Capacitive field sensor with sigma-delta modulator

• US 9,442,144 B1
• Filed: 10/21/2013
• Issued: 09/13/2016
• 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:
  US14/058,464
• Time in Patent Office: 1,058 Days
• Field of Search: 324679-688
• US Class Current: 1/1
• 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 comprising:

charging a sensing capacitor and a modulation capacitor in a first non-overlapping phase;

discharging the sensing capacitor in a second non-overlapping phase;

generating a modulation signal in response to accumulated charge on the modulation capacitor, the modulation signal having at least one characteristic;

determining a presence of an object proximate to the sensing capacitor based on the at least one characteristic of the modulation signal.

2. The method of claim 1, wherein in the first non-overlapping phase, the charging capacitor and the modulation capacitor are coupled to the charge source.

3. The method of claim 1, wherein, in the second non-overlapping phase, the sensing capacitor is coupled to a discharge voltage potential while the modulation capacitor is decoupled from the sensing capacitor.

4. The method of claim 1, wherein the generating of the modulation signal comprises:

coupling the modulation capacitor to a comparison circuit; and

comparing a voltage potential across the modulation capacitor to a reference voltage potential.

5. The method of claim 1 wherein the determining the presence of the object proximate to the sensing capacitor comprises:

comparing the characteristic of the modulation signal to a baseline characteristic; and

if the characteristic of the modulation signal differs substantially from the baseline characteristic, determining the presence of the object proximate to the sensing capacitor.

6. The method of claim 1, wherein the characteristic of the modulation signal is a duty cycle.

7. The method of claim 1, wherein the first and second non-overlapping phases are defined by a pseudo-random pulse generator.

8. A method comprising:

responsive to a charging signal, charging a modulation capacitor based on a capacitance of a sensing capacitor;

generating a modulation signal having a characteristic 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.

9. The method of claim 8, further comprising discharging the sensing capacitor in response to a discharge signal by coupling the sensing capacitor to a discharge voltage potential, wherein the discharging the sensing capacitor does not affect the charge on the modulation capacitor.

10. The method of claim 9, wherein the charging and discharging of the sensing capacitor is controlled by a pseudo-random signal generator.

11. The method of claim 8, wherein the modulation signal is configured to discharge the modulation capacitor through a discharge circuit in a first state and is configured to allow charging of the modulation capacitor based on the capacitance of the sensing capacitor in a second state.

12. The method of claim 8, wherein the characteristic of the modulation signal is a duty cycle.

13. The method of claim 8, further comprising:

comparing the characteristic of the modulation capacitor to a baseline characteristic; and

determining a presence of a conductive object on the sensing capacitor based on the comparison of the characteristic of the modulation capacitor to the baseline characteristic.

14. The method of claim 8, wherein the conductive object is a finger.

15. The method of claim 8, wherein the sensing capacitor is one capacitor of a plurality of capacitors.

16. The method of claim 8, wherein the sensing capacitor comprises a first drive electrode and second receive electrode, wherein the drive electrode is coupled to a charge voltage and wherein the receive electrode is coupled to the modulation capacitor.

17. An apparatus comprising:

a sensing capacitor alternately coupleable to a charge voltage and a discharge voltage in non-overlapping phases;

a modulation capacitor coupled to the sensing capacitor and to a modulation circuit, the modulation circuit configured to generate a modulation signal based on the charge on the modulation capacitor; and

a detection circuit configured to determine a presence of an object proximate to the sensing capacitor based on a characteristic of the modulation signal.

18. The apparatus of claim 17, wherein the characteristic of the modulating signal is asymmetrically responsive to noise and the presence of the object.

19. The apparatus of claim 17, wherein the sensing capacitor is one of a plurality of capacitor formed by the intersection of at least one electrode disposed along a first axis at a plurality of electrodes disposed along a second axis.

20. The apparatus of claim 17, wherein the sensing capacitor comprises to parallel wires, a first wire coupled to a charging/discharging circuit and a second wire coupled to the modulation capacitor.