Fingerprint Sensing Circuit Quiet Ground Noise Reduction
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Solution Overview
Problem
Fingerprint sensing circuits, particularly capacitive-type sensors, are susceptible to noise and parasitic capacitive coupling, which degrade the quality of acquired fingerprint images.
Innovation Solution
The implementation of a fingerprint sensing circuit with multiple transmitting and receiving elements that utilize a quiet ground to reduce parasitic capacitive coupling and noise, where inactive elements are connected to this quiet ground and active elements are disconnected from it, along with control logic to manage the connection and disconnection of these elements to minimize noise transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitive sensing elements are used to detect fingerprint ridges and valleys, then fingerprint images can be generated, but noise and parasitic capacitive coupling degrade the quality of the acquired fingerprint images
Solution Approach 1:
The ground system is segmented into two separate grounds: a first ground connected to digital components and a second ground (quiet ground) connected to inactive transmitting elements. This segmentation isolates noisy digital ground from the sensitive capacitive sensing elements, reducing parasitic capacitive coupling and noise interference while maintaining effective fingerprint image acquisition
Solution Approach 2:
A second ground (quiet ground) is introduced as an intermediary between the noisy digital ground and the inactive transmitting elements. This intermediary ground acts as a noise barrier, preventing parasitic capacitive coupling from degrading the fingerprint image quality while allowing the sensing circuit to function properly
2Productivity
If multiple transmitting elements are used to scan fingerprint lines, then complete fingerprint images can be acquired, but inactive transmitting elements generate noise and parasitic coupling
Solution Approach 1:
The connection of transmitting elements to grounds is made dynamic rather than static. Switches control the connection state of each transmitting element, dynamically switching between first ground (during active scanning) and second ground (when inactive). This dynamic reconfiguration reduces noise generation from inactive elements while maintaining complete fingerprint scanning capability
Solution Approach 2:
The transmitting elements operate in periodic cycles of active scanning and inactive states. During each cycle, elements are selectively connected to appropriate grounds based on their operational state. This periodic switching reduces cumulative noise from inactive elements while ensuring complete fingerprint line acquisition over time
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
This approach effectively reduces parasitic capacitive coupling and noise, enhancing the quality of fingerprint images by isolating inactive elements from noise sources and maintaining a quiet ground, thereby improving the reliability and accuracy of fingerprint sensing.
Implementation Method 1
capacitive sensors may be particularly susceptible to noise and parasitic capacitive coupling, which may degrade the quality of the acquired fingerprint image
Data Source
AI summary
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.