Apparatus and Method for Reducing Parasitic Capacitive Coupling and Noise in Fingerprint Sensing Circuits

• US 20090252386A1
• Filed: 04/04/2008
• Published: 10/08/2009
• Est. Priority Date: 04/04/2008
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Validity Sensors Incorporated (Synaptics Incorporated)
• Inventors: Thompson, Erik Jonathon, Dean, Gregory Lewis, Jandu, Jaswinder, Erhart, Richard Alexander
• Granted Patent: US 8,005,276 B2
• Time in Patent Office: Days
• US Class Current: 382/124
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing

Abstract

A fingerprint sensing circuit for reducing noise and parasitic capacitive coupling is disclosed in one embodiment of the invention as including a plurality of transmitting elements to sequentially emit a probing signal. A digital ground is provided to ground digital components in the fingerprint sensing circuit. A quiet ground, separate from and quieter than the digital ground, is provided to ground transmitting elements that are not transmitting the probing signal. Similarly, control logic is provided to connect, to the quiet ground, transmitting elements that are not transmitting the probing signal, while disconnecting, from the quiet ground, transmitting elements that are emitting the probing signal. The quiet ground helps to reduce the adverse effects of parasitic capacitive coupling and noise on the inactive transmitting elements.

Claims

1. A fingerprint sensing circuit for reducing parasitic capacitive coupling and noise when scanning a fingerprint, the fingerprint sensing circuit comprising:

a plurality of transmitting elements to sequentially emit a probing signal;

a digital ground to provide a ground for digital components in the fingerprint sensing circuit;

a quiet ground, substantially separate from the digital ground, to provide a ground for selected transmitting elements that are not currently transmitting the probing signal; and

control logic configured to connect, to the quiet ground, transmitting elements that are not currently transmitting the probing signal, while disconnecting, from the quiet ground, transmitting elements that are emitting the probing signal.

2. The fingerprint sensing circuit of claim 1, further comprising a plurality of switches to selectively connect and disconnect each transmitting element from the quiet ground.

3. The fingerprint sensing circuit of claim 2, wherein the control logic controls the operation of the switches.

4. The fingerprint sensing circuit of claim 1, wherein the plurality of transmitting elements are arranged in a substantially linear array.

5. The fingerprint sensing circuit of claim 4, wherein the control logic is further configured to connect, to the digital ground, transmitting elements that are adjacent to the transmitting elements emitting the probing signal.

6. The fingerprint sensing circuit of claim 1, wherein the digital ground and quiet ground are connected to one another at a ground plane.

7. The fingerprint sensing circuit of claim 1, further comprising a receiving element configured to sense the probing signal from the plurality of transmitting elements.

8. A method for reducing parasitic capacitive coupling and noise in a fingerprint sensing circuit, the method comprising:

providing a plurality of transmitting elements to sequentially emit a probing signal;

providing a digital ground to ground digital components in the fingerprint sensing circuit;

providing a quiet ground, substantially separate from the digital ground, to ground selected transmitting elements that are not currently transmitting the probing signal;

connecting, to the quiet ground, the selected transmitting elements that are not transmitting the probing signal; and

disconnecting, from the quiet ground, transmitting elements that are emitting the probing signal.

9. The method of claim 8, further comprising providing a plurality of switches to selectively connect and disconnect each transmitting element from the quiet ground.

10. The method of claim 9, further comprising providing control logic to control the operation of the switches.

11. The method of claim 8, further comprising arranging the plurality of transmitting elements in a substantially linear array.

12. The method of claim 11, further comprising providing control logic to connect, to the digital ground, transmitting elements that are adjacent to the transmitting elements emitting the probing signal.

13. The method of claim 8, further comprising connecting the digital ground and quiet ground to one another at a ground plane.

14. The method of claim 8, further comprising providing a receiving element to sense the probing signal emitted by the plurality of transmitting elements.

15. A fingerprint sensing circuit for reducing parasitic capacitive coupling and noise when scanning a fingerprint, the fingerprint sensing circuit comprising:

a plurality of receiving elements to sequentially sense a probing signal;

a digital ground to provide a ground for digital components in the fingerprint sensing circuit;

a quiet ground, substantially separate from the digital ground, to provide a ground for selected receiving elements that are not currently sensing the probing signal; and

control logic configured to connect, to the quiet ground, receiving elements that are not currently sensing the probing signal, while disconnecting, from the quiet ground, receiving elements that are sensing the probing signal.

16. The fingerprint sensing circuit of claim 15, further comprising a plurality of switches to selectively connect and disconnect each receiving element from the quiet ground.

17. The fingerprint sensing circuit of claim 16, wherein the control logic controls the operation of the switches.

18. The fingerprint sensing circuit of claim 15, wherein the plurality of receiving elements are arranged in a substantially linear array.

19. The fingerprint sensing circuit of claim 18, wherein the control logic is further configured to connect, to the digital ground, receiving elements that are adjacent to the receiving elements sensing the probing signal.

20. The fingerprint sensing circuit of claim 15, wherein the digital ground and quiet ground are connected to one another at a ground plane.