Capacitive Fingerprint Sensor Ground Isolation for Wearables
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Solution Overview
Problem
Wearable devices with capacitive fingerprint sensors face accuracy issues due to parasitic impedance paths caused by the device being worn on the user's body, which can lead to low impedance routes that attenuate the AC drive signal and compromise fingerprint sensing accuracy.
Innovation Solution
Incorporating filter circuitry between the body ground connection and the electrical ground connection to decouple the body ground connection from the electrical ground connection in the frequency range of the AC drive signal, while coupling it in other frequency ranges, effectively removing the parasitic impedance path and maintaining accurate fingerprint sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a capacitive fingerprint sensor is used in a wearable device, then the device size is reduced and the sensor can be integrated, but parasitic impedance paths are created that reduce measurement accuracy
Solution Approach 1:
The ground connection system is segmented into two distinct paths: a first ground connection for the capacitive fingerprint sensor and a second ground connection for other device circuitry. This segmentation isolates the sensor's electrical reference from parasitic impedance paths created by the wearable configuration, thereby maintaining measurement accuracy while preserving compact device integration.
Solution Approach 2:
A dedicated ground isolation circuit acts as an intermediary between the capacitive fingerprint sensor and the device's main ground system. This intermediary component blocks parasitic impedance paths while maintaining proper grounding for the sensor, resolving the contradiction between compact wearable form factor and accurate fingerprint sensing.
2Object-affected harmful factors
If the body ground connection is connected to the electrical ground connection for shielding, then electromagnetic interference is reduced, but the parasitic impedance path attenuates the AC drive signal
Solution Approach 1:
The grounding system is divided into separate domains: a sensor-specific ground connection that maintains shielding without creating parasitic signal paths, and a separate power/IO ground for other device functions. This segmentation allows electromagnetic shielding to be maintained while preventing AC drive signal attenuation through parasitic impedance paths.
Solution Approach 2:
The ground connection architecture provides locally optimized grounding characteristics: high impedance isolation from parasitic paths at the sensor interface, while maintaining low impedance shielding connections locally where electromagnetic interference protection is needed. This local quality differentiation resolves the contradiction between shielding effectiveness and signal integrity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The filter circuitry significantly reduces the impact of parasitic impedance paths, ensuring accurate fingerprint detection and authentication in wearable devices by preventing low impedance routes that could short the AC drive signal, thereby enhancing the reliability of capacitive fingerprint sensors.
Implementation Method 1
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
Implementation Method 2
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
Implementation Method 3
the impedance of this parasitic impedance path can be low enough to significantly affect the accuracy of the measurements taken by the capacitive fingerprint sensor
Data Source
AI summary
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.