Capacitive field sensor with sigma-delta modulator

• US 8,089,289 B1
• Filed: 07/02/2008
• Issued: 01/03/2012
• Est. Priority Date: 07/03/2007
• Status: Active Grant

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Kremin, Viktor, Ryshtun, Andriy
• Primary Examiner(s): Nguyen, Vincent Q
• Application Number:
  US12/167,100
• Time in Patent Office: 1,280 Days
• Field of Search: 324/658, 324/676, 324/678, 324/679, 324/683
• US Class Current: 324/678
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/96074 Switched capacitor

Abstract

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.

Claims

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.