Capacitive Fingerprint Sensor Timing Circuitry
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Solution Overview
Problem
Capacitive fingerprint sensors face limitations in achieving high signal-to-noise ratio and robustness due to the need for thin protective coatings, making them susceptible to wear and electro-static discharge, and existing active systems have room for improvement in performance and protective coating thickness.
Innovation Solution
A capacitive fingerprint sensing device with a metal plate sensing structure and a thick protective coating, utilizing localized timing circuitry for faster operation and improved measurement performance, allowing for thicker coatings and reduced energy consumption, and enabling measurement through thicker protective layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thin protective coating is used in passive capacitive sensors, then the capacitance measurement is possible, but the sensor becomes sensitive to scratching and electro-static discharge
Solution Approach 1:
The patent changes the operating mode from passive to active capacitive sensing, which fundamentally alters the measurement parameters and enables the use of thicker protective coatings. The active sensing method applies a driving signal to the finger and measures the modulated response, allowing reliable operation with coatings 50-200 μm thick that would be impossible in passive mode.
Solution Approach 2:
The patent employs periodic driving signals (pulses) applied to the finger through a conductive structure. This periodic excitation allows the system to measure capacitance changes at specific time intervals, enabling active sensing operation that can penetrate through thicker protective coatings while maintaining measurement accuracy and robustness.
2Measurement precision
If active fingerprint sensing is implemented, then the signal-to-noise ratio is improved, but the device complexity increases
Solution Approach 1:
The patent segments the sensing system into distinct functional components: a conductive structure for signal injection, an array of independent sensing structures, and a control unit for coordinating operation. This segmentation allows each component to be optimized independently and simplifies the overall control architecture, reducing device complexity while maintaining high signal-to-noise ratio performance.
Solution Approach 2:
The patent introduces a conductive structure as an intermediary element between the user's finger and the sensing array. This intermediary serves multiple functions: it delivers the driving signal to the finger, receives the modulated response, and transfers it to the sensing structures. This intermediary approach simplifies the direct interaction between finger and sensors while improving measurement precision.
3Reliability
If thicker protective coatings are used, then the sensor robustness is improved, but the measurement performance through the coating deteriorates
Solution Approach 1:
The patent employs dynamic active sensing with time-varying driving signals and adjustable measurement parameters. The system can adapt its operating frequency, pulse width, and signal amplitude to optimize performance for different coating thicknesses. This dynamic operation enables maintaining measurement precision even through thick protective coatings that would static systems.
Solution Approach 2:
The patent applies a driving signal to the finger before the actual capacitance measurement takes place. This preliminary action charges the capacitive coupling through the protective coating, creating a stronger initial signal that can be more accurately measured. The pre-charging effect compensates for the signal attenuation caused by thicker coatings, maintaining measurement precision.
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 enhances signal-to-noise ratio, allows for thicker protective coatings, reduces energy consumption, and facilitates faster fingerprint recognition, making the device more robust and efficient for use in electronic devices like mobile phones.
Implementation Method 1
capacitive fingerprint sensing device... measuring a change of a charge carried by the sensing structure resulting from a change in a potential difference between a potential of the finger and a potential of the sensing structure
Implementation Method 2
The protective coating may advantageously be at least 20 μm thick and have a high dielectric strength to protect the underlying structures of the fingerprint sensing device from wear and tear as well as ESD (electro-static discharge)
Data Source
Figure 1~2
Figure 3a~3b
Figure 3c
AI summary
The present invention relates to a capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, the capacitive fingerprint sensor comprising a plurality of sensing elements, each including: a protective dielectric top layer to be touched by the finger;an electrically conductive sensing structure arranged underneath the top layer;charge measuring circuitry connected to the sensing structure for sequentially transitioning between at least a first measurement state and a second measurement state to perform a measurement sequence resulting in an output signal from the charge measuring circuitry being indicative of a change of a charge carried by the sensing structure resulting from a change in a potential difference between the finger and the sensing structure; and timing circuitry connected to the charge measuring circuitry for controlling a timing of at least one of the measurement states.