Capacitive Fingerprint Sensor Parasitic Capacitance Feedback
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive fingerprint sensors face issues with parasitic capacitors that distort measurements, leading to noise interference and reduced image resolution due to large parasitic capacitance at the input of the charge amplifier and inefficient use of layers.
Innovation Solution
The lower electrode is connected to the output terminal of the charge amplifier, converting the large parasitic capacitance into a feedback capacitor, allowing for tuning of gain and reducing noise, while efficiently using the available layers and minimizing lateral parasitic capacitance effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the lower electrode is connected to ground level, then the parasitic capacitor can be eliminated to some extent, but the parasitic capacitor remains in parallel with the finger capacitance and continues to disturb the measurement
Solution Approach 1:
The patent connects the lower electrode to the output terminal of the charge amplifier, creating a feedback connection. This feedback arrangement converts the parasitic capacitor into a feedback capacitor, which is then subtracted from the total capacitance measurement, eliminating its interfering effect on the finger capacitance measurement.
Solution Approach 2:
The patent transforms the harmful parasitic capacitor into a beneficial feedback capacitor. By connecting the lower electrode to the charge amplifier output, the parasitic capacitance between the fingerprint sensor electrode and lower electrode becomes a controlled feedback element that can be mathematically removed from the measurement, converting a source of error into a useful component for compensation.
2Device complexity
If a large parasitic capacitor is present at the input of the charge amplifier, then the measurement can be simplified, but the noise performance deteriorates significantly
Solution Approach 1:
The feedback connection to the charge amplifier output converts the large parasitic input capacitor into a feedback capacitor. This transformation maintains the simplified sensor structure while dramatically improving noise performance, as the feedback capacitor's effect can be subtracted from the measurement and it does not degrade the signal-to-noise ratio like an input capacitor would.
Solution Approach 2:
The charge amplifier output serves as an intermediary connection point. By routing the lower electrode connection to this intermediary point rather than directly to ground, the system achieves both structural simplicity and improved noise performance through the mathematical compensation provided by the charge amplifier's feedback mechanism.
3Device complexity
If multiple sensor elements are connected to a single charge amplifier, then the device complexity is reduced, but the measurement bandwidth is shared and resolution may be affected
Solution Approach 1:
The patent makes a single charge amplifier serve multiple functions by connecting it to multiple sensor elements through the shared lower electrode connection. Each sensor element's signal is processed by the same amplifier, reducing the total number of amplifiers needed while maintaining measurement precision through the parasitic capacitor compensation mechanism that works for each element independently.
Solution Approach 2:
The patent merges multiple sensor element processing paths by using a single charge amplifier for multiple elements. The lower electrode connection acts as a common reference that allows multiple sensor elements to share the same amplification and compensation resources, reducing device complexity while preserving individual element measurement accuracy through the feedback mechanism.
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 achieves a high signal-to-noise ratio, enabling the use of thicker protective coatings for enhanced ESD protection, improved image resolution, and reduced interference noise, with the ability to connect multiple sensor elements to a single charge amplifier for tunable resolution.
Implementation Method 1
the fingerprint sensor electrode and the lower electrode are arranged in such a way that a capacitance is formed between them, and where the sensor electrode is arranged for being connected to the negative input terminal of the charge amplifier
Implementation Method 2
The lower electrode is connected to the output terminal of the charge amplifier, converting the large parasitic capacitance into a feedback capacitor, allowing for tuning of gain and reducing noise
Data Source
AI summary
The present invention relates to a fingerprint sensor element, comprising a sensor electrode formed in an upper conducting layer, a lower electrode formed in a lower conducting layer and at least one insulating layer between the upper conducting layer and the lower conducting layer. It further comprises a charge amplifier having a negative and a positive input terminal and an output terminal. An upper side of the fingerprint sensor electrode is arranged for facing a finger and a lower side is arranged for facing the lower electrode, and the fingerprint sensor electrode and the lower electrode are arranged in such a way that a capacitance is formed between them. The sensor electrode is arranged for being connected to the negative input terminal of the charge amplifier, and the lower electrode is arranged for being connected to the output terminal of the charge amplifier.