CAPACITIVE TOUCH SENSE ARCHITECTURE

• US 20130162586A1
• Filed: 02/23/2012
• Published: 06/27/2013
• Est. Priority Date: 02/25/2011
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Erdogan, Ozan E., Shen, Guozhong, Anantharaman, Rajesh, Taparia, Ajay, Javid, Behrooz, Mahmud, Syed T., Chowdhury, Rahim
• Granted Patent: US 8,878,797 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 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

An analog front end circuit utilizes coherent detection within a capacitance measurement application. The analog front end circuit uses coherent detection to measure capacitance of a touch screen display. An analog excitation signal is modulated by a capacitor to be measured. The modulated signal is synchronously demodulated using a correlator, which includes a discrete mixer and a discrete integrator. The excitation signal is also input to the mixer such that the modulated signal is multiplied by the excitation signal. The excitation signal is an analog signal having a sine wave function or other waveform.

Claims

1. A capacitance measurement circuit comprising:

a. an external capacitor; and

b. a coherent detection circuit coupled to the external capacitor and configured to measure a capacitance of the external capacitor and to convert the measured capacitance to a corresponding voltage output.

2. The capacitance measurement circuit of claim 1 wherein the coherent detection circuit comprises a discrete mixing circuit and a discrete integration circuit.

3. The capacitance measurement circuit of claim 2 wherein the coherent detection circuit further comprises a signal generator coupled to the external capacitor, wherein the signal generator is configured to generate an analog excitation signal.

4. The capacitance measurement circuit of claim 3 wherein the coherent detection circuit further comprises an amplifier and a feedback capacitor coupled to an input and to an output of the amplifier, wherein the input of the amplifier is coupled to the external capacitor and the output of the amplifier is coupled to the mixing circuit.

5. The capacitance measurement circuit of claim 4 wherein a modulated analog excitation signal is input to a first input of the mixing circuit, wherein the modulated analog signal comprises the analog excitation signal modulated by a capacitance of the external capacitor.

6. The capacitance measurement circuit of claim 5 wherein the analog excitation signal generated by the signal generator is input to a second input of the mixing circuit, wherein the mixing circuit is configured to multiply the modulated analog excitation signal and the analog excitation signal and to output a mixed analog signal.

7. The capacitance measurement circuit of claim 6 wherein the integration circuit is configured to receive the mixed analog signal and to integrate the mixed analog signal over a discrete period of time to output the corresponding voltage output.

8. The capacitance measurement circuit of claim 7 wherein the period of time is a multiple of 1/f, where f is a frequency of the analog excitation signal.

9. The capacitance measurement circuit of claim 2 wherein the mixing circuit comprises a continuous time mixing circuit.

10. The capacitance measurement circuit of claim 2 wherein the integration circuit comprises a continuous time integration circuit.

11. The capacitance measurement circuit of claim 1 wherein the coherent detection circuit comprises a continuous time signal path, wherein the continuous time signal path is configured to receive a modulated analog signal from the external capacitor and convert the modulated analog signal to the corresponding voltage output.

12. The capacitance measurement circuit of claim 1 further comprising a touch screen display, wherein the touch screen display includes the external capacitor.

13. The capacitance measurement circuit of claim 1 wherein the coherent detection circuit comprises an analog synchronous demodulator.

14. A capacitance measurement circuit comprising:

a. a signal generator configured to generate an analog excitation signal;

b. a touch screen display having at least one capacitor, wherein the capacitor is configured to input the analog excitation signal and to output a modulated analog excitation signal, wherein the analog excitation signal is modulated according to a capacitance of the capacitor;

c. an amplifier coupled to the capacitor and a feedback capacitor coupled to an input and to an output of the amplifier, wherein the amplifier is configured to input the modulated analog excitation signal and to output an amplified modulated analog excitation signal;

d. a mixing circuit coupled to the amplifier and to the signal generator, wherein the mixing circuit is configured to input the amplified modulated analog excitation signal and an analog correlation signal that is correlated to the analog excitation signal, and to output a mixed analog signal; and

e. an integration circuit coupled to the mixing circuit, wherein the integration circuit is configured to receive the mixed analog signal and to integrate the mixed analog signal over a discrete period of time to output a voltage signal, wherein the voltage signal corresponds to the capacitance of the capacitor.

15. A method of measuring capacitance, the method comprising:

a. generating an analog excitation signal;

b. applying the analog excitation signal to a capacitor to be measured, thereby modulating the analog excitation signal; and

c. correlating the modulated analog excitation signal to a correlation signal to demodulate the modulated analog excitation signal resulting in an output voltage corresponding to a capacitance of the capacitor, wherein the correlation signal is correlated to the analog excitation signal.

16. The method of claim 15 wherein correlating the modulated analog excitation signal to the correlation signal comprises:

a. mixing the modulated analog excitation signal with the correlation signal resulting in a mixed analog signal; and

b. integrating the mixed analog signal over a period of time resulting in the output voltage.

17. The method of claim 15 wherein the output voltage comprises a DC voltage.

18. The method of claim 15 wherein correlating the modulated analog excitation signal to the correlation signal filters noise and interference from the modulated analog excitation signal.

19. The method of claim 15 wherein the capacitor measures a capacitance of a touch screen display.