Capacitor Group Signal Processing Circuit for Fingerprint Sensor
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Solution Overview
Problem
Capacitive fingerprint recognition systems face signal saturation due to large variations in common-mode (CM) signals, which are not effectively removed, leading to inefficient processing of differential-mode (DM) signals in follow-up circuits.
Innovation Solution
A signal processing circuit and method utilizing a capacitor group and amplifier to couple and amplify sensing signals, with a compensation signal canceling out CM variations, maintaining a constant common-mode voltage level to prevent saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the sensing signal is directly amplified without removing the common-mode part, then the amplification process can be simplified, but the voltage signal becomes saturated and cannot be properly processed
Solution Approach 1:
The patent extracts and removes the common-mode part from the sensing signal before amplification. A common-mode rejection circuit is implemented that separates the common-mode voltage from the differential-mode signal, eliminating the saturation problem while maintaining a relatively simple overall circuit structure.
Solution Approach 2:
The signal processing is divided into distinct stages: first removing the common-mode component, then amplifying the differential-mode signal. This segmentation allows each stage to be optimized independently, ensuring reliable processing without excessive complexity.
2Reliability
If the common-mode part is removed using traditional methods, then signal saturation is prevented, but the circuit complexity increases and cannot effectively handle large CM variations
Solution Approach 1:
The patent changes the parameter handling approach by dynamically adjusting the common-mode rejection based on the magnitude of CM variations. The circuit adapts its rejection capability according to the sensed common-mode voltage level, effectively handling large variations without requiring overly complex fixed-structure circuits.
Solution Approach 2:
A feedback mechanism is introduced where the common-mode voltage is sensed and used to control the rejection process. This feedback loop enables the circuit to automatically adjust and maintain optimal performance across varying operating conditions while keeping the overall circuit design manageable.
3Speed
If the sensing signal is processed without compensation, then the processing speed is faster, but the differential-mode signal cannot be accurately extracted due to CM variations
Solution Approach 1:
The common-mode rejection is performed as a preliminary action before the differential-mode signal extraction and amplification. By pre-removing the common-mode component, the subsequent processing stages can focus solely on the differential signal, maintaining fast processing speed while ensuring accurate extraction.
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
Effectively eliminates CM signal variations, allowing the differential-mode signal to be accurately processed and preventing signal saturation in analog front-end circuits, ensuring proper operation and data integrity.
Implementation Method 1
The capacitor group, coupled to a node, comprises an input capacitor for coupling the sensing signal to the node and a compensation capacitor for coupling a compensation signal to the node
Implementation Method 2
The amplifier, coupled to the node, is configured to amplify the sensing signal coupled from the input capacitor
Data Source
AI summary
A signal processing circuit for receiving a sensing signal from a sensor includes a capacitor group, an amplifier and a switch. The sensing signal consists of a common-mode part and a differential-mode part. The capacitor group, coupled to a node, includes an input capacitor for coupling the sensing signal to the node and a compensation capacitor for coupling a compensation signal to the node. The amplifier, coupled to the node, is configured to amplify the sensing signal coupled from the input capacitor. The switch is coupled between the node and the amplifier.