Capacitive field sensor with sigma-delta modulator
View Patent ↗A capacitive sensor includes a switching capacitor circuit, a comparator, and a charge dissipation circuit. The switching capacitor circuit reciprocally couples a sensing capacitor in series with a modulation capacitor during a first switching phase and discharges the sensing capacitor during a second switching phase. The comparator is coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response. The charge dissipation circuit is coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.
1. A method of operating a capacitive sensor, the method comprising:
charging first and second series coupled capacitors during a first operation phase;
discharging the first capacitor while holding charge on the second capacitor during a second operation phase;
comparing a voltage potential on the second capacitor to a reference; and
selectively discharging the second capacitor in response to the comparing.
2. The method of claim 1 , further comprising:
generating a feedback signal in response to the comparing; and
controlling the selective discharging of the second capacitor based on the feedback signal.
3. The method of claim 1 , further comprising:
generating a signal based on the comparing;
measuring a duty cycle of the signal; and
determining whether a capacitance of the first capacitor changed based on a change in the duty cycle.
4. The method of claim 1 , wherein holding charge on the second capacitor comprises reverse biasing a diode coupled between the first and second capacitors and wherein charging the first and second capacitors comprises forward biasing the diode.
5. The method of claim 4 , further comprising:
driving a pseudo-random signal into the first capacitor; and
driving a control terminal of a switch coupled between a circuit node and a ground with an inversion of the pseudo-random signal, wherein the circuit node is positioned between the first capacitor and the diode.
6. The method of claim 4 , further comprising:
filtering noise during the second phase of operation through a resistor coupled between the ground and the first capacitor.
7. The method of claim 1 , wherein the voltage potential on the second capacitor oscillates about the reference voltage.