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 comprising:
charging a sensing capacitor and a modulation capacitor in a first non-overlapping phase;
discharging the sensing capacitor in a second non-overlapping phase;
generating a modulation signal in response to accumulated charge on the modulation capacitor, the modulation signal having at least one characteristic;
determining a presence of an object proximate to the sensing capacitor based on the at least one characteristic of the modulation signal.
2. The method of claim 1 , wherein in the first non-overlapping phase, the charging capacitor and the modulation capacitor are coupled to the charge source.
3. The method of claim 1 , wherein, in the second non-overlapping phase, the sensing capacitor is coupled to a discharge voltage potential while the modulation capacitor is decoupled from the sensing capacitor.
4. The method of claim 1 , wherein the generating of the modulation signal comprises:
coupling the modulation capacitor to a comparison circuit; and
comparing a voltage potential across the modulation capacitor to a reference voltage potential.
5. The method of claim 1 wherein the determining the presence of the object proximate to the sensing capacitor comprises:
comparing the characteristic of the modulation signal to a baseline characteristic; and
if the characteristic of the modulation signal differs substantially from the baseline characteristic, determining the presence of the object proximate to the sensing capacitor.
6. The method of claim 1 , wherein the characteristic of the modulation signal is a duty cycle.
7. The method of claim 1 , wherein the first and second non-overlapping phases are defined by a pseudo-random pulse generator.
8. A method comprising:
responsive to a charging signal, charging a modulation capacitor based on a capacitance of a sensing capacitor;
generating a modulation signal having a characteristic that changes in a first direction in response to a change in capacitance of the sensing capacitor and changes in a second direction in response to noise.
9. The method of claim 8 , further comprising discharging the sensing capacitor in response to a discharge signal by coupling the sensing capacitor to a discharge voltage potential, wherein the discharging the sensing capacitor does not affect the charge on the modulation capacitor.
10. The method of claim 9 , wherein the charging and discharging of the sensing capacitor is controlled by a pseudo-random signal generator.
11. The method of claim 8 , wherein the modulation signal is configured to discharge the modulation capacitor through a discharge circuit in a first state and is configured to allow charging of the modulation capacitor based on the capacitance of the sensing capacitor in a second state.
12. The method of claim 8 , wherein the characteristic of the modulation signal is a duty cycle.
13. The method of claim 8 , further comprising:
comparing the characteristic of the modulation capacitor to a baseline characteristic; and
determining a presence of a conductive object on the sensing capacitor based on the comparison of the characteristic of the modulation capacitor to the baseline characteristic.
14. The method of claim 8 , wherein the conductive object is a finger.
15. The method of claim 8 , wherein the sensing capacitor is one capacitor of a plurality of capacitors.
16. The method of claim 8 , wherein the sensing capacitor comprises a first drive electrode and second receive electrode, wherein the drive electrode is coupled to a charge voltage and wherein the receive electrode is coupled to the modulation capacitor.
17. An apparatus comprising:
a sensing capacitor alternately coupleable to a charge voltage and a discharge voltage in non-overlapping phases;
a modulation capacitor coupled to the sensing capacitor and to a modulation circuit, the modulation circuit configured to generate a modulation signal based on the charge on the modulation capacitor; and
a detection circuit configured to determine a presence of an object proximate to the sensing capacitor based on a characteristic of the modulation signal.
18. The apparatus of claim 17 , wherein the characteristic of the modulating signal is asymmetrically responsive to noise and the presence of the object.
19. The apparatus of claim 17 , wherein the sensing capacitor is one of a plurality of capacitor formed by the intersection of at least one electrode disposed along a first axis at a plurality of electrodes disposed along a second axis.
20. The apparatus of claim 17 , wherein the sensing capacitor comprises to parallel wires, a first wire coupled to a charging/discharging circuit and a second wire coupled to the modulation capacitor.