Solid-State Imaging Device Dual-Comparison ADC for Dynamic Range
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Solution Overview
Problem
CMOS image sensors face limitations in achieving a broader dynamic range and higher frame rate due to charge overflow from photodiodes not being utilized in real time, random noise issues, and challenges in expanding the effective pixel region while maintaining cost-effectiveness.
Innovation Solution
A solid-state imaging device with a pixel structure that includes a photoelectric conversion element, a transfer element, an output buffer, and a comparator performing dual comparison processes to generate combined signals with expanded dynamic range, allowing for real-time utilization of overflow charges and improved noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an ADC including a comparator is arranged in each pixel to enable global shutter function, then global shutter capability is improved, but charges overflowing from photodiodes cannot be utilized in real time and dynamic range is limited
Solution Approach 1:
The pixel structure is segmented into distinct functional regions: photodiode for charge accumulation, transfer transistor for charge movement, floating diffusion for charge reception, and output buffer for signal conversion. This segmentation allows overflow charges to be separately managed and utilized while maintaining global shutter capability through coordinated operation of these segments.
Solution Approach 2:
The transfer transistor is configured to transfer charges from the photodiode to the floating diffusion before the ADC conversion process. This preliminary action enables overflow charges to be moved to a dedicated region (floating diffusion) where they can be detected and utilized separately from the main signal path, allowing real-time utilization of overflow charges while preserving global shutter operation.
2Reliability
If transistor size is enlarged to reduce flicker noise, then random noise is reduced, but area increases
Solution Approach 1:
The patent extracts the noise-sensitive functions into dedicated circuits: the output buffer separately converts charge to voltage, and the comparator performs ADC conversion. By taking out the signal conversion functions from the main pixel transistor, smaller transistors can be used in the photodiode readout path while maintaining low noise through the dedicated buffer and comparator design.
Solution Approach 2:
The floating diffusion acts as an intermediary between the photodiode and the output buffer. It receives charges from the photodiode and transfers them to the output buffer for voltage conversion. This intermediary structure allows the use of smaller photodiode transistors while the noise performance is maintained by the optimized floating diffusion and output buffer design.
3Reliability
If capacity is added to comparator output to lower bandwidth for noise filtering, then random noise is reduced, but inversion delay of comparator degrades
Solution Approach 1:
The reading operation is structured in periodic phases: integration period for charge accumulation, transfer period for charge movement to floating diffusion, and readout period for ADC conversion. By organizing the operation periodically with distinct phases, the system achieves noise filtering through correlated double sampling without requiring excessive bandwidth reduction, thereby maintaining fast comparator response and minimizing inversion delay.
4Adaptability or versatility
If ADC and memory part are arranged in each pixel, then global shutter is enabled, but effective pixel region cannot be expanded to maximum limit
Solution Approach 1:
The patent merges the ADC function into the pixel structure by using the floating diffusion as both a charge storage node and an input to the output buffer/comparator. This merging eliminates the need for separate large memory structures within each pixel, allowing the effective pixel region to be maximized while still enabling global shutter operation through the integrated readout path.
Solution Approach 2:
The floating diffusion serves multiple functions: it acts as a charge accumulation node during integration, a transfer destination during the transfer period, and an input signal source for the output buffer. This multi-functionality reduces the need for separate dedicated structures, thereby maximizing the effective pixel region while enabling global shutter capability.
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
Enables smooth signal switching, broader dynamic range, higher frame rates, and expanded effective pixel regions while reducing noise and optimizing cost-effectiveness.
Implementation Method 1
pixels for performing photoelectric conversion
Data Source
AI summary
A comparator in an AD conversion part, under the control of a reading part, performs a first comparison processing outputting a digitized first comparison result signal with respect to a voltage signal corresponding to an overflow charge overflowing from a photodiode PD1 to FD1 in an integration period and performs a second comparison processing outputting a digitized second comparison result signal with respect to a voltage signal corresponding to an accumulated charge of the photodiode PD1 transferred to the FD1 after a transfer period after the integration period, and a signal processing part performs combinational processing applying FWC information and joining a first AD conversion transfer curve TC1 corresponding to the first comparison processing and a second AD conversion transfer curve TC2 corresponding to the second comparison processing. Thus, it is possible to smoothly switch (connect) a plurality of signals to be combined and to suppress deterioration of an image.