Apparatus and method for reducing noise in fingerprint sensing circuits

• US 8,116,540 B2
• Filed: 04/04/2008
• Issued: 02/14/2012
• Est. Priority Date: 04/04/2008
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Validity Sensors Incorporated (Synaptics Incorporated)
• Inventors: Dean, Gregory Lewis, Erhart, Richard Alexander, Jandu, Jaswinder, Thompson, Erik Jonathan
• Primary Examiner(s): WERNER, BRIAN P
• Application Number:
  US12/098,367
• Publication Number:
  US 20090252385A1
• Time in Patent Office: 1,411 Days
• Field of Search: None
• US Class Current: 382/124
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

An apparatus for reducing noise in fingerprint sensing circuits is disclosed in one embodiment of the invention as including a fingerprint sensing area onto which a user can apply a fingerprint. An analog front end is coupled to the fingerprint sensing area and is configured to generate an analog response signal. An analog-to-digital converter (ADC) samples the analog response signal and converts the sample to a digital value, which may be received by a digital device such as a processor or CPU. To reduce the amount of the noise that is present in the analog response signal and therefore reflected in the digital value, the digital device may be shut down while the ADC is sampling the analog response signal.

Claims

1. An apparatus for reducing noise in fingerprint sensing circuits, the apparatus comprising:

a fingerprint sensing area onto which a user can apply a fingerprint;

an analog front end coupled to the fingerprint sensing area to generate an analog response signal;

an analog-to-digital converter (ADC) to sample the analog response signal and convert the sample to a digital value;

a digital device to receive the digital value from the ADC, the digital device further configured to shut down while the ADC is sampling the analog response signal;

wherein the digital device is shut down by turning off a clock signal to the digital device; and

wherein the digital device is further configured to re-start while the ADC is converting a sample to a digital value.

2. The apparatus of claim 1, wherein the digital device is shut down by disabling the digital device.

3. The apparatus of claim 1, wherein the digital device is shut down by turning off power to the digital device.

4. The apparatus of claim 1, wherein the digital device is configured to shut down some interval prior to the time the ADC samples the analog response signal.

5. The apparatus of claim 4, wherein the interval is selected to allow the analog response signal to attain substantially steady state prior to sampling the analog response signal.

6. The apparatus of claim 1, wherein the digital device is configured to re-start after the ADC has finished sampling the analog response signal.

7. The apparatus of claim 1, wherein the digital device is a processor.

8. The apparatus of claim 7, wherein the digital device is a CPU.

9. A method for reducing noise in fingerprint sensing circuits, the method comprising:

providing a fingerprint sensing area onto which a user can apply a fingerprint;

generating an analog response signal associated with finger activity over the fingerprint sensing area;

sampling the analog response signal;

converting the sample to a digital value;

receiving the digital value at a digital device;

shutting down the digital device while sampling the analog response signal;

wherein the digital device is shut down by turning off a clock signal to the digital device; and

turning on the clock signal to the digital device while the ADC is converting a sample to a digital value.

10. The method of claim 9, wherein shutting down the digital device comprises disabling the digital device.

11. The method of claim 9, wherein shutting down the digital device comprises turning off power to the digital device.

12. The method of claim 9, wherein shutting down the digital device comprises shutting down the digital device some interval prior to sampling the analog response signal.

13. The method of claim 12, further comprising selecting the interval to allow the analog response signal to attain substantially steady state prior to sampling the analog response signal.

14. The method of claim 9, further comprising re-starting the digital device after sampling the analog response signal.

15. The method of claim 9, wherein the digital device is a processor.

16. A method for reducing noise in fingerprint sensing circuits, the method comprising:

providing a fingerprint sensing area onto which a user can apply a fingerprint;

generating an analog response signal associated with a finger being applied onto the fingerprint sensing area;

sampling the analog response signal to form an analog sample signal;

converting the analog sample signal to a digital value;

receiving the digital value at a CPU;

turning off a clock signal to the CPU while sampling the analog response signal; and

turning on the clock signal to a digital device while the ADC is converting a sample to a digital value.