Capacitive Fingerprint Sensor Noise Reduction via Adjustable Current Source
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Solution Overview
Problem
Capacitive fingerprint sensing technologies face challenges in maintaining high-quality image representation, especially when a thick protective layer is used, and are unreliable due to reduced excitation voltage caused by ground impedance paths, leading to increased complexity and cost in active capacitive techniques.
Innovation Solution
A capacitive sensing unit with a sensing electrode, a voltage follower, and a MOS transistor, operated in the saturation region, utilizing an adjustable current source to provide distinct current levels for biasing and reading, and a reference capacitor to compensate for process variations, ensuring high sensitivity and immunity to noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If passive capacitive technique is used with thick protective layer, then sensor protection is improved, but signal strength and image quality decline significantly
Solution Approach 1:
The patent introduces an intermediary excitation mechanism using a metal ring or strip that applies voltage to the epidermal skin layer. This intermediary approach allows the sensing system to penetrate through the thick protective layer by exciting the skin's natural capacitance, thereby maintaining image quality despite the protective layer's signal attenuating effect
Solution Approach 2:
The patent employs periodic excitation by applying different voltages to the metal ring or strip in sequence. This periodic voltage application creates time-varying electric fields that interact with the skin's capacitance, enabling the system to extract fingerprint information through the thick protective layer by measuring the capacitive response at different excitation phases
2Measurement precision
If active capacitive technique is used to improve image quality, then excitation voltage is significantly reduced when low impedance path exists, causing system unreliability
Solution Approach 1:
The patent incorporates feedback mechanisms that monitor the actual capacitive response and adjust the excitation voltage dynamically. By measuring the skin's capacitive characteristics and feeding this information back to the control system, the excitation voltage can be optimized in real-time to maintain reliable operation even when low impedance paths cause voltage reduction
Solution Approach 2:
The patent makes the excitation system dynamic by allowing the excitation voltage and timing to be adjusted based on detected conditions. The system can adapt its excitation parameters in response to varying skin conditions and impedance paths, maintaining measurement reliability through dynamic control rather than fixed excitation parameters
3Reliability
If excitation voltage is increased to overcome ground impedance path, then system complexity and cost increase
Solution Approach 1:
The patent changes the parameters of the excitation signal, specifically using different voltage levels applied in sequence to the metal ring or strip. By varying the voltage parameters rather than continuously increasing voltage magnitude, the system achieves reliable excitation through ground impedance paths while maintaining controlled complexity through parameter modulation rather than hardware escalation
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 enables high-quality fingerprint representation without excitation voltage, reducing noise and maintaining reliability even with thick protective layers, while simplifying the design and reducing costs.
Implementation Method 1
The MOS transistor is operated in the saturation region
Implementation Method 2
a voltage follower, including: an adjustable current source, for providing at least two distinct current levels; and a MOS transistor
Implementation Method 3
capacitive fingerprint sensors utilize distribution of electrical field which is affected by the capacitance between the fingertip and the sensor to generate a fingerprint image
Data Source
AI summary
A capacitive fingerprint sensor which includes capacitive sensing units is disclosed. Each of the capacitive sensing unit includes a sensing electrode; a first switch; a voltage follower; and a reference capacitor. The voltage follower includes an adjustable current source, for providing at least two distinct current levels; and a MOS transistor. The MOS transistor includes a source node, connected to ground via the adjustable current source and serves as an output node of the voltage follower; a gate node, connected to the sensing electrode and serves as an input node of the voltage follower; a drain node, connected to a power source, for providing power to the voltage follower; and a bulk node, connected to the source node.