Pixel Ramp Generator Current Steering for Low-Noise Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors face challenges in reducing noise from ramp generators and comparators, which affects image quality and power efficiency, especially in portable devices where noise from thousands of pixels can be correlated and noticeable even at low levels, leading to image blurring and distortion.
Innovation Solution
The implementation of a low-noise pixel ramp voltage generator using current steering, buffer multiplier techniques, sample-and-hold methods, and slope calibration to mitigate low-frequency noise and kickback noise, isolating noise sources and maintaining consistent noise levels across phases of the conversion cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a same ramp voltage is shared across an entire row of pixels, then device complexity is reduced, but noise from the ramp generator and comparators increases and affects image quality
Solution Approach 1:
The patent divides the single shared ramp generator into multiple segmented ramp generators, each serving a portion of the pixel row. This segmentation isolates noise sources so that noise from one segment does not correlate with noise from other segments, reducing the overall impact on image quality while maintaining reasonable device complexity
Solution Approach 2:
Each segmented ramp generator is designed with local optimization to minimize its specific noise contribution. The patent applies different noise reduction techniques to different segments based on their specific characteristics, such as using dummy comparators in certain segments to cancel noise, thereby improving overall image quality without uniformly increasing complexity across the entire system
2Object-affected harmful factors
If current steering is used to toggle between ramp-run and ramp-stop modes, then low-frequency noise components are mitigated, but device complexity increases due to additional control circuitry
Solution Approach 1:
The patent implements periodic toggling between ramp-run and ramp-stop modes using current steering circuitry. By periodically switching the current path, the system cancels out low-frequency noise components that would otherwise accumulate. The periodic nature of this switching is carefully designed to match the pixel conversion cycle, ensuring noise cancellation while maintaining efficient operation
Solution Approach 2:
The patent incorporates feedback mechanisms that monitor the ramp generator operation and adjust the current steering control accordingly. This feedback ensures that the ramp-stop mode is activated at precisely the right moments to cancel noise, while preventing unnecessary switching that would increase complexity and power consumption
3Object-affected harmful factors
If buffer multiplier techniques are used to isolate kickback noise, then image smearing is prevented, but device complexity and power consumption increase
Solution Approach 1:
The patent introduces buffer multiplier circuits as intermediary elements between the ramp generator and the pixel comparators. These buffers act as isolation barriers that prevent kickback noise from the comparators from coupling back into the ramp generator. The buffers are designed with high input impedance and low output impedance to effectively decouple the two stages while minimizing power consumption
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
This approach significantly reduces ramp noise, improving image quality by minimizing correlated noise across pixels and preventing image smearing, while maintaining power efficiency and meeting stringent noise specifications.
Implementation Method 1
a current steering block coupled with the current generator to selectively toggle between a ramp-run mode and a ramp-off mode based on a ramp start/stop signal, the current steering block being configured, when operating in the ramp-run mode, to steer the reference current to a load path to generate a ramp voltage
Implementation Method 2
a buffer multiplier block comprising a plurality of buffers, each buffer coupled with the ramp voltage output node and configured to buffer the ramp voltage for provision as a respective buffered-ramp input voltage to each of a respective set of pixel conversion comparators
Data Source
AI summary
Techniques are described for implementing reduced-noise pixel ramp voltage generators for image sensors. The described pixel ramp voltage generators include circuit blocks to address various conventional causes of ramp generator noise. For example, a reference current is generated. Current steering can be used to toggle between a ramp-run mode for generation of a ramp voltage, and a ramp-stop mode for stopping the generation, thereby mitigating changes in low-frequency noise components between pixel conversion cycles, or phases or such cycles. Buffer multiplier techniques can be used to isolate kickback noise of the pixel conversion comparators from components generating the ramp voltage, such as to mitigate image smearing. Some implementations include additional features, such as sample-and-hold techniques used to isolate ramp generation components from current generation component noise, slope calibration techniques to dynamically tune voltage ramp slope, reset techniques, etc.