Capacitive Fingerprint Sensor Circuit for High-Sensitivity Array Imaging
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Solution Overview
Problem
Capacitive fingerprint sensors face challenges in sensitivity and noise characteristics when transitioning from macro-scale to micro-scale electronic components, and are limited by large measurement circuit sizes, which restrict their ability to achieve high image frame rates and commercialization potential.
Innovation Solution
The introduction of a 'C-Q-T' type capacitive sensor circuit, featuring a 'C-Q' converter and a 'Q-T' converter, which converts spatial differences in fingerprint ridges and valleys into charge differences on the sensing electrode, and uses an integrating capacitor to charge/discharge at varying rates, allowing for improved sensitivity and noise reduction through optimized circuit design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If capacitive fingerprint sensors are scaled from macro-scale to micro-scale electronic components, then device size is reduced, but sensitivity and noise characteristics deteriorate
Solution Approach 1:
The patent changes the measurement parameter from direct capacitance measurement to charge transfer quantity measurement. By converting the capacitive signal to a charge transfer process, the system achieves micro-scale dimensions while maintaining high sensitivity through precise charge quantization and transfer counting, effectively resolving the sensitivity deterioration issue at micro-scale
Solution Approach 2:
The patent replaces traditional voltage-based capacitive measurement with a charge transfer mechanism. Instead of measuring capacitance changes through voltage variations, the system uses charge packets that are transferred and counted, providing superior noise immunity and sensitivity at micro-scale dimensions
2Device complexity
If measurement circuit size is reduced for array sensors, then device integration is improved, but sampling time is limited and image frame rate is restricted
Solution Approach 1:
The patent segments the measurement process into discrete charge transfer steps. Each pixel's charge is transferred through a shared circuit in sequential time slots, allowing compact circuit design while maintaining adequate sampling time through time-division multiplexing of the charge transfer process
Solution Approach 2:
The patent implements continuous charge transfer operation across the sensor array. While one pixel group is being read out, another group is already transferring charge, ensuring that the useful measurement action continues without interruption, thereby maintaining high frame rates despite reduced circuit size
3Device complexity
If array sensors use single-channel design with time division multiplexing, then device complexity is reduced, but sampling time is greatly limited
Solution Approach 1:
The patent employs periodic charge transfer cycles where charge is transferred in regular time intervals across different pixel groups. This periodic operation allows the use of a single shared measurement circuit while ensuring each pixel receives adequate sampling time through systematic time-division allocation, resolving the time loss issue
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 'C-Q-T' sensor circuit enhances sensitivity and reduces thermal noise, achieving performance beyond previous capacitive distance sensors, with improved resolution and linearity, and is suitable for high-demand consumer electronics applications.
Implementation Method 1
measuring the differences between the coupling capacitance formed between ridgelines of fingerprints and planar sensing electrode array units and that formed between valley lines of the fingerprints and the planar sensing electrode array units
Implementation Method 2
a reference capacitor (equivalent to the integrating capacitor) is first fully charged, and then discharges to the capacitance measuring plates (equivalent to the sensing electrode) with increased levels
Data Source
AI summary
On the basis of the physical principle that a capacitance value is inversely proportional to the distance between capacitive electrodes, the spatial structure of the surface of an object can be imaged by measuring a coupling capacitance between the surface of measured object and the electrode arrays on a surface of a sensor; for example, imaging may be performed to uneven spaces between ridge lines and valley lines of fingerprints. The present application provides a C-Q-T type capacitive fingerprint sensor. Firstly, coupling capacitance differences between the fingerprints and the electrodes of the sensor are converted into charge quantity differences, then the charge quantity differences are converted into time differences, and edge signals carrying the time differences are output. Fingerprint sensors are grouped into an array, reading and data combination may be performed to the edge signals, and imaging may be performed to fingerprints.