Capacitive touch sense architecture having a correlator for demodulating a measured capacitance from an excitation signal

Patent metadata

Field	Value
Publication number	US-9857932-B2
Application number	US-201715476745-A
Country	US
Kind code	B2
Filing date	Mar 31, 2017
Priority date	Feb 25, 2011
Publication date	Jan 2, 2018
Grant date	Jan 2, 2018

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.

First claim

What is claimed is:

1. A capacitance measurement circuit comprising:
an external capacitor; and
a coherent detection circuit coupled to the external capacitor and configured to measure a capacitance of the external capacitor, the coherent detection circuit including a signal generator coupled to the external capacitor, a mixing circuit and an integration circuit, wherein:
the signal generator is configured to generate an analog excitation signal having a first frequency “f”;
a modulated analog excitation signal, being the analog excitation signal modulated by a capacitance of the external capacitor, is input to a first input of the mixing circuit;
the analog excitation signal is input to a second input of the mixing circuit, the mixing circuit being configured to multiply the modulated analog excitation signal and the analog excitation signal and to output a mixed analog signal, the mixed analog signal including a DC voltage term plus a sine term having a second frequency approximately 2 f; and
the integration circuit filters out noise by providing a band-pass filtering function with a peak around f and is configured to output a corresponding voltage output that is proportional to a capacitance value of the capacitance.

2. The capacitance measurement circuit of claim 1 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.

3. The capacitance measurement circuit of claim 1 wherein the mixing circuit comprises a continuous time mixing circuit.

4. The capacitance measurement circuit of claim 1 wherein the integration circuit comprises a continuous time integration circuit.

5. 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.

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

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

8. A capacitance measurement circuit comprising:
a signal generator configured to generate an analog excitation signal having a first frequency “f”;
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;
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;
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, the mixed analog signal including a DC voltage term plus a sine term having a second frequency approximately 2 f; and
an integration circuit coupled to the mixing circuit, wherein the integration circuit filters out noise by providing a band-pass filtering function with a peak around f and is configured to receive the mixed analog signal and to output a voltage signal, wherein the voltage signal corresponds to the capacitance value of the capacitor.

9. A method of measuring capacitance, the method comprising:
generating an analog excitation signal having a first frequency “f”; and
applying the analog excitation signal to a capacitor to be measured, wherein:
the analog excitation signal is modulated based upon a capacitance of the capacitor; and multiplying, via a mixing circuit, the modulated analog excitation signal by the analog excitation signal to generate a mixed signal, the mixed signal including a DC voltage term plus a sine term having a second frequency approximately 2 f; and integrating the mixed signal, via an integrating circuit that filters out noise by providing a band-pass filtering function with a peak around f, and is configured to generate an output voltage corresponding to the capacitance of the capacitor.

10. The method of claim 9 wherein the output voltage comprises a DC voltage.

11. The method of claim 9 wherein integrating the mixed signal filters noise and interference from the modulated analog excitation signal.

12. The method of claim 9 wherein the capacitance is a capacitance of a touch screen display.

Assignees

• Qualcomm Inc

Inventors

• Erdogan Ozan E
• Shen Guozhong
• Anantharaman Rajesh
• Taparia Ajay
• Javid Behrooz
• Mahmud Syed T
• Chowdhury Rahim F

Classifications

• G06F3/0416 Primary

  Control or interface arrangements specially adapted for digitisers · CPC title

• G01R27/2605

  Measuring capacitance (capacitive sensors G01D5/24) · CPC title

• G06F3/044 Primary

  by capacitive means · CPC title

• G06F3/04182 Primary

  Filtering of noise external to the device and not generated by digitiser components · CPC title