Correlated Double Sampling Circuit Leakage Current Cutoff
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Solution Overview
Problem
Conventional image sensors employing correlated double sampling (CDS) technology face challenges in reducing fixed pattern noise (FPN) due to leakage currents, which distort output signals and increase the complexity of signal processing units, leading to larger sensor sizes and incomplete noise reduction.
Innovation Solution
A CDS circuit is designed with an auto-zero switch and boosting switches to cutoff leakage currents during comparison intervals, using a ramp signal to perform comparisons between reset and image components, thereby reducing noise and maintaining signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional CDS technology is used to reduce fixed pattern noise, then noise reduction is achieved, but leakage currents distort output signals and increase signal processing unit complexity
Solution Approach 1:
The patent extracts and removes the harmful leakage current component from the signal processing path by introducing a dedicated cutoff switch (S2) that isolates the leakage current source from the output signal path. This extraction approach eliminates the distortion caused by leakage currents without requiring complex compensation algorithms, thereby reducing signal processing unit complexity while maintaining noise reduction effectiveness.
Solution Approach 2:
The patent applies preliminary action by preemptively cutting off leakage currents before they can distort the output signals. The cutoff switch (S2) is activated during specific time intervals (reset phase and signal reading phase) to prevent leakage currents from entering the signal path, thereby eliminating the need for complex post-processing compensation and simplifying the overall signal processing architecture.
2Device complexity
If leakage currents are not cutoff, then signal processing can be simplified, but output signals become distorted and fixed pattern noise reduction becomes incomplete
Solution Approach 1:
The patent introduces an intermediary component (cutoff switch S2) that mediates between the signal processing unit and the leakage current source. This intermediary selectively connects or disconnects the leakage current path based on operational phase, allowing the signal processing unit to remain relatively simple while effectively preventing leakage-induced distortion and achieving complete fixed pattern noise reduction.
3Measurement precision
If auto-zero switch connects input terminal to output node during comparison intervals, then auto-zero operation is achieved, but leakage currents flow to the switch and distort signals
Solution Approach 1:
The patent employs periodic action by controlling the auto-zero switch (S1) and cutoff switch (S2) to operate in alternating phases. During the auto-zero phase, switch S1 connects the input terminal to the output node for zeroing operation while switch S2 remains open to block leakage currents. During comparison and reading phases, switch S1 is open and switch S2 closes to cut off leakage currents. This periodic switching strategy achieves both auto-zero functionality and leakage current elimination without signal distortion.
Data Source
AI summary
A correlated double sampling (CDS) circuit may include a sampling circuit and a switching circuit. The CDS circuit may perform a CDS operation on a reset component of an input signal and an image component of the input signal based on a ramp signal, and the CDS circuit may generate an output signal. The switching circuit may include an auto-zero switch that connects a first input terminal receiving the input signal to an output node in response to an auto-zero control signal during an auto-zero interval. The switching circuit may connect a first terminal of the auto-zero switch to a reference voltage in order to cutoff a leakage current flowing to the auto-zero switch in response to a first boosting control signal during a first comparison interval and a second comparison interval. The output signal may be provided at the output node.