Capacitive to voltage sensing circuit

• US 8,390,361 B2
• Filed: 03/02/2011
• Issued: 03/05/2013
• Est. Priority Date: 12/28/2010
• Status: Active Grant

• Current Assignee: STMicroelectronics International N.V. (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Inventors: Ningrat, Kusuma Adi
• Primary Examiner(s): Donovan, Lincoln
• Assistant Examiner(s): Hernandez, William
• Application Number:
  US13/038,526
• Publication Number:
  US 20120161846A1
• Time in Patent Office: 734 Days
• Field of Search: 327/336, 327/337, 327/517, 345/173, 345/174
• US Class Current: 327/337
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

A circuit for converting a measured variable capacitance to an output voltage signal includes a charge amplifier circuit selectively coupled to an integrator circuit. The charge amplifier circuit, in one implementation, is configured as a high pass filter. In another implementation, the charge amplifier circuit is configured as a combination high pass and low pass filter. The charge amplifier circuit is selectively coupled to the integrator circuit when the circuit forces a switch in voltage across a measurement capacitor.

Claims

1. A circuit, comprising:

a sense node, where the sense node is adapted to supply a capacitive charge resulting from a forced switching of a voltage across a measurement capacitor;

an amplifier circuit having a first input coupled to the sense node and further having a first output, the amplifier circuit including a filtering structure adapted to amplify and filter the capacitive charge from the sense node;

an integrator circuit having a second input and a second output, the integrator circuit adapted to integrate the amplified capacitive charge and produce an output voltage at the second output;

a first switch adapted to selectively couple the first output of the amplifier circuit to the second input of the integrator circuit; and

a control circuit adapted to control actuation of the first switch to selectively couple the first output to the second input when forced switching of the voltage across the measurement capacitor occurs.

2. The circuit of claim 1, wherein the filtering structure of the amplifier circuit is a high pass filtering structure.

3. The circuit of claim 2, wherein the filtering structure of the amplifier circuit is further a low pass filtering structure.

4. The circuit of claim 1, wherein the filtering structure of the amplifier circuit is a low pass filtering structure.

5. The circuit of claim 4, wherein the filtering structure of the amplifier circuit is further a high pass filtering structure.

7. The circuit of claim 1, wherein the control circuit is further adapted to control forced switching of the voltage across the measurement capacitor.

6. A circuit, comprising:

a sense node, where the sense node is adapted to supply a capacitive charge resulting from a forced switching of a voltage across a measurement capacitor;

an amplifier circuit having a first input coupled to the sense node and further having a first output, the amplifier circuit adapted to amplify the capacitive charge from the sense node;

an integrator circuit having a second input and a second output, the integrator circuit adapted to integrate the amplified capacitive charge and produce an output voltage at the second output;

a first switch adapted to selectively couple the first output of the amplifier circuit to the second input of the integrator circuit; and

a control circuit adapted to control actuation of the first switch to selectively couple the first output to the second input when forced switching of the voltage across the measurement capacitor occurs,

wherein the amplifier circuit further comprises a second switch adapted to reset the amplifier circuit, and wherein the integrator circuit further comprises a third switch adapted to reset the integrator circuit, and wherein the control circuit is further adapted to control actuation of the second and third switches to reset the amplifier circuit and integrator circuit, respectively, prior to forced switching of the voltage across the measurement capacitor.

8. A circuit, comprising:

a sense node, where the sense node is adapted to supply a capacitive charge resulting from a forced switching of a voltage across a measurement capacitor;

an amplifier circuit having a first input coupled to the sense node and further having a first output, the amplifier circuit adapted to amplify the capacitive charge from the sense node and comprising;

an operational amplifier circuit having an input and an output,

a capacitor coupled between the input and output,

a resistor coupled between the input and output in parallel with the capacitor, and

a second switch coupled between the input and output in parallel with the capacitor;

an integrator circuit having a second input and a second output, the integrator circuit adapted to integrate the amplified capacitive charge and produce an output voltage at the second output;

a first switch adapted to selectively couple the first output of the amplifier circuit to the second input of the integrator circuit; and

a control circuit adapted to control actuation of the first switch to selectively couple the first output to the second input when forced switching of the voltage across the measurement capacitor occurs.

9. The circuit of claim 8, wherein the control circuit is further adapted to control actuation of the second switch to reset the amplifier circuit prior to forced switching of the voltage across the measurement capacitor.

