Shared Readout Low Noise Global Shutter Image Sensor
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Solution Overview
Problem
Conventional CMOS image sensors with electronic global shutters face challenges in reducing readout noise and increasing resolution due to the need for floating diffusions and multiple control signals, which compromise the active fill factor and efficiency.
Innovation Solution
The implementation of shared floating diffusions and charge-coupled gate devices allows for sequential readout of charges during rolling shutter operations, eliminating the need for floating diffusions in each pixel and reducing the number of control signals, thereby enhancing the active fill factor and light collection efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an additional floating diffusion is added to each pixel for CDS readout operation, then readout noise is reduced, but device complexity and pixel area increase
Solution Approach 1:
Multiple pixels share a common floating diffusion node for CDS readout operations. Instead of each pixel having its own dedicated floating diffusion, the patent combines the readout function across multiple pixels, reducing per-pixel component count while maintaining low-noise CDS capability.
Solution Approach 2:
The shared floating diffusion serves multiple pixels simultaneously, making a single component perform the function that would otherwise require multiple separate components. This universal node handles charge transfer and signal readout for an array of pixels.
2Measurement precision
If floating diffusions are added to each pixel for CDS operation, then readout noise is reduced, but active fill factor decreases
Solution Approach 1:
By merging the floating diffusion resources across multiple pixels, the patent eliminates redundant components in each pixel, thereby increasing the light-sensitive area while preserving the noise reduction functionality through shared infrastructure.
3Adaptability or versatility
If four control lines are provided per pixel for GS and CDS operations, then both operations are supported, but device complexity increases
Solution Approach 1:
The patent designs control lines to serve multiple pixels and multiple functions. Control signals are routed to share across pixel groups, allowing the same control infrastructure to manage both global shutter timing and CDS readout sequences for multiple pixels simultaneously.
Solution Approach 2:
Control functions are merged at the circuit level, where single control lines manage coordinated operations across pixel groups, reducing the total number of control signals required while maintaining full operational flexibility.
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 enables low-noise correlated double sampling readout operations and increased active fill factor, facilitating higher resolution image capture while minimizing control lines and improving light collection efficiency.
Implementation Method 1
Each pixel includes a sensing element that is capable of converting a portion of an optical image into an electronic signal
Implementation Method 2
each pixel of an associated pixel group includes a charge coupled gate (CCG) device (or other memory node) for storing the captured charge generated by that pixel's photodiode
Implementation Method 3
the CCG devices of the group's pixels are operably coupled to the shared floating diffusion. During a first phase of the RS readout operation, a first captured charge, which was generated and stored in a first pixel in the pixel group during the GS image capture operation, is transferred from the first pixel to the shared floating diffusion
Data Source
AI summary
A method for operating a global shutter image sensor includes performing both a global shutter (image capture) operation and a rolling shutter (readout) operation. During the global shutter operation, image information (charges) are captured by photodiodes in every pixel, and then simultaneously transferred to charge coupled gate (CCG) devices provided in each pixel. The rolling shutter operation includes performing multiple correlated double sampling (CDS) readout phases utilizing readout circuits that are shared by groups of pixels (e.g., four pixels share each readout circuit) having CCG devices connected in a chain. After resetting a floating diffusion in the readout circuit, a first captured charge is transferred to floating diffusion for readout, and the remaining charges are shifted along the CCG chain. The remaining CCG devices are then sequentially read out by repeating the read-and-shift operation. The readout operation is then repeated for each row of pixel groups.