Capacitance measurement system and methods

• US 9,500,686 B1
• Filed: 07/27/2011
• Issued: 11/22/2016
• Est. Priority Date: 06/29/2007
• Status: Active Grant

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Fuller, Thomas Middleton Rutledge, Lee, Mark, Wilson, Cole, Bokma, Louis, Best, Andrew
• Primary Examiner(s): Baldridge, Benjamin M
• Application Number:
  US13/191,806
• Time in Patent Office: 1,945 Days
• Field of Search: 324/663, 345/173, 345/174
• US Class Current: 1/1
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/26 Measuring inductance or capacitance; Measuring quality factor, e.g. by using the resonance method; Measuring loss factor; Measuring dielectric constants ; Measuring impedance or related variables
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G01R 31/28 Testing of electronic circuits, e.g. by signal tracer EMC, EMP or similar testing of electronic circuits G01R31/002; testing for short-circuits, discontinuities, leakage or incorrect line connection G01R31/50; checking computers or computer components G06F11/00; checking static stores for correct operation G11C29/00 ; testing receivers or transmitters of transmission systems H04B17/00
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960715 Rc-timing; e.g. measurement of variation of charge time or discharge time of the sensor

Abstract

A first capacitor and a second capacitor are charged until voltage at the second capacitor settles to a settling voltage. While charging, the first capacitor is alternately switched between a current source and ground. When the settling voltage is reached, charging of the first capacitor is halted. The second capacitor continues to be charged until voltage at the second capacitor reaches a reference voltage. The amount of time it takes for the settling voltage to reach the reference voltage corresponds to a measure of capacitance on the first capacitor.

Claims

1. A system for measuring capacitance comprising:

a current source coupled to a first node of a first capacitor and to a first node of a second capacitor, the current source configured to supply charge to the first and second capacitors, wherein the charge supplied to the first and second capacitors generates a voltage potential across the first and second capacitors;

a first switch configured to couple the current source and the first node of the first capacitor to the first node of the second capacitor;

a second switch configured to couple the first node of the first capacitor to a second node of the second capacitor; and

a circuit configured to measure the voltage potential across the first and second capacitors, wherein,

the second capacitor is coupled to the measurement circuit after the current source is configured and coupled to the first node of the first capacitor and after a voltage potential across the first capacitor has reached a settling voltage, and

the current source is configured to provide a charge on the first capacitor according to a predetermined charge rate.

2. The system of claim 1 wherein the first and second capacitors are coupled to the current source and to the voltage measurement circuit through a multiplexer bus.

3. The system of claim 1, wherein the second capacitor is coupled to the voltage measurement circuit and the current source through a pin tap.

4. The system of claim 1, wherein the voltage measurement circuit comprises a first comparator comprising a first input coupled to the first and second capacitors, a second input coupled to a first reference voltage, and an output coupled to a processing means.

5. The system of claim 4, wherein the voltage measurement circuit further comprises a second comparator comprising a first input coupled to the first and second capacitors, a second input coupled to a second reference voltage, and an output coupled to a processing means.

6. The system of claim 1 further comprising a reference buffered coupled intermediate to the current source and the second capacitor.

7. The system of claim 6, wherein the reference buffer is configured to charge the second capacitor at a rate substantially greater than the programmable current source.

8. The system of claim 1, wherein the first capacitor is a capacitance sensing input configured to have a variable capacitance in response to capacitance coupling to an activating element.

9. The system of claim 8, wherein the variable capacitance is capacitance between the first node of the first capacitor and a ground potential.

10. The system of claim 8, wherein the variable capacitance is a mutual capacitance between the first node of the first capacitor and a drive electrode, the drive electrode configured to provide a variable voltage signal.

