Capacitive Fingerprint Sensor Common Mode Suppression
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Solution Overview
Problem
Capacitive fingerprint sensors face challenges in measuring fingerprints through thick protective coatings, such as glass, due to similar capacitive coupling between the finger and sensing structures, leading to a small difference component compared to the common mode component, making it difficult to extract fingerprint information effectively.
Innovation Solution
The solution involves a fingerprint sensing device with a large array of sensing structures and a charge amplifier configuration where the output of one charge amplifier is capacitively coupled to the input of another, allowing for suppression of the common mode component and amplification of the difference component, achieved through symmetric circuitry without additional signal generating circuitry or resistors, enabling effective fingerprint sensing through thick protective coatings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick protective coating is used to protect the sensing structures, then the robustness and durability of the sensor are improved, but the capacitive coupling between the finger and sensing structures becomes uniform, making it difficult to extract fingerprint information
Solution Approach 1:
The patent uses feedback by measuring the common mode signal from all sensing structures and feeding it back to suppress the common mode component. The processing circuit measures the common mode signal, inverts it, and applies it back to the sensing structures to cancel out the uniform capacitive coupling effect, thereby extracting the difference component that contains fingerprint information.
Solution Approach 2:
The patent extracts the difference component from the total signal by separating the common mode signal (which contains no fingerprint information) from the difference component (which contains fingerprint information). The processing circuit selectively processes only the difference component after suppressing the common mode signal, thereby extracting the useful fingerprint information.
2Device complexity
If passive capacitive sensing is used to simplify the sensor structure, then the device complexity is reduced, but the signal-to-noise ratio deteriorates and requires a very thin protective layer
Solution Approach 1:
The patent segments the sensing function into two independent components: common mode signal measurement and difference component extraction. By separating these functions and processing them independently, the system achieves high signal-to-noise ratio while maintaining manageable complexity. The common mode signal is measured separately and used for suppression, while the difference component is extracted for fingerprint recognition.
Solution Approach 2:
The patent introduces an intermediary processing circuit that mediates between the sensing structures and the fingerprint extraction process. This processing circuit includes common mode signal measurement, inversion, and difference component extraction functions that act as intermediaries to transform the raw sensor signals into useful fingerprint information while suppressing noise.
3Reliability
If the protective coating is made thicker to increase robustness, then the sensor can withstand more wear and ESD, but the common mode component becomes dominant over the difference component
Solution Approach 1:
The system uses feedback to counteract the dominant common mode component. By measuring the common mode signal and applying an inverted version back to the sensing structures, the system creates a feedback loop that actively suppresses the common mode component, preventing it from overwhelming the fingerprint information in the difference component.
Solution Approach 2:
The patent converts the harmful dominant common mode component into a useful tool for fingerprint extraction. By measuring the common mode signal and using it to generate an inverted suppression signal, the system transforms the previously harmful uniform capacitive coupling into a controlled parameter that can be actively canceled, thereby revealing the hidden fingerprint information.
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 approach allows for improved sensing performance by suppressing the common mode component and amplifying the difference component, resulting in better fingerprint image quality even with thick protective coatings, facilitating robust and reliable fingerprint sensing in electronic devices.
Implementation Method 1
the output of a charge amplifier is capacitively coupled to an input of the charge amplifier
Data Source
Figure 1~2
Figure 3
Figure 4a~4b
AI summary
The present invention relates to a fingerprint sensing device for sensing a fingerprint pattern of a finger, the fingerprint sensing device comprising at least a first sensing structure and a second sensing structure; and at least a first charge amplifier and a second charge amplifier. Each charge amplifier comprises: a first input; a second input; an output; and at least one amplifier stage between the first and second inputs, and the output. The output is capacitively coupled to the first input. The first input of the first charge amplifier is connected to the first sensing structure; the first input of the second charge amplifier is connected to the second sensing structure; and the output of the first charge amplifier is capacitively coupled to the first input of the second charge amplifier to suppress the common mode component.