, a comparator, and a charge dissipation circuit. The switching capacitor circuit reciprocally couples a sensing capacitor in series with a modulation capacitor during a first switching phase and discharges the sensing capacitor during a second switching phase. The comparator is coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response. The charge dissipation circuit is coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.

Claims

1. A method for determining a presence of an object, the method comprising:

charging a first capacitor and a second capacitor during a first phase;

discharging said first capacitor during a second phase;

measuring a charge of said second capacitor;

modulating a signal based on said charge of said second capacitor;

determining based on a duty cycle of said signal whether an object is proximate to said first capacitor, wherein said duty cycle of said signal is asymmetrically responsive to noise and a presence of an object.

2. The method of claim 1, wherein said duty cycle of said signal decreases relative to baseline duty cycle when a finger is proximate to said first capacitor.

3. The method of claim 1, wherein said duty cycle of said signal increases relative to a baseline duty in response to noise.

4. The method of claim 1, wherein a diode is used to block discharge from said second capacitor.

5. The method of claim 4, wherein said diode functions to provide noise immunity to said second capacitor, during said second phase.

6. A capacitive sensor circuit, comprising:

a modulation capacitor coupled with a sensing capacitor;

a plurality of switches configured to charge the sensing capacitor and the modulation capacitor during a first phase, and discharge the sensing capacitor during a second phase;

a modulator circuit coupled with the modulation capacitor and configured to modulate a signal based on an amount of charge stored on the modulation capacitor;

a measurement circuit coupled with the modulator circuit, wherein the measurement circuit is configured to determine based on a duty cycle of said signal whether an object is proximate to said sensing capacitor, wherein said duty cycle of said signal is asymmetrically responsive to noise and a presence of the object, wherein during the second phase, wherein the plurality of switches are configured to couple the sensing capacitor to ground.

7. The capacitive sensor circuit of claim 6, wherein the measurement circuit is configured to determine that the object is within a threshold proximity to the sensing capacitor when the duty cycle decreases relative to a baseline duty cycle.

8. The capacitive sensor circuit of claim 6, wherein the modulator circuit is further configured to increase the duty cycle of the signal in response to noise.

9. The capacitive sensor circuit of claim 8, wherein the modulator circuit comprises a comparator configured to generate an output based on comparing a voltage of the modulation capacitor with a reference voltage.

10. The capacitive sensor circuit of claim 6, further comprising a diode configured to block discharge from the modulation capacitor.

11. The capacitive sensor circuit of claim 10, wherein the diode is configured to provide noise immunity to the modulation capacitor during the second phase.

12. The capacitive sensor circuit of claim 10, wherein during the first phase, the plurality of switches is configured to couple the sensor capacitor to the modulation capacitor and to the diode.

13. A capacitive sensor circuit, comprising:

a modulation capacitor coupled with a sensing capacitor;

a plurality of switches configured to charge the modulation capacitor based on a capacitance of the sensing capacitor;

a modulation circuit coupled with the modulation capacitor and configured to generate a modulated signal having a duty cycle that changes in a first direction in response to a change in capacitance of the sensing capacitor and changes in a second direction in response to noise.

14. The capacitive sensor of claim 13, wherein the plurality of switches is further configured to:

charge the sensing capacitor and the modulation capacitor during a first phase, and

discharge the sensing capacitor during a second phase.

15. The capacitive sensor of claim 13, further comprising a diode coupled between the sensing capacitor and the modulation capacitor, wherein the diode is configured to rectify a current resulting from the noise.

16. The capacitive sensor of claim 13, further comprising a diode coupled between the sensing capacitor and the modulation capacitor, wherein the diode is configured to block discharge from the modulation capacitor.

17. The capacitive sensor of claim 13, wherein the plurality of switches is configured to charge the sensing capacitor and the modulation capacitor during the first phase by connecting the sensing capacitor and the modulation capacitor to a voltage source.

18. The capacitive sensor of claim 13, wherein the modulation circuit is configured to decrease the duty cycle relative to a baseline duty cycle when the capacitance of the sensing capacitor is increased by an object proximate to the sensing capacitor.

19. The capacitive sensor of claim 13, wherein the modulator circuit is further configured to increase the duty cycle of the signal in response to noise.

20. The capacitive sensor of claim 13, wherein the modulator circuit comprises a comparator configured to generate an output based on comparing a voltage of the modulation capacitor with a reference voltage.