Fingerprint Sensor Differential Noise Cancellation
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Solution Overview
Problem
Existing electronic fingerprint sensing systems are prone to parasitic coupling and noise interference from finger ridges outside the sensor gap, which affects the accuracy of fingerprint imaging.
Innovation Solution
The system employs a reference plate parallel to the image pickup plate and spaced from the image drive plates to cancel common mode noise, with a comb plate connected to ground to further reduce parasitic signals, using differential noise cancellation techniques to isolate the fingerprint image signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a capacitive sensing array with drive plates and pickup plate is used for fingerprint sensing, then the sensing capability is improved, but parasitic coupling and noise interference from finger ridges outside the sensor gap increase
Solution Approach 1:
A reference plate is introduced as an intermediary element between the drive plates and pickup plate. The reference plate is coupled to ground and positioned to receive parasitic coupling and noise interference from finger ridges outside the sensor gap. This intermediary structure allows the harmful factors to be captured and cancelled through differential sensing, resolving the contradiction between sensing capability and noise rejection.
Solution Approach 2:
The parasitic coupling and noise interference, which were previously harmful factors, are converted into a useful reference signal. By coupling the reference plate to ground and using it in a differential measurement setup, the harmful noise is transformed into a reference that can be subtracted from the main signal, thereby eliminating the harmful effects and improving sensing accuracy.
2Measurement precision
If the reference plate is spaced from the image drive plates to permit differential image signal development, then noise cancellation is improved, but the device complexity increases
Solution Approach 1:
The sensor structure is segmented into distinct functional regions: drive plates for signal generation, pickup plate for signal collection, reference plate for noise reference, and comb plate for ground coupling. This segmentation allows each component to perform its specific function optimally while maintaining a modular architecture that manages complexity through functional separation.
Solution Approach 2:
The reference plate serves multiple functions: it acts as a noise reference for differential sensing, provides a ground coupling path through the comb plate, and maintains a structured geometric relationship with the drive and pickup plates. This multi-functionality reduces the need for additional separate components, thereby managing device complexity while achieving effective noise cancellation.
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 significantly reduces noise and parasitic signals, enhancing the accuracy and reliability of fingerprint sensing by isolating the desired fingerprint image signal from unwanted interference.
Implementation Method 1
The reference plate is spaced from the image pickup plate to permit common mode noise and coupling to be cancelled and is spaced from the image drive plates to permit a differential image signal to develop between the image pickup plate and the reference plate.
Implementation Method 2
Fingerprint sensors of this type provide satisfactory performance but are subject to parasitic coupling and noise combined with interference coupled through the body of the finger from finger ridges outside the sensor gap.
Implementation Method 3
The comb plate may comprise substantially parallel, interconnected conductors disposed perpendicular to the reference plate and spaced from the reference plate. In some embodiments, an arrangement of the parallel, interconnected conductors of the comb plate relative to the reference plate may substantially match an arrangement of the image drive plates relative to the image pickup plate.
Data Source
AI summary
Image sensing apparatus includes an image pickup plate disposed generally orthogonally with respect to an expected direction of movement of an object, such as a finger, multiple image drive plates in spaced relation to the image pickup plate to define sensor gaps between respective image drive plates and the image pickup plate, and a reference plate disposed substantially parallel to the image pickup plate. The reference plate is spaced from the image pickup plate to permit common mode noise and coupling to be cancelled and is spaced from the image drive plates to permit a differential image signal to develop between the image pickup plate and the reference plate. A differential amplifier coupled to the image pickup plate and the reference plate provides noise cancellation. The apparatus may further include a comb plate spaced from the reference plate and coupled to a reference potential, such as ground.