Capacitive Sensing Unit Noise Reduction via Bias Switching
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Solution Overview
Problem
Capacitive-type fingerprint sensors face challenges in achieving high accuracy due to noise interference, particularly in the absence of a separate conductive drive structure, which affects the quality of fingerprint images.
Innovation Solution
A noise-reduced capacitive sensing unit is designed with a sensing plate, bias voltage sources, a switch unit, an excitation signal source, a reference capacitor, and a voltage follower, along with a sample-and-hold circuit and signal conditioning circuit to calculate and amplify voltage differences between sensing stages, effectively reducing noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a separate conductive drive structure is used to improve sensing accuracy, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent extracts the drive structure function from a separate component and integrates it into the sensing electrode itself. The sensing electrode is configured to serve dual purposes: sensing capacitance changes and providing drive signals to the finger, thereby eliminating the need for a separate conductive drive structure while maintaining sensing accuracy
Solution Approach 2:
The sensing electrode is designed to perform multiple functions simultaneously: it acts as both the sensing element for detecting fingerprint patterns and the drive structure for injecting excitation signals into the finger. This multi-functionality reduces device complexity while preserving measurement precision
2Measurement precision
If noise reduction techniques are applied to improve image quality, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent employs periodic excitation signals (sinusoidal or square waves) to drive the sensing electrode. By using periodic action, the system can implement lock-in detection techniques that selectively amplify signals at the excitation frequency while rejecting noise at other frequencies, thereby improving image quality without requiring complex noise filtration circuits
Solution Approach 2:
The patent implements feedback mechanisms where the sensed capacitance values are processed and used to adjust subsequent measurements or signal processing parameters. This feedback approach enables adaptive noise reduction and enhances fingerprint image quality through intelligent signal processing rather than hardware complexity
3Volume of moving object
If capacitive sensing is used to achieve compact form factor, then device size is reduced, but measurement precision deteriorates due to noise
Solution Approach 1:
The patent replaces optical sensing mechanisms with capacitive sensing, enabling compact form factor suitable for portable devices. To compensate for noise issues inherent in capacitive sensing, the patent employs signal processing techniques including periodic excitation, lock-in detection, and differential measurement methods that enhance measurement precision without increasing physical size
Solution Approach 2:
The patent utilizes parameter changes in the excitation signal (frequency, amplitude, waveform) to optimize the signal-to-noise ratio. By carefully selecting and adjusting these parameters, the system achieves high measurement precision in a compact capacitive sensor configuration, overcoming the noise limitations typically associated with small-form-factor sensors
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 the accuracy of fingerprint images by minimizing noise, allowing for high-quality image acquisition without a separate conductive drive structure, making it suitable for portable applications.
Implementation Method 1
A capacitive image sensor consists of an array of sensing units. Each sensing unit contains a sensing electrode. By using the sensing electrode as one plate of the two-plated capacitor and a dermal tissue as another plate, ridges and valleys of a fingerprint can be located by measuring the different capacitances.
Implementation Method 2
A noise-reduced value of the capacitive sensing unit is obtained by calculating a voltage difference between the first sensing stage and the second sensing stage.
Data Source
AI summary
A noise-reduced capacitive sensing unit is disclosed. The noise-reduced capacitive sensing unit includes: a sensing plate; a first bias voltage source for providing a first bias voltage; a second bias voltage source for providing a second bias voltage; a switch unit, connected between two bias voltage sources and the sensing plate, for selectively providing one of the bias voltages to the sensing plate; an excitation signal source for providing a bi-level waveform; a reference capacitor, formed between the excitation signal source and the sensing plate, for injecting the excitation signal to the sensing plate; and a voltage follower for providing sensing results, wherein an input node of the voltage follower is connected to the sensing plate.