Compensation circuit for a TX-RX capacitive sensor

Patent metadata

Field	Value
Publication number	US-9778301-B2
Application number	US-201414543529-A
Country	US
Kind code	B2
Filing date	Nov 17, 2014
Priority date	Dec 14, 2007
Publication date	Oct 3, 2017
Grant date	Oct 3, 2017

Abstract

A capacitive sensor may include a transmit electrode and a receive electrode capacitively coupled with the transmit electrode. A capacitance sensing circuit senses a capacitance between the transmit and receive electrodes by applying a signal to the transmit electrode and rectifying a current waveform induced at the receive electrode. A compensation circuit reduces the effect of a mutual and parasitic capacitances of the transmit and receive electrode pair by adding a compensation current to the rectified current.

First claim

What is claimed is:

1. A device, comprising:
a sensor including a first electrode and a second electrode coupled capacitively, wherein the first electrode is driven by a transmit signal to induce a receive (RX) signal at the second electrode, wherein a baseline capacitance between the driven first electrode and the second electrode is defined when no external input is present;
a demodulation circuit that generates a modified signal based on the RX signal, wherein in a first configuration the demodulation circuit outputs a positive modified signal based on a positive portion of the RX signal, and in a second configuration the demodulation circuit outputs a negative modified signal based on a negative portion of the RX signal; and
a compensation circuit receiving the modified signal, wherein the compensation circuit is configured to generate a compensation signal to offset at least a portion of the modified signal to output a compensated RX signal, wherein the compensation signal is calibrated in accordance with the baseline capacitance, the compensation circuit further including a first current source configured to provide a first compensation signal for combining with the positive modified signal in the first configuration, and a second current source configured to provide a second compensation signal for combining with the negative modified signal in the second configuration.

2. The device of claim 1 , wherein the modified signal is generated by a half wave rectification operation.

3. The device of claim 1 , wherein the modified signal is generated by a full wave rectification operation and the demodulation circuit comprises:
a first half wave rectifier receiving the RX signal and configured to output the positive modified signal based on the positive portion of the RX signal; and
a second half wave rectifier receiving the RX signal and configured to output the negative modified signal based on the negative portion of the RX signal.

4. The device of claim 1 wherein the first electrode includes a transmit (TX) electrode and the second electrode includes a receive (RX) electrode, the sensor further includes a plurality of the TX electrodes and a plurality of the RX electrodes.

5. The device of claim 4 , further comprising a calibration circuit configured to adjust the compensation signal based on measured capacitance values between the plurality of TX electrodes and the plurality of RX electrodes.

6. The device of claim 5 , further comprising a memory configured to store a compensation signal value which is determined by the measured capacitance value between a pair of electrodes including the TX electrode and the RX electrode.

7. The device of claim 1 , wherein the compensation signal is approximately equal to the modified signal in magnitude.

8. The device of claim 1 , wherein the compensation circuit comprises a programmable current digital to analog converter (IDAC).

9. The device of claim 8 , wherein the compensation signal is a current value for the programmable IDAC.

10. The device of claim 1 , further comprising a transimpedance amplifier configured to generate an output voltage based on the compensated RX signal.

11. The device of claim 10 , further comprising a filter configured to reduce a ripple voltage of the output voltage.

12. A method, comprising:
applying a transmit signal to a first electrode capacitively coupled to a second electrode to generate a receive (RX) signal at the second electrode;
determining a baseline capacitance as a measured capacitance value between the first electrode carrying the transmit signal and the second electrode when no external input is present;
receiving the RX signal from the second electrode;
outputting a modified RX signal based on the RX signal, including: outputting a positive modified signal based on a positive portion of the RX signal in a first configuration and outputting a negative modified signal based on a negative portion of the RX signal in a second configuration;
generating a compensation signal corresponding to the baseline capacitance, wherein the compensation signal is calibrated to offset at least a portion of the modified RX signal, including providing a first compensation signal for combining with the positive modified signal in the first configuration and providing a second compensation signal for combining with the negative modified signal in the second configuration; and
generating a compensated RX signal, including combining the first compensation signal to the positive modified RX signal in the first configuration, and combining the second compensation signal to the negative modified RX signal in the second configuration.

13. The method of claim 12 , further comprising:
generating an output voltage based on the compensated RX signal; and
filtering the output voltage to reduce a ripple voltage.

14. The method of claim 13 , further comprising:
generating a digital code based on the output voltage; and
determining a presence of the external input based on the digital code.

15. The method of claim 13 , further comprising reducing a ripple voltage of the output voltage.

16. The method of claim 12 , wherein the compensation signal is approximately equal to the modified RX signal in magnitude.

17. The method of claim 12 , wherein the compensation signal is a current value for a programmable IDAC.

18. The method of claim 12 , wherein the outputting the modified RX signal includes rectifying the RX signal.

19. An apparatus, comprising:
a signal generator to provide a transmit signal;
a sensor comprising a plurality of first electrodes wherein each first electrode is driven by the transmit signal configured to induce a receive (RX) signal at each of a plurality of second electrodes, wherein each capacitive coupling of a pair of the first electrodes and the second electrodes produces a baseline capacitance value when no external input is present;
a demodulation circuit, coupled with the sensor, that generates a modified RX signal based on the RX signal for each of the pair of the first and second electrodes, wherein in a first configuration the demodulation circuit outputs a positive modified signal based on a positive portion of the RX signal, and in a second configuration the demodulation circuit outputs a negative modified signal based on a negative portion of the RX signal; and
a compensation circuit, coupled with the demodulation circuit, to add a compensation signal to the modified RX signal to generate a compensated RX signal, wherein the compensation signal is calibrated to cancel a portion of the modified RX signal corresponding to the baseline capacitance value of the pair of the first and second electrodes, wherein the compensation circuit further includes a first current source configured to provide a first compensation signal for combining with the positive modified signal in the first configuration, and a second current source configured to provide a second compensation signal for combining with the negative modified signal in the second configuration.

Assignees

• Parade Tech Ltd

Inventors

• Maharyta Andriy
• Kremin Viktor

Classifications

• G01R27/2605 Primary

  Measuring capacitance (capacitive sensors G01D5/24) · CPC title

• G06F3/044

  by capacitive means · CPC title

• G01R23/165

  using filters · CPC title

• H03K17/955

  using a capacitive detector · CPC title

• G01R19/257

  using analogue/digital converters of the type with comparison of different reference values with the value of voltage or current, e.g. using step-by-step method · CPC title