Receive demodulator for capacitive sensing

• US 8,575,947 B1
• Filed: 01/11/2013
• Issued: 11/05/2013
• Est. Priority Date: 11/21/2008
• Status: Active Grant

• Current Assignee: Parade Technologies, Ltd.
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Walsh, Paul, Zhang, Zheyao, Kremin, Viktor
• Primary Examiner(s): NATALINI, JEFF WILLIAM
• Application Number:
  US13/739,991
• Time in Patent Office: 298 Days
• Field of Search: None
• US Class Current: 324/658
• CPC Class Codes: G06F 2203/0339 Touch strips, e.g. orthogonal touch strips to control cursor movement or scrolling; single touch strip to adjust parameter or to implement a row of soft keys
• CPC Class Codes: G06F 2203/0382 Plural input, i.e. interface arrangements in which a plurality of input device of the same type are in communication with a PC
• CPC Class Codes: G06F 3/03547 Touch pads, in which fingers can move on a surface
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

A first integration capacitor stores charge from a positive signal portion. A second integration capacitor stores charge from negative signal portion. The voltage across the first and second integration capacitors is measured differentially. The presence of a conductive object proximate to a capacitance sensing element is detected based on the measured differential voltage between the first and second integration capacitors.

Claims

1. A method, comprising:

detecting a current flowing at a terminal of a capacitance sensing element;

coupling the current to a charge storage circuit to generate a first output voltage signal and a second output voltage signal; and

detecting a presence of a conductive object proximate to the capacitance sensing element based on a difference between the first output voltage signal and the second output voltage signal.

2. The method of claim 1, further comprising:

alternately driving a first electrode of the capacitance sensing element with a high supply voltage and a low supply voltage; and

holding a second electrode of the capacitance sensing element at a fixed voltage level, wherein from the second electrode flows current to the charge storage circuit.

3. The method of claim 1, wherein the charge storage circuit comprises a capacitor circuit.

4. The method of claim 1, wherein the charge storage circuit comprises a quasi-differential capacitor circuit comprising a positive integration capacitor and a negative integration capacitor.

5. The method of claim 4, further comprising:

pre-charging the positive integration capacitor to a first voltage level; and

pre-charging the negative integration capacitor to a second voltage level.

6. The method of claim 4, further comprising:

mirroring the current across the capacitance sensing element and coupling a mirrored current to the charge storage circuit.

7. The method of claim 6, further comprising storing a charge from the mirrored current in the charge storage circuit, the storing comprising:

charging the positive integration capacitor when a flow of current is positive; and

discharging the negative integration capacitor when the flow of current is negative.

8. The method of claim 4, further comprising:

outputting a first voltage level stored on the positive integration capacitor to a processing circuit as the first output voltage signal; and

outputting a second voltage level stored on the negative integration capacitor to the processing circuit as the second output voltage signal.

9. The method of claim 8, further comprising:

performing one or more subconversions to output the first and second voltage levels stored on the positive and negative integration capacitors to the processing circuit; and

accumulating results of the one or more subconversions to determine a total change in capacitance.

10. An apparatus, comprising:

a terminal coupled to an electrode of a capacitive sensing device;

a coupling circuit coupled to the electrode of the capacitive sensing device; and

a charge storage circuit coupled to the coupling circuit, wherein the charge storage circuit is configured to generate a first output voltage signal from a first portion of the signal received from the electrode and a second output voltage signal from a second portion of the signal received from the electrode for a detection of a presence of a conductive object proximate to the capacitive sensing device based on a difference between the first output voltage signal and the second output voltage signal.

11. The apparatus of claim 10, wherein the coupling circuit comprises a low impedance input coupled to the electrode of the capacitive sensing device, the low impedance input configured to hold the electrode at a fixed voltage level.

12. The apparatus of claim 11, wherein the coupling circuit further comprises:

a positive bipolar output coupled to the charge storage circuit, the positive bipolar output configured to mirror a current sensed at the low impedance input; and

a negative bipolar output, the negative bipolar output configured to invert the current sensed at the low impedance input.

13. The apparatus of claim 12, wherein the charge storage circuit further comprises an integration capacitor coupled between an output node and a ground node.

14. The apparatus of claim 13, wherein the charge storage circuit further comprises a second integration capacitor coupled between the output node and the ground node.

15. The apparatus of claim 12, wherein the charge storage circuit comprises a quasi-differential capacitor circuit, the quasi-differential capacitor circuit comprising a first integration capacitor coupled between a first output node and a ground node and a second integration capacitor coupled between a second output node and the ground node.

16. The apparatus of claim 15, further comprising a synchronous detect circuit coupled to the charge storage circuit, the synchronous detect circuit configured to charge the first integration capacitor when the current at the bipolar output is positive and to discharge the second integration capacitor when the current is negative.

17. The apparatus of claim 10, further comprising a processing circuit coupled to the charge storage circuit, the processing circuit configured to detect the presence of the conductive object based on the first output voltage signal and the second output voltage signal.

18. A method, comprising:

charging a first integration capacitor with a positive signal portion of a signal received from an electrode of a capacitive sensing device;

charging a second integration capacitor with a negative signal portion of the signal received from the electrode of the capacitive sensing device;

measuring a differential voltage between the first and second integration capacitors; and

detecting a presence of a conductive object proximate to the capacitive sensing device based on the measured differential voltage.

19. The method of claim 18, wherein the positive signal portion and the negative signal portion are different components of a single signal received from the capacitance sensing element capacitive sensing device.

20. The method of claim 18, wherein the first integration capacitor is a positive integration capacitor and the second integration capacitor is a negative integration capacitor.