Noise rejection and parasitic capacitance removal implementations

• US 8,395,395 B2
• Filed: 11/27/2008
• Issued: 03/12/2013
• Est. Priority Date: 11/27/2007
• Status: Active Grant

• Current Assignee: Azoteq Holdings Limited
• Original Assignee: Azoteq Pty Ltd.
• Inventors: Bruwer, Frederick Johannes, Viljoen, Jean, Harker, Grant Cecil, Van Der Merwe, Douw Gerbrandt
• Primary Examiner(s): Koval, Melissa
• Assistant Examiner(s): Baldridge, Benjamin M
• Application Number:
  US12/593,956
• Publication Number:
  US 20100060301A1
• Time in Patent Office: 1,566 Days
• Field of Search: 324/686
• US Class Current: 324/686
• CPC Class Codes: H03K 17/16 Modifications for eliminating interference voltages or currents
• CPC Class Codes: H03K 17/955 using a capacitive detector

Abstract

A capacitive sensing circuit including a microchip and a sense plate, wherein the microchip comprises a hardware Q RF noise detector module that provides a logical indication of when noise impacts on the integrated circuit.

Claims

1. A capacitive sensing circuit comprising:

a sense plate for reflecting a capacitance to be measured; and

a capacitance measurement circuit including;

a charge transfer circuit that, during a charge transfer cycle, introduces a charge on said sense plate which reflects a capacitance associated with said sense plate, said charge being introduced by applying a charging voltage of a fixed value to said sense plate, and transfers an amount of charge from said sense plate to said capacitance measurement circuit, said charge transfer cycle being repeated a number of times to form a complete capacitance measurement cycle; and

a charge subtracting circuit that effectively reduces the measured capacitance by removing an amount of charge from said capacitance measurement circuit during each charge transfer cycle, wherein, during each charge transfer cycle, the amount of said charge removed from said capacitance measurement circuit by the charge subtracting circuit is in a proportion to the amount of said charge transferred from said sense plate to said capacitance measurement circuit, which proportion remains the same for each charge transfer cycle in the complete capacitance measurement cycle, irrespective of an amount of charge accumulated in the measurement circuit throughout the complete capacitance measurement cycle.

2. The capacitive sensing circuit of claim 1 wherein the charge subtraction circuit is employed for tuning the sense plate.

3. The capacitive sensing circuit of claim 1 wherein said capacitance measurement circuit includes a reference capacitor that is employed to accumulate the charge received from said sense plate during each charge transfer cycle and the amount of the charge removed by said subtraction circuit from said measurement circuit is reduced as a voltage of said reference capacitor increases.

4. The capacitive sensing circuit of claim 1 wherein the amount of the charge removed from said measurement circuit is not influenced by a value of capacitance associated with the sense plate.

5. The capacitive sensing circuit of claim 1, further including a separate sense channel that is designed not to be influenced by a user, and wherein said sense channel is used for detection of noise.

6. The capacitive sensing circuit of claim 5 further including means for detecting noise and identifying and eliminating capacitive measurement data gathered at the same time that the noise is detected.

7. The capacitive sensing circuit of claim 1 which includes a noise detector that enables the removal of a specific capacitance measurement value if noise was detected by the noise detector during a specific capacitive measurement cycle during which said measurement value was obtained.

8. The capacitive sensing circuit of claim 1 wherein said capacitance measurement circuit includes a reference capacitor that is employed to accumulate the charge received from said sense plate during each charge transfer cycle;

and

the charge removed by said subtraction circuit from said measurement circuit during each charge transfer cycle is removed from said reference capacitor.

9. The capacitive sensing circuit of claim 8 wherein said subtraction circuit includes a compensation capacitor and is configured such that during each charge transfer cycle, an amount of charge which is equal to the amount of charge to be removed from said reference capacitor is applied to the compensation capacitor;

and said subtraction circuit further includes means for discharging said compensation capacitor and thereby causing an amount of charge equal to the charge discharged from said compensation capacitor to be removed from the charge accumulated in said reference capacitor.

10. The capacitive sensing circuit of claim 9 wherein said charge subtraction circuit includes an op-amp circuit that is used to implement the charge removal from the reference capacitor during each charge transfer cycle.

11. A method of sensing capacitance which includes the steps of:

a) introducing a charge on a sense plate which reflects a capacitance associated with said sense plate, said charge being introduced by applying a charging voltage of a fixed value to said sense plate; and

b) transferring a first amount of said sense plate charge to a capacitance measurement circuit;

wherein steps a) and b) combined define a charge transfer cycle and said charge transfer cycle is repeated a number of times to form a complete capacitance measurement cycle; and

removing a second amount of charge from the capacitance measurement circuit during each charge transfer cycle,

wherein said first amount of charge that is transferred from said sense plate to said capacitance measurement circuit and said second amount of charge that is removed from said capacitance measurement circuit, stay in a proportion to one another that remains the same for all of said charge transfer cycles throughout the complete capacitance measurement cycle.

12. The method of claim 11 wherein said capacitance measurement circuit includes a reference capacitor that is employed to accumulate charge received from said sense plate during each charge transfer cycle and wherein the second amount of the charge removed from said capacitance measurement circuit is reduced as a voltage of said reference capacitor increases during the complete capacitance measurement cycle.

13. The method of claim 12 wherein the amount of said charge transferred from said sense plate to the capacitive measurement circuit is proportional to the value of the charging voltage minus a present value of the voltage of said reference capacitor, and

said second amount of charge removed from said capacitance measurement circuit during each charge transfer cycle is inversely proportional to the present value of the voltage of said reference capacitor.

14. The method of claim 11 wherein the second amount of the charge removed from said capacitance measurement circuit is not influenced by a value of capacitance associated with the sense plate.

15. The method of claim 11, further including the steps of detecting noise and in response to detection of noise, identifying and eliminating capacitive measurement data gathered at the same time that the noise is detected.

16. The method of claim 15 wherein capacitive measurement data gathered at the same time that noise is detected is eliminated.

17. The method of claim 11 wherein said capacitance measurement circuit includes a reference capacitor that is employed to accumulate the charge received from said sense plate during each charge transfer step, and the charge removed from said capacitance measurement circuit during each charge transfer cycle is removed from said reference capacitor.

18. The method of claim 17, wherein a compensation capacitor and an op-amp buffer circuit are provided to remove said second amount of charge from said reference capacitor during each charge transfer cycle and wherein, for each charge transfer cycle, said op-amp buffer circuit is configured to carry out the steps of:

applying an amount of charge to said compensation capacitor that is equal to the amount of charge to be removed from the reference capacitor; and

discharging said compensation capacitor in such manner that the amount of charge removed from said compensation capacitor during said discharging is also removed from said the charge accumulated in said reference capacitor.