A WEARABLE DEVICE HAVING A FINGERPRINT SENSOR

• US 20170124371A1
• Filed: 06/12/2015
• Published: 05/04/2017
• Est. Priority Date: 06/13/2014
• Status: Active Grant

• Current Assignee: Arm IP Limited
• Original Assignee: Arm IP Limited
• Inventors: VINCENT, Hugo John Martin
• Granted Patent: US 10,248,828 B2
• Time in Patent Office: Days
• CPC Class Codes: G06V 40/1306 non-optical, e.g. ultrasonic or capacitive sensing
• CPC Class Codes: G06V 40/1365 Matching; Classification

Abstract

A device worn by a user has a device body providing a first surface that is located adjacent to skin of the user when the device is being worn. Fingerprint sensor circuitry on the device comprises a capacitive fingerprint sensor and associated control circuitry, a primary impedance path being established between electrodes of the capacitive fingerprint sensor when a user places a finger on the capacitive fingerprint sensor. The associated control circuitry passes an AC drive signal through the capacitive fingerprint sensor in order to measure a property whose value varies as a function of impedance. A body ground connection provided by the device body is arranged such that when the user touches the fingerprint sensor, a parasitic impedance path is established from the finger through the body of the user and via the first surface of the device body to the body ground connection. To avoid this parasitic impedance path having the potential to adversely affect the accuracy of the fingerprint sensing operation, the device further comprises filter circuitry between the body ground connection and the electrical ground connection which decouples the body ground connection from the electrical ground connection in a first frequency range including the frequency of the AC drive signal, whilst coupling the body ground connection to the electrical ground connection in at least one further frequency range.

Claims

1. A device configured to be worn by a user when in operation, comprising:

a device body having a first surface located adjacent to skin of a user when the device is worn by the user; and

fingerprint sensor circuitry comprising a capacitive fingerprint sensor and associated control circuitry, the capacitive fingerprint sensor having electrodes between which a primary impedance path is established when a user places a finger on the capacitive fingerprint sensor, the associated control circuitry being connected to an electrical ground connection and configured to pass an AC drive signal through the capacitive fingerprint sensor when a user places said finger on the capacitive fingerprint sensor in order to measure a property whose value varies as a function of impedance;

the device body providing a body ground connection;

the body ground connection being arranged such that when the user places said finger on the capacitive fingerprint sensor, a parasitic impedance path is established from the finger through the body of the user and via said first surface of the device body to the body ground connection;

the device further comprising filter circuitry between the body ground connection and the electrical ground connection and configured to decouple the body ground connection from the electrical ground connection in a first frequency range including the frequency of the AC drive signal, and configured to couple the body ground connection to the electrical ground connection in at least one further frequency range.

2. A device as claimed in claim 1, wherein said first surface is conductive.

3. A device as claimed in claim 2, wherein said first surface is metallic.

4. A device as claimed in claim 2, wherein the device body includes a conductive strap used to attach the device to the user.

5. A device as claimed in claim 2, wherein said body ground connection is provided by said first surface.

6. A device as claimed in claim 1, wherein said filter circuitry comprises band stop filter circuitry;

and said at least one further frequency range comprises a second frequency range higher than said first frequency range and a third frequency range lower than said first frequency range.

7. A device as claimed in claim 1, further comprising electrostatic discharge protection circuitry configured in parallel with the filter circuitry between said body ground connection and said electrical ground connection.

8. A device configured to be worn by a user when in operation, comprising:

a device body having first surface means for locating adjacent to skin of a user when the device is worn by the user; and

a fingerprint sensor comprising a capacitive fingerprint sensor means and associated control means, the capacitive fingerprint sensor means for providing electrodes between which a primary impedance path is established when a user places a finger on the capacitive fingerprint sensor means, the associated control means for connecting to an electrical ground connection and for passing an AC drive signal through the capacitive fingerprint sensor means when a user places said finger on the capacitive fingerprint sensor means in order to measure a property whose value varies as a function of impedance;

the device body providing a body ground connection;

the body ground connection being arranged such that when the user places said finger on the capacitive fingerprint sensor means;

a parasitic impedance path is established from the finger through the body of the user and via said first surface means of the device body to the body ground connection;

the device further comprising filter means between the body ground connection and the electrical ground connection for decoupling the body ground connection from the electrical ground connection in a first frequency range including the frequency of the AC drive signal, and for coupling the body ground connection to the electrical ground connection in at least one further frequency range.

9. A method of detecting a fingerprint of a user by a device configured to be worn by the user when in operation, the device having a first surface located adjacent to skin of a user when the device is worn by the user, and fingerprint sensor circuitry comprising a capacitive fingerprint sensor and associated control circuitry, the method comprising:

establishing a primary impedance path between electrodes of the capacitive fingerprint sensor when a user places a finger on the capacitive fingerprint sensor;

connecting the associated control circuitry to an electrical ground connection and issuing an AC drive signal from the associated control circuitry and through the capacitive fingerprint sensor when a user places said finger on the capacitive fingerprint sensor in order to measure a property whose value varies as a function of impedance;

providing a body ground connection which is arranged such that when the user places said finger on the capacitive fingerprint sensor, a parasitic impedance path is established from the finger through the body of the user and via said first surface of the device body to the body ground connection; and

employing filter circuitry between the body ground connection and the electrical ground connection in order to decouple the body ground connection from the electrical ground connection in a first frequency range including the frequency of the AC drive signal, and in order to couple the body ground connection to the electrical ground connection in at least one further frequency range.

10. (canceled)