Isolation Ramp Buffer for CMOS Image Sensor Horizontal Banding
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Solution Overview
Problem
Conventional CMOS image sensors suffer from charge kickback noise in readout circuitry, leading to unwanted horizontal banding or smear in images due to the interaction among columns, degrading the performance of analog-to-digital converters.
Innovation Solution
A system ramp generator is coupled with an isolation ramp buffer to generate isolated column ramp signals, which are received by each analog-to-digital converter, separating the system ramp generator output and reducing unwanted interactions among columns, thereby minimizing horizontal banding and smear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional shared ramp generator is used for all columns, then device complexity is reduced, but charge kickback noise from ADCs causes horizontal banding and degrades image quality
Solution Approach 1:
The patent divides the shared ramp generator into multiple column-specific ramp generators, with each column having its own isolated ramp signal source. This segmentation prevents charge kickback noise from one column's ADC from affecting other columns, thereby eliminating horizontal banding while maintaining manageable circuit complexity through modular design
Solution Approach 2:
The patent introduces isolation circuitry (such as transmission gates or buffer stages) as intermediary elements between the ramp generator and each column's ADC. This intermediary structure blocks the direct coupling that allows charge kickback noise to propagate across columns, thereby preventing horizontal banding while maintaining signal integrity
2Object-affected harmful factors
If columns are isolated to prevent charge kickback noise, then horizontal banding is reduced, but device complexity increases due to separate ramp generators for each column
Solution Approach 1:
The patent segments the ramp generation function into column-specific units, where each column has its own ramp generator or isolated ramp signal path. This segmentation achieves column isolation to prevent horizontal banding while keeping each segment's complexity low and manageable
Solution Approach 2:
The patent employs universal isolation circuitry designs (such as standard transmission gate configurations or buffer stages) that can be replicated across multiple columns. This multi-functional approach allows the same isolation mechanism to serve all columns, reducing overall complexity compared to designing unique isolation circuits for each column
3Device complexity
If the ramp generator output is directly coupled to all ADCs, then circuit complexity is minimized, but charge kickback noise from one ADC affects other ADCs causing horizontal smear
Solution Approach 1:
The patent introduces isolation circuitry (transmission gates, buffer stages, or switch matrices) as intermediary elements between the ramp generator and each ADC. These intermediaries block the direct coupling that allows charge kickback noise to propagate from one ADC to other ADCs, thereby preventing horizontal smear while maintaining signal integrity
Solution Approach 2:
The patent segments the shared ramp generator into multiple isolated ramp signal paths, with each path serving a specific column's ADC. This segmentation prevents charge kickback noise from affecting other ADCs while keeping the overall circuit complexity manageable through modular design
Data Source
AI summary
A readout circuit for use in an image sensor includes a system ramp generator coupled to generate a system ramp signal. A plurality of analog-to-digital converters is coupled to a plurality of column bitlines from a pixel array to receive corresponding analog column image signals. An isolation ramp buffer is coupled between the system ramp generator and the analog-to-digital converters. The isolation ramp buffer includes a single input to receive the system ramp signal, and a plurality of isolated outputs. Each of the isolated outputs is coupled to provide an isolated column ramp signal to a corresponding analog-to-digital converter. Each of the of analog-to-digital converters is coupled to generate a corresponding digital column image signal in response to the corresponding analog column image signal and corresponding isolated column ramp signal.