Signal Processing Circuit With Compensation Capacitors for Charge Injection
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Solution Overview
Problem
Capacitive fingerprint recognition systems face issues with abnormal voltage ranges and charge injection problems due to variations in sensing signals, leading to signal saturation and performance degradation.
Innovation Solution
A signal processing circuit with a pair of compensation capacitors is introduced to reduce or cancel signal-dependent components at floating nodes, maintaining them within a small voltage range and mitigating charge injection issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the switched-capacitor integrator continuously receives sensing signals from sensing pixels, then the fingerprint recognition function is achieved, but the floating nodes may enter abnormal voltage ranges causing signal saturation
Solution Approach 1:
The patent applies preliminary action by performing a reset operation before the sensing operation. The reset switch is turned on during a reset period to clear any residual charge at the floating nodes before the sensing pixel starts exposure. This preliminary reset action ensures that the floating nodes start from a known voltage state, preventing them from entering abnormal voltage ranges during subsequent sensing operations.
Solution Approach 2:
The patent utilizes parameter changes by dynamically controlling the switching states of the reset switch and sensing pixel. The reset switch is activated during the reset period to change the voltage parameter at floating nodes to a baseline value. During the sensing period, the reset switch is turned off and the sensing pixel is activated, changing the parameter state to allow signal accumulation. This periodic parameter change maintains voltage stability while enabling fingerprint recognition.
2Adaptability or versatility
If large variations of sensing signals are received due to different brightness, then the photodiode responds to different light intensities, but the charge injection problem becomes more serious
Solution Approach 1:
The reset operation serves as a preliminary action that clears residual charge and establishes a baseline voltage state before sensing begins. By performing this reset action for each sensing cycle, the circuit prepares the floating nodes to handle varying signal levels without accumulating excessive charge that would worsen the charge injection problem.
Solution Approach 2:
The patent implements feedback through the switched-capacitor integrator structure, where the output voltage is fed back to the input through the feedback capacitor. This feedback mechanism automatically adjusts the integration process based on the accumulated charge, preventing excessive voltage buildup at floating nodes even when large variations in sensing signals are received, thereby mitigating charge injection effects.
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 compensation capacitors effectively stabilize the voltage at floating nodes, preventing signal saturation and improving the charge injection problem, ensuring the circuit operates within a normal voltage range and maintains signal integrity.
Implementation Method 1
During a sensing period of the sensing pixel, the photodiode of the sensing pixel starts exposure and thereby generates electronic signals
Implementation Method 2
The first compensation capacitor is coupled between a first floating node among the plurality of floating nodes and a first output node among the pair of output nodes
Data Source
AI summary
A signal processing circuit, which has a pair of input nodes and a pair of output nodes, includes a first switch pair, a second switch pair, an amplifier, a first compensation capacitor and a second compensation capacitor. The first switch pair is coupled between the pair of input nodes and a plurality of floating nodes. The second switch pair is coupled between the plurality of floating nodes and the pair of output nodes. The amplifier is coupled between the plurality of floating nodes and the pair of output nodes. The first compensation capacitor is coupled between a first floating node among the plurality of floating nodes and a first output node among the pair of output nodes. The second compensation capacitor is coupled between a second floating node among the plurality of floating nodes and the first output node.