Fingerprint Sensing Circuit for Parasitic Capacitance Elimination
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Solution Overview
Problem
Capacitive fingerprint recognition systems face accuracy degradation due to parasitic capacitance formed between the touch layer and the shielding layer, which interferes with the capacitance determining circuit and degrades the performance of fingerprint recognition.
Innovation Solution
A capacitance determining circuit and fingerprint recognition system that utilizes a switch to either make the shielding layer floating or ensure it has the same voltage as the touched layer during the sensing phase, thereby eliminating the parasitic capacitance effect and enhancing accuracy and performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a shielding layer is disposed under the touched layer to prevent interference from circuitry, then the shielding effect is improved, but parasitic capacitance is formed between the touched layer and shielding layer which degrades measurement precision
Solution Approach 1:
The patent applies preliminary action by performing a reset operation before the sensing operation. The reset circuit charges the touched layer and shielding layer to the same voltage level before measurement, eliminating the voltage difference that causes parasitic capacitance interference. This preliminary voltage equalization enables accurate capacitance sensing to proceed without the shielding layer's parasitic effects
Solution Approach 2:
The patent introduces a reset circuit as an intermediary component between the driving circuit and the sensing circuit. This reset circuit includes reset switches and capacitors that mediate the voltage distribution between the touched layer and shielding layer, temporarily equalizing their voltages to eliminate parasitic capacitance during the sensing phase
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 solution effectively reduces noise and improves the accuracy of capacitance sensing and fingerprint recognition by eliminating the impact of parasitic capacitance, leading to enhanced performance in identifying finger ridges and valleys.
Implementation Method 1
senses a capacitance variation of the touch layer to determine whether it is a finger ridge or a finger valley of a fingerprint of the user
Implementation Method 2
a parasitic capacitance is formed between the touch layer and the shielding layer
Data Source
AI summary
The present invention provides a capacitance determining circuit comprising a touched layer configured to receive a touch from a finger; a shielding layer disposed under the touched layer, wherein a parasitic capacitance is formed between the shielding layer and the touched layer; a driving circuit coupled to the touched layer and the shielding layer, configured to provide a first voltage to the touched layer and the shielding layer at a first time; a sensing circuit electrically connected to the touched layer, configured to sense the touch capacitance at a second time; and a first switch with a terminal electrically connected to the shielding layer and another terminal electrically connected to the driving circuit or the touched layer. The capacitance determining circuit and the fingerprint recognition system provided by the present invention is capable of eliminating parasitic capacitance effect and enhancing accuracy and performance of capacitance sensing and fingerprint recognition.