Capacitive Fingerprint Sensor Shielding Parasitic Capacitance
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Solution Overview
Problem
Capacitive fingerprint sensors face issues with parasitic capacitors causing a large base signal that saturates the integrator, reducing the dynamic range and making it difficult to accurately cancel the base signal, leading to a small output dynamic range and inefficient fingerprint acquisition.
Innovation Solution
The introduction of a fourth electrode plate layer between the first and second electrode plate layers in the capacitive fingerprint sensor configuration, which shields the parasitic capacitors from the integrating capacitor, allowing only the fingerprint capacitor charges to be stored, thereby increasing the integrator's dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a parasitic capacitor is present in the fingerprint sensor, then the base signal is amplified, but the integrator reaches saturation state and output dynamic range becomes small
Solution Approach 1:
A shielding electrode is introduced as an intermediary element between the sensing electrode and the reference electrode. This shielding electrode acts as a mediator to block the harmful electric field coupling between the sensing electrode and parasitic capacitors, thereby preventing the parasitic capacitance from affecting the fingerprint signal measurement while allowing the useful signal to pass through
Solution Approach 2:
The harmful parasitic capacitance effect is extracted and isolated from the main measurement circuit by introducing the shielding electrode. The shielding electrode captures and contains the parasitic electric fields, separating them from the fingerprint sensing path, thus removing their harmful influence on the integrator saturation
2Power
If the base signal is at least 100 times of the valid signal, then amplification is needed, but the amplified base signal causes integrator saturation
Solution Approach 1:
The shielding electrode serves as a protective intermediary that shields the integrator input from the large base signal generated by parasitic capacitors. By blocking the harmful electric field coupling, it allows the integrator to amplify the valid fingerprint signal without being overwhelmed by the base signal, preventing saturation while maintaining amplification capability
3Productivity
If the output dynamic range of the integrator is small, then fingerprint acquisition efficiency is reduced, but increasing amplification worsens saturation
Solution Approach 1:
The shielding electrode acts as a protective barrier that enables the integrator to maintain a larger effective output dynamic range by blocking the base signal interference from parasitic capacitors. This allows the system to achieve both high fingerprint acquisition efficiency and avoid integrator saturation simultaneously
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
This configuration effectively reduces the impact of parasitic capacitors on the integrating capacitor, preventing saturation and enhancing the dynamic range of the integrator, allowing for more accurate and efficient fingerprint acquisition.
Implementation Method 1
the fourth electrode plate layer arranged between the first electrode plate layer and the second electrode plate layer, the fourth electrode plate layer makes the first parasitic capacitor and the second parasitic capacitor have no impact on the integrating capacitor
Data Source
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AI summary
Embodiments of the present invention provide a capacitive fingerprint sensor. The capacitive fingerprint sensor includes: a first electrode plate layer, a second electrode plate layer and a third electrode plate layer that are sequentially arranged. The first electrode plate layer forms a fingerprint capacitor with a finger, at least one fourth electrode plate layer is arranged between the first electrode plate layer and the second electrode plate layer, a first parasitic capacitor is formed between the first electrode plate layer and the fourth electrode plate layer, and a second parasitic capacitor is formed between the second electrode plate layer and the fourth electrode plate layer; and the capacitive fingerprint sensor further comprises an integrator having an integrating capacitor, and the integrating capacitor is formed between the second electrode plate layer and the third electrode plate layer, wherein the first parasitic capacitor and the second parasitic capacitor have no impact on the integrating capacitor, and the integrating capacitor is configured to store charges from a fingerprint capacitor.