Method and apparatus for sensing capacitance value and converting it into digital format

A capacitance sensing device coupled to operate a keyless entry system comprising:

• a. a sensed capacitor;
• b. a signal generator configured to generate a digital local oscillator signal and an excitation signal;
• c. a capacitance-to-amplitude converter coupled to the sensed capacitor and to the signal generator, wherein the capacitance-to-amplitude converter is configured to convert a measured capacitance received from the sensed capacitor to an AC signal according to the excitation signal, the AC signal having an amplitude proportional to the measured capacitance;
• d. a demodulator coupled to the capacitance-to-amplitude converter to receive the AC signal and to the signal generator to receive the digital local oscillator signal, wherein the demodulator is configured to output a DC component proportional to the AC signal and to output an AC component;
• and
• e. an analog-to-digital converter coupled to the output of the demodulator, wherein the analog-to-digital converter is configured to output a digital conversion value proportional to the DC component, which is proportional to the measured capacitance, wherein the keyless entry system is coupled to receive the digital conversion value.

A method of converting a measured capacitance for operating a keyless entry system, the method comprising:

• a. measuring a capacitance;
• b. generating a digital local oscillator signal and an excitation signal;
• c. converting the measured capacitance according to the excitation signal to form an AC signal having an amplitude proportional to the measured capacitance;
• d. multiplying the AC signal and the digital local oscillator signal to form a demodulated signal with a DC component proportional to the AC signal and an AC component;
• and
• e. converting the demodulated signal to a digital conversion value proportional to the DC component, which is proportional to the measured capacitance;
• and
• f. operating the keyless entry system in response to the digital conversion value.

A capacitive sensing device comprising:

• a. a sensed capacitor;
• b. a first circuit for generating a signal to excite the capacitor;
• c. a second circuit to sense an amplitude of a resulting signal on the sensed capacitor;
• d. an analog-to-digital converter to form a digital conversion value proportional to the amplitude of the resulting signal.