10. A circuit, comprising:

a sense node, where the sense node is adapted to supply a capacitive charge resulting from a forced switching of a voltage across a measurement capacitor;

an amplifier circuit having a first input coupled to the sense node and further having a first output, the amplifier circuit adapted to amplify the capacitive charge from the sense node and comprising;

an input node coupled to the sense node,

an operational amplifier circuit having an input and an output;

a first capacitor coupled between the input and the output of the operational amplifier circuit,

first resistor coupled between the input node and the output of the operational amplifier circuit,

a second capacitor coupled between the input node and a reference node,

a second resistor coupled between the input node and the input of the operational amplifier circuit,

a second switch coupled between the input node and the output of the operational amplifier circuit in parallel with the first resistor, and

a third switch coupled between the input node and the input of the operational amplifier circuit in parallel with the second resistor;

an integrator circuit having a second input and a second output, the integrator circuit adapted to integrate the amplified capacitive charge and produce an output voltage at the second output;

a first switch adapted to selectively couple the first output of the amplifier circuit to the second input of the integrator circuit; and

a control circuit adapted to control actuation of the first switch to selectively couple the first output to the second input when forced switching of the voltage across the measurement capacitor occurs.

11. The circuit of claim 10, wherein the control circuit is further adapted to control actuation of the second and third switches to reset the amplifier circuit prior to forced switching of the voltage across the measurement capacitor.

12. The circuit of claim 10 wherein the second capacitor is a parasitic capacitance associated with a device including the measurement capacitor.

13. A circuit, comprising:

a capacitive touch device including;

a sense node;

a measurement capacitor coupled to the sense node; and

switching circuitry adapted to support forced switching of a voltage across the measurement capacitor, wherein the sense node is adapted to supply a capacitive charge resulting from forced switching of the voltage across the measurement capacitor;

a capacitance to voltage conversion circuit, comprising;

an amplifier circuit having a first input coupled to the sense node and further having a first output, the amplifier circuit including a filtering structure adapted to amplify and filter the capacitive charge from the sense node;

an integrator circuit having a second input and a second output, the integrator circuit adapted to integrate the amplified capacitive charge and produce an output voltage at the second output; and

a first switch adapted to selectively couple the first output of the amplifier circuit to the second input of the integrator circuit; and

a control circuit adapted to control actuation of the switching circuitry in the capacitive touch device to perform forced switching of the voltage across the measurement capacitor and control actuation of the first switch in the capacitance to voltage conversion circuit to selectively couple the first output to the second input when the switching circuitry in the capacitive touch device is actuated to perform forced switching of the voltage across the measurement capacitor.

14. The circuit of claim 13, wherein the filtering structure of the amplifier circuit includes a high pass filtering structure.

15. The circuit of claim 14, wherein the filtering structure of the amplifier circuit further includes a low pass filtering structure.

16. The circuit of claim 13, wherein the amplifier circuit comprises:

an operational amplifier circuit having an input and an output;

a capacitor coupled between the input and output;

a resistor coupled between the input and output in parallel with the capacitor; and

a second switch coupled between the input and output in parallel with the capacitor; and

wherein the control circuit is further adapted to control actuation of the second switch to shunt the capacitor prior to actuating the switching circuitry in the capacitive touch device to perform forced switching of the voltage across the measurement capacitor.

17. The circuit of claim 13 wherein the amplifier circuit comprises:

an input node coupled to the sense node;

an operational amplifier circuit having an input and an output;

a first capacitor coupled between the input and the output;

a first resistor coupled between the input node and the output;

a second capacitor coupled between the input node and a reference node;

a second resistor coupled between the input node and the input of the operational amplifier;

a second switch coupled between the input node and the output in parallel with the first resistor; and

a third switch coupled between the input node and the input of the operational amplifier in parallel with the second resistor; and

wherein the control circuit is further adapted to control actuation of the second and third switches to shunt the first capacitor and first and second resistors prior to actuating the switching circuitry in the capacitive touch device to perform forced switching of the voltage across the measurement capacitor.

18. The circuit of claim 17 wherein the second capacitor is a parasitic capacitance associated with a device including the measurement capacitor.

19. A method, comprising:

resetting a charge amplifier and an integrator;

dis-coupling an output of the charge amplifier from an input of the integrator;

filtering a sense node coupled to a measurement capacitor;

forcing a switch in voltage across the measurement capacitor to supply a capacitive charge to the sense node;

when forcing, coupling an output of the charge amplifier to an input of the integrator;

amplifying the supplied capacitive charge in the charge amplifier; and

integrating the amplifier capacitive charge in the integrator to produce a voltage output.

20. The method of claim 19, wherein filtering comprises filtering an offset arising from charge amplifier resetting.

21. The method of claim 19, further comprising, after integrating, dis-coupling the charge amplifier from the integrator and converting the voltage output from the integrator to a digital output signal.

22. The method of claim 19, wherein filtering the sense node coupled to the measurement capacitor comprises performing high-pass filtering.

23. The method of claim 22, wherein filtering the sense node coupled to the measurement capacitor further comprises performing low-pass filtering.

24. The method of claim 19, wherein filtering is performed by operation of the charge amplifier.