Presettable Ramp Driver Circuit for Low-Noise Image Sensor ADCs
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Solution Overview
Problem
Existing image sensors face challenges in achieving high-quality image data by effectively reducing noise, particularly fixed pattern noise and temporal noise, which are not adequately addressed by current correlated double sampling methods.
Innovation Solution
The implementation of a presettable ramp driver in the ADC circuitry for image sensors, which includes a ramp generator, presettable ramp drivers, and ADC comparators, allows for precise control of the ramp voltage to enhance the correlated double sampling process, thereby improving noise reduction and image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional ADC circuitry is used for image sensor readout, then the basic conversion function is provided, but fixed pattern noise and temporal noise are not adequately reduced
Solution Approach 1:
The patent applies preliminary action by performing correlated double sampling operations before the final ADC conversion. The circuit captures and stores reset level data and signal level data separately, then processes them through differential amplification and multiple ADC conversions to eliminate fixed pattern noise and temporal noise before generating the final digital image data.
Solution Approach 2:
The patent segments the ADC conversion process into multiple independent conversion operations. Each pixel column has its own ADC that performs separate conversions for reset data and signal data, allowing independent processing and noise reduction for each segment, which improves overall image quality by eliminating correlated noise components.
2Measurement precision
If the number of pixels in the array is increased to achieve higher image quality, then resolution improves, but noise in the image data increases
Solution Approach 1:
The patent implements feedback through correlated double sampling, where the reset level data is captured and stored, then used as a reference for subtracting from the signal level data. This feedback mechanism allows the circuit to identify and eliminate fixed pattern noise and temporal noise that would otherwise accumulate with higher pixel counts, maintaining image quality despite increased pixel density.
3Object-affected harmful factors
If correlated double sampling is implemented to reduce noise, then fixed pattern noise and temporal noise are reduced, but the device complexity increases
Solution Approach 1:
The patent merges the correlated double sampling functionality directly into the ADC circuitry itself, rather than using separate processing stages. The differential amplification, sample-and-hold circuits, and multiple ADC conversion operations are integrated into a unified readout circuit for each pixel column, reducing overall device complexity while maintaining effective noise reduction capabilities.
Data Source
AI summary
An analog-to-digital converter (ADC) converts analog image signal received from a bitline to a digital signal through an ADC comparator. The ADC comparator compares an image signal voltage at its first input against a ramp voltage at its second input to trigger a latch and to cause a digital signal output from the ADC. Each pixel voltage is coupled to only one comparator at its first input through a sample capacitor. A ramp generator generates a global ramp voltage as an input to a plurality of presettable ramp drivers. Each presettable ramp driver outputs a buffered ramp voltage to drive multiple ADC comparators simultaneously at their second inputs. Model circuits of the presettable ramp drivers are disclosed, and respective procedures of presetting the presettable ramp drivers are presented to demonstrate how each of featured presettable ramp drivers is conditioned to drive multiple ADC comparators of the ADCs.