Capacitive touch sense architecture having a correlator for demodulating a measured capacitance from an excitation signal
View Patent ↗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.
1. A capacitance measurement circuit comprising:
an external capacitor;
an amplifier having an input and an output, the amplifier comprising a feedback capacitor disposed between the input and output, the input of the amplifier coupled to the external capacitor, the amplifier configured to amplify a modulated excitation signal furnished to the input of the amplifier by the external capacitor, the modulated excitation signal modulated according to a capacitance of the external capacitor; and
a correlator configured to synchronously demodulate the modulated excitation signal, the correlator comprising a mixing circuit coupled to the output of the amplifier, the mixing circuit configured to multiply the modulated excitation signal by an original non-modulated analog excitation signal to generate a mixed signal, and an integrator circuit coupled to the mixing circuit, the integrator circuit configured to integrate the mixed signal over a discrete period of time to generate an output voltage corresponding to the capacitance of the external capacitor,
wherein the period of time is a multiple of 1/f, where f is a frequency of the analog excitation signal.
2. The capacitance measurement circuit of claim 1 further comprising a signal generator coupled to the external capacitor, the signal generator configured to generate an analog excitation signal.
3. The capacitance measurement circuit of claim 2 wherein the analog excitation signal generated by the signal generator is input to the mixing circuit.
4. The capacitance measurement circuit of claim 1 wherein the mixing circuit comprises a continuous time mixing circuit.
5. The capacitance measurement circuit of claim 1 wherein the integration circuit comprises a continuous time integration circuit.
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 modulated excitation signal includes a noise portion, and wherein the integrator circuit is configured to filter the noise portion such that the output voltage does not include the noise portion.
8. A capacitance measurement circuit comprising:
a signal generator configured to generate an analog excitation signal;
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 the analog excitation signal, and to output a mixed analog signal based upon the amplified modulated analog excitation signal and the analog excitation signal; and
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, 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 8 wherein the analog excitation signal includes a noise portion, and wherein the integration circuit is configured to filter the noise portion such that the voltage signal does not include the noise portion.
10. A method of measuring capacitance, the method comprising:
generating an analog excitation signal;
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; and
integrating, via an integrating circuit, the mixed signal over a period of time to generate an output voltage corresponding to the capacitance of the capacitor, wherein the period of time is a multiple of 1/f, where f is a frequency of the analog excitation signal.
11. The method of claim 10 wherein the output voltage comprises a DC voltage.
12. The method of claim 10 wherein the capacitor measures a capacitance of a touch screen display.