Capacitive fingerprint sensor with quadrature demodulator and multiphase scanning

Patent metadata

Field	Value
Publication number	US-9864894-B2
Application number	US-201715397502-A
Country	US
Kind code	B2
Filing date	Jan 3, 2017
Priority date	Nov 17, 2014
Publication date	Jan 9, 2018
Grant date	Jan 9, 2018

Abstract

A fingerprint sensing circuit, system, and method is disclosed. The fingerprint sensor maybe include a plurality of inputs coupled to a plurality of fingerprint sensing electrodes and to an analog front end. The analog front end may be configured to generate at least one digital value in response to a capacitance of at least one of the plurality of fingerprint sensing electrodes. Additionally, the analog front end may include a quadrature demodulation circuit to generate at least one demodulated value for processing by a channel engine. The channel engine may generate a capacitance result value that is based, in part, on the demodulated value and is stored in a memory.

First claim

What is claimed is:

1. A fingerprint sensing circuit comprising:
at least two inputs of a plurality of inputs coupled to an analog front end (AFE) and to a plurality of fingerprint sensing electrodes, wherein the at least two inputs are coupled to the AFE through a multiplexor, the AFE configured to generate at least one digital value in response to a mutual capacitance between the at least two of the plurality of fingerprint sensing electrodes and at least two drive electrodes, wherein the AFE comprises a quadrature demodulation circuit configured to generate at least one demodulated value;
a channel engine configured to generate a mutual capacitance result value based on the at least one demodulated value; and
a plurality of non-transitory storage elements configured to store the mutual capacitance result generated by the channel engine.

2. The fingerprint sensing circuit of claim 1 , wherein the quadrature demodulation circuit comprises:
at least two demodulator inputs, each input coupled to a modifier, the modifier driven by a phase-shifted reference signal;
a plurality of low pass filters coupled to an output of the multiplier, and
a plurality of analog-to-digital converters (ADCs) coupled to an output of the low pass filters, the ADCs configured to generate a digital value derived from a capacitance on the at least one of the plurality of fingerprint sensing electrodes.

3. The fingerprint sensing circuit of claim 2 , wherein the low pass filters are reconfigurable for each of the plurality of fingerprint sensing electrodes.

4. The fingerprint sensing circuit of claim 1 , further comprising a variable gain amplifier coupled between the plurality of fingerprint sensing electrodes and the quadrature demodulation circuit.

5. The fingerprint sensing circuit of claim 1 , wherein the outputs are combined by the channel engine.

6. The fingerprint sensing circuit of claim 5 , wherein the combination includes:
a squaring step, wherein each of the outputs is squared;
a summing step, wherein the squared outputs are combined;
a square root step, wherein the square root of the combined output is generated.

7. The fingerprint sensing circuit of claim 1 , wherein the plurality of fingerprint sensing electrodes includes:
a first plurality of drive (TX) electrodes; and
a second plurality of receive (RX) electrodes, wherein:
at least two of the first plurality of TX electrodes are driven concurrently, and
receive signals on at least two of the second plurality of RX electrodes are received concurrently, the receive signal generated from a current induced on the at least two of the second plurality of RX electrodes in response to a TX signal on the at least two of the first plurality of TX electrodes.

8. The fingerprint sensing circuit of claim 1 , wherein the at least two inputs are coupled to the AFE differentially through the multiplexor.

9. A method comprising:
receiving a pair of signals from at least two sensing electrodes of a plurality of sensing electrodes of a fingerprint sensing array disposed along a first axis in response to a drive signal from a pair of drive electrodes disposed substantially orthogonal to and intersecting the at least two sensing electrodes, a received signal on each of the sensing electrodes being derived from the drive signal on the pair of drive electrodes and a mutual capacitance between the drive electrodes and the sensing electrodes;
modifying the received signal with at least one phase-shifted reference signal to produce a phase-shifted signal;
converting the phase-shifted signal to a phase-shifted digital value; and
processing the digital value to produce a result representative of the mutual capacitance between one of the pair of sensing electrodes and one of the pair of drive electrodes.

10. The method of claim 9 , wherein the received signal for each of the plurality of sensing electrodes is modified by at least one phase-shifted signal, and wherein the phase-shifted signals are shifted 90 degrees relative to each other.

11. The method of claim 9 , wherein the phase-shifted signal is filtered by a reconfigurable low pass filter prior to conversion to the phase-shifted digital value.

12. The method of claim 9 , wherein the processing of the digital value comprises:
squaring the phase-shifted digital value;
summing the phase-shifted digital value with at least one other phase-shifted digital value; and
calculating a square root of the sum of the phase-shifted digital values.

13. The method of claim 9 , wherein the digital value is stored in a memory and is processed to determine the presence of a ridge or valley of a fingerprint on the fingerprint sensing array.

14. The method of claim 9 , wherein the receiving of the signal from the at least two sensing electrodes of the fingerprint sensing array is controlled by a multiplexer coupled between at least one sensing electrode and an analog front end (AFE).

15. The method of claim 9 , wherein the pair of signals from the at least two sensing electrodes are received differentially at least a pair of inputs of an analog front end (AFE).

16. A fingerprint detection system comprising:
a plurality of drive (TX) electrodes disposed along a first axis;
a plurality of receive (RX) electrodes disposed along a second axis, wherein the TX electrodes and the RX electrodes are configured to have a mutual capacitance at each intersection of the TX electrodes and the RX electrodes;
a drive circuit coupled to the TX electrodes, the drive circuit configured to produce a TX signal on the TX electrodes, wherein at least two TX electrodes are driven by the drive circuit concurrently;
an analog front end (AFE) coupled to the RX electrodes, the AFE configured to generate a digital value representative of the mutual capacitance, wherein the AFE comprises a quadrature demodulation circuit configured to generate a demodulated signal;
a channel engine configured to generate a capacitance result value based on the digital value;
a scan control circuit to configure a first multiplexer coupled between the drive circuit and the plurality of TX electrodes and to configure a second multiplexer coupled between the AFE and the plurality of RX electrodes; and
a memory configured to store the capacitance result value generated by the channel engine.

17. The fingerprint detection system of claim 16 , wherein the channel engine is configured to operate using a hardware accelerator circuit.

18. The fingerprint detection system of claim 16 , wherein the quadrature demodulator circuit comprises:
at least two modulator inputs, each input coupled to a modifier, the modifier driven by a phase-shifted reference signal;
a plurality of low pass filters coupled to an output of the modifiers, and
a plurality of analog-to-digital converters (ADCs) coupled to an output of the low pass filters, the ADCs configured to generate a digital value derived from a capacitance on the at least one of the plurality of RX electrodes.

19. The fingerprint detection system of claim 16 , wherein the capacitances of the sensing electrodes are processed by the AFE concurrently, each on a differential channel to produce a differential output for each sensing electrode.

20. The fingerprint detection system of claim 16 , wherein at least two RX electrodes are coupled to the AFE differentially.

Assignees

• Cypress Semiconductor Corp

Inventors

• Kremin Viktor
• Walsh Paul M
• Hosseini Kaveh
• Johal Jaskarn Singh
• Hancioglu Erhan
• Ozbek Onur

Classifications

• G01R27/2605

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

• G06K9/0002 Primary

  Physics · mapped topic

• G06K9/001

  Physics · mapped topic

• G06V40/1376

  Matching features related to ridge properties or fingerprint texture · CPC title

• G06V40/1306 Primary

  non-optical, e.g. ultrasonic or capacitive sensing · CPC title