11. A method for measuring a capacitive input comprising:

configuring a current source to charge a first capacitor, wherein a first node of the first capacitor is the capacitive input and wherein the current source has an adjustable output;

coupling the first capacitor and the current source to a multiplexer bus;

coupling a second capacitor to the multiplexer bus;

charging a first capacitor and a second capacitor until voltage at said second capacitor settles to a settling voltage derived from the current source, a settling time, and the capacitance of the first capacitor, wherein during said charging said first capacitor is switched back and forth between a current source and ground;

when said settling voltage is reached, halting charging of said first capacitor while continuing to charge said second capacitor until voltage at said second capacitor reaches a reference voltage that is greater than said settling voltage; and

determining a measure of time for said settling voltage to reach said reference voltage, wherein said measure of time corresponds to a measure of capacitance on said first capacitor.

12. The method of claim 11 further comprising:

charging a third capacitor until voltages at said second and third capacitors settle to said settling voltage; and

after said settling voltage is reached and said charging of said first capacitor is halted, continuing charging said third capacitor until voltage at said third capacitor reaches said reference voltage.

13. The method of claim 11 wherein said charging further comprises supplying a constant charging current to said first and second capacitors using a digital current source.

14. The method of claim 11 wherein said determining comprises counting a first number of oscillatory cycles until said settling voltage reaches said reference voltage, wherein said first number corresponds to said measure of capacitance on said first capacitor.

15. The method of claim 14 further comprising:

halting charging of said second capacitor and reducing voltage at said second capacitor to less than said threshold voltage after voltage at said second capacitor reaches said threshold voltage;

charging said first and second capacitors until voltage at said second capacitor settles to said settling voltage;

halting charging of said first capacitor while continuing to charge said second capacitor until voltage at said second capacitor again reaches said reference voltage when said settling voltage is again reached; and

counting a second number of oscillatory cycles until voltage at said second capacitor reaches said reference voltage.

16. The method of claim 15 further comprising comparing said first and second numbers to identify a change in said measure of capacitance on said first capacitor.

17. The method of claim 11 wherein said first capacitor is one of a plurality of capacitors, wherein said method further comprises measuring capacitance for each capacitor of said plurality of capacitors.

18. The method of claim 11 further comprising detecting an element in sensing range of at least one of said capacitors based on capacitances measured for said plurality of capacitors.

19. The method of claim 11 further comprising switching said first capacitor to ground and disconnecting said first capacitor from said second capacitor after said settling voltage is reached.

20. The method of claim 11 further comprising:

comparing the measure of time to a reference measure of time;

adjusting an output of an internal main oscillator in response to the comparison;

adjusting at least one of a plurality of trim values for the internal main oscillator after adjusting the output of the internal main oscillator;

configuring an oscillator for the measuring of the capacitive input after adjusting the at least one the plurality of trim values; and

repeating the measuring of the capacitive input.

21. An apparatus comprising a programmable current source coupled to a first variable capacitor and a switch network, the switch network configured to couple the programmable current source alternately to a ground potential and to a second capacitor and an input of a voltage measurement circuit, wherein the programmable current source is coupled to the second capacitor only after the voltage on the first variable capacitor and the second capacitor reaches a settling voltage.

22. The apparatus of claim 21 wherein the voltage measurement comprises a first comparator comprising a first input coupled to the first and second capacitors, a second input coupled to a first reference voltage, and an output coupled to a processing means.

23. The apparatus of claim 22 wherein the voltage measurement further comprises a second comparator comprising a first input coupled to the first and second capacitors, a second input coupled to a second reference voltage, and an output coupled to a processing means.

24. The apparatus of claim 21 further comprising a reference buffer coupled intermediate to the switch network and the second capacitor, wherein the reference buffer is configured to charge the second capacitor at a rate substantially greater than the programmable current source.

25. The system of claim 21, wherein the variable capacitor comprises a first node coupled to the measurement current source and a second node to a ground potential.

26. The system of claim 21, wherein the variable capacitor is a mutual capacitor between a first node of the variable capacitor and a drive electrode, the drive electrode configured to provide a variable voltage signal.