ACTIVE INTEGRATOR FOR A CAPACITIVE SENSE ARRAY

• US 20120256869A1
• Filed: 09/30/2011
• Published: 10/11/2012
• Est. Priority Date: 04/05/2011
• Status: Active Grant

• Current Assignee: Parade Technologies, Ltd.
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Anderson, Erik, Rohilla, Gajender, Walsh, Paul, Klein, Hans W., O'Donoghue, Keith, Hancioglu, Erhan
• Granted Patent: US 9,268,441 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 2203/04103 Manufacturing, i.e. details related to manufacturing processes specially suited for touch sensitive devices
• CPC Class Codes: G06F 2203/04111 Cross over in capacitive digitiser, i.e. details of structures for connecting electrodes of the sensing pattern where the connections cross each other, e.g. bridge structures comprising an insulating layer, or vias through substrate
• CPC Class Codes: G06F 3/03547 Touch pads, in which fingers can move on a surface
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• 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 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• 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: G06F 3/0446 using a grid-like structure of electrodes in at least two directions, e.g. using row and column electrodes

Abstract

An active integrator for sensing capacitance of a touch sense array is disclosed. The active integrator is configured to receive from the touch sense array a response signal having a positive portion and a negative portion. The response signal is representative of a presence or an absence of a conductive object on the touch sense array. The active integrator is configured to continuously integrate the response signal.

Claims

1. A circuit, comprising

an active integrator configured to measure a capacitance of a touch sense array, wherein the active integrator is configured to receive from the touch sense array a response signal having a positive portion and a negative portion, wherein the response signal is representative of a presence or an absence of a conductive object on the touch sense array, and wherein the active integrator is configured to continuously integrate the response signal.

2. The circuit of claim 1, wherein the active integrator comprises a switched capacitor integrator.

3. The circuit of claim 1, wherein the active integrator is configured to have a pass-band centered about a fundamental frequency of the response signal.

4. The circuit of claim 1, further comprising:

a first feedback capacitor coupled to an output terminal of the active integrator and a first input terminal of the active integrator, wherein the first feedback capacitor is configured to store a charge responsive to the positive portion; and

a second feedback coupled to the output terminal of the active integrator and the first input terminal of the active integrator, wherein the second feedback capacitor is configured to store a charge responsive to the negative portion.

5. The circuit of claim 4, further comprising:

a first switch configured to connect or disconnect the first feedback capacitor between the output terminal and the first input terminal; and

a second switch connect to connect or disconnect the second feedback capacitor between the output terminal and the first input terminal.

6. The circuit of claim 5,

wherein the first switch is closed responsive to the positive portion and open responsive to the negative portion, and

wherein the second switch is closed responsive to the negative portion and open responsive to the positive portion.

7. The circuit of claim 1, further comprising a baseline compensation circuit coupled to a second input terminal of the active integrator and configured to minimize a direct current (DC) component of the response signal.

8. The circuit of claim 7, wherein the baseline compensation circuit comprises:

a digital-to-analog (D/A) converter;

a gain circuit coupled to the D/A converter; and

a first switch and a second switch, each coupled to the gain circuit and the second input terminal of the active integrator,

wherein the first switch is configured to apply a negative signal to the second input terminal of the active integrator when the positive portion is present, and wherein the second switch is configured to apply a positive signal to the second input terminal of the active integrator when the negative portion is present.

9. The circuit of claim 1, further comprising a sample-and-hold circuit coupled to an output terminal of the active integrator, wherein the output signal of the active integrator is impressed upon the sample-and-hold circuit.

10. The circuit of claim 9, wherein the sample-and-hold circuit comprising:

a first capacitor and a first switch connected in series and coupled between the output terminal of the active integrator and a first input terminal of an analog-to-digital converter (ADC); and

a second capacitor and a second switch connected in series and coupled between the output terminal of the active integrator and a second input terminal of the ADC,

wherein the first switch is configured to cause the first capacitor to hold a positive signal on the output terminal of the active integrator when the positive portion is present, and wherein the second switch is configured to cause the second capacitor to hold a negative signal on the output terminal of the active integrator when the negative portion is present.

11. The circuit of claim 1, further comprising a self-calibration circuit coupled between an input signal source and the touch sense array.

12. The circuit of claim 11, wherein the self-calibration circuit comprises:

a first capacitor coupled between the input signal source and the touch sense array;

a second capacitor coupled between the input signal source and the touch sense array; and

a switch in series with the second capacitor,

wherein the switch is configured to insert or remove the first capacitor to simulate a presence or an absence of a touch object.

13. The circuit of claim 1, further comprising a mode select circuit coupled to the active integrator and configured to permit the active integrator to integrate in a self-capacitance sensing mode and a mutual-capacitance sensing mode.

14. A method, comprising:

receiving a response signal at an active integrator from a touch sense array, the response signal having a positive portion and a negative portion; and

continuously integrating the response signal using the active integrator to measure a change in capacitance, wherein the change in capacitance is representative of an absence or a presence of the conductive object.

15. The method of claim 14, wherein continuously integrating the response signal further comprises:

(a) accumulating a charge responsive to the positive portion using a first capacitor coupled between an input terminal and an output terminal of the active integrator;

(b) accumulating a charge responsive to the negative portion using a second capacitor coupled between the input terminal and the output terminal of the active integrator; and

(c) repeating steps (a) and (b) for a predetermined number of cycles.

16. The method of claim 15, wherein said steps (a) and (b) further comprise:

sample-and-holding the charge responsive to the positive portion on a third capacitor coupled between the output terminal of the integration circuit and a first input terminal of an analog-to-digital converter (ADC); and

sample-and-holding the charge responsive to the negative portion to on fourth coupled between the output terminal of the integration circuit and a second input terminal of the ADC.

17. The method of claim 16, wherein said steps (a) and (b) further comprise:

(d) measuring a differential voltage across a combination of the third capacitor and the fourth capacitor;

(e) discharging charge across each of the first, second, third, and fourth capacitors; and

(f) repeating steps (a)-(e).

18. The method of claim 14, further comprising:

applying a baseline compensation to a first input terminal of the integration circuit, wherein said applying comprises;

applying a negative signal to the a first input terminal when the positive portion is present; and

applying a positive signal to the a first input terminal when the negative portion is present.

19. An apparatus, comprising:

a processing device configured to detect a presence of a conductive object on a touch sense array, wherein the processing device comprises a capacitive sensing circuit, comprising an active integrator configured to measure a capacitance of the touch sense array, wherein the active integrator is configured to receive from the touch sense array a response signal having a positive portion and a negative portion, wherein the response signal is representative of a presence or an absence of the conductive object on the touch sense array, and wherein the active integrator is configured to continuously integrate the response signal.

20. The apparatus of claim 19, further comprising:

a first feedback capacitor coupled to an output terminal of the active integrator and a first input terminal of the active integrator, wherein the first feedback capacitor is configured to store a charge responsive to the positive portion; and

a second feedback coupled to the output terminal of the active integrator and the first input terminal of the active integrator, wherein the second feedback capacitor is configured to store a charge responsive to the negative portion.