Logarithmic Pixel Correlated Double Sampling Circuit
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Solution Overview
Problem
Logarithmic pixels in imaging systems face challenges with noise removal due to their continuous measurement of light intensity, making them susceptible to pixel fixed pattern noise, as they lack the ability to perform correlated double sampling.
Innovation Solution
Incorporating an anti-blooming transistor that enables correlated double sampling in logarithmic mode by sampling the voltage at the floating diffusion region both with and without the anti-blooming transistor, allowing for noise isolation and reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If logarithmic pixels continuously measure light intensity, then light intensity measurement capability is improved, but noise susceptibility increases
Solution Approach 1:
The patent applies preliminary action by performing a reset sampling before the actual light measurement. The reset sample captures the pixel output before light exposure, establishing a baseline that accounts for fixed pattern noise and other offsets. This preliminary measurement enables subsequent subtraction to remove noise from the actual light intensity measurement, resolving the contradiction between continuous measurement capability and noise susceptibility.
2Adaptability or versatility
If logarithmic pixels continuously measure light intensity, then dynamic range capability is improved, but pixel fixed pattern noise increases
Solution Approach 1:
The patent extracts the fixed pattern noise component by separately sampling it during a reset phase when no light is present. By taking out the noise component (reset sample) from the overall measurement process, it can be subtracted from the actual light measurement to leave only the true light intensity signal. This extraction approach maintains dynamic range capability while eliminating pixel fixed pattern noise.
3Object-affected harmful factors
If correlated double sampling is implemented in logarithmic mode, then noise reduction is improved, but device complexity increases
Solution Approach 1:
The patent implements multi-functionality by designing the pixel circuit to operate in multiple modes (reset mode and measurement mode) using the same hardware components. The anti-blooming transistor and sampling circuitry serve dual purposes: preventing charge overflow during measurement and enabling correlated double sampling for noise reduction. This universal design achieves noise reduction without proportionally increasing device complexity.
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 effectively reduces noise in logarithmic pixels, improving the accuracy of light intensity measurement and enabling the creation of high dynamic range images with minimized artifacts from flickering lighting and objects with changing illumination.
Implementation Method 1
Each pixel includes a photosensitive layer that receives incident photons (light) and converts the photons into electrical charge
Data Source
AI summary
An imaging pixel may be operated in either a linear mode or a logarithmic mode. In the logarithmic mode, the voltage at a floating diffusion region may be proportional to the logarithm of the intensity of incident light. In order to enable correlated double sampling (CDS) in the logarithmic mode, a transistor may be provided that couples the photodiode to a bias voltage. When the transistor is turned off, the photodiode may be able to operate in a logarithmic mode. When the transistor is turned on, the floating diffusion region may be reset to a baseline voltage level. Images from the linear mode and the logarithmic mode may be combined to form high dynamic range images with flicker mitigation.