CMOS Image Sensor Pixel with Shared Output Chain for Wide Dynamic Range
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Solution Overview
Problem
CMOS image sensors struggle to capture a wide dynamic range, falling short of the human eye's capabilities, particularly in uncontrolled lighting environments, due to limitations in their linear operating range and saturation points, which result in significant spatial noise and inability to accurately represent bright and dark details in a single exposure.
Innovation Solution
Incorporating a Wide Dynamic Range (WDR) detection circuit, specifically a simplified time-to-saturation (TTS) method, alongside a correlated double sampling (CDS) circuit within each pixel, sharing a single output chain to minimize transistor count and facilitate high-resolution imaging with low fill-factor pixels, allowing for accurate light measurement across a broader dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional pinned photodiode with CDS methodology is used, then low noise capability is achieved, but dynamic range is limited to 60-72 db
Solution Approach 1:
The pixel is segmented into multiple independent photodiodes: a pinned photodiode for low-light detection and a pinned photodiode with WDR circuitry for high-light detection. Each photodiode is optimized for specific lighting conditions, allowing the system to handle both shadow and highlight details simultaneously within the extended dynamic range of 90 db or greater.
2Adaptability or versatility
If WDR detection circuitry is added to each pixel, then dynamic range is extended to 90 db or greater, but transistor count increases
Solution Approach 1:
The WDR detection circuit and CDS circuit share common components including the pinned photodiode, transfer gates, floating diffusion nodes, and output chain. This merging of functions allows the pixel to achieve extended dynamic range capability while minimizing the additional transistor count by reusing existing circuit elements for multiple purposes.
3Adaptability or versatility
If more transistors are used per pixel for WDR and CDS circuits, then image processing capability is improved, but fill factor increases
Solution Approach 1:
The shared output chain and common circuit elements serve multiple functions: they handle both WDR signal detection and CDS noise reduction operations. This multi-functionality eliminates the need for separate dedicated circuits for each function, thereby reducing the total transistor count per pixel and maintaining a low fill factor while preserving both WDR and low-noise capabilities.
4Measurement precision
If photodiode operates in linear range only, then measurement accuracy is maintained, but bright light details are lost due to saturation
Solution Approach 1:
Different photodiodes within the same pixel array are assigned different operational characteristics: the pinned photodiode is optimized for linear operation in low-light conditions, while the pinned photodiode with WDR circuitry is optimized for high-light detection. This local differentiation allows each photodiode type to operate in its optimal range, preventing saturation in bright areas while maintaining accuracy in dark areas.
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 CMOS image sensors to achieve a dynamic range of 90 db or greater, combining low noise capabilities for shadow details with the ability to record high illumination, thereby enhancing image capture accuracy and resolution in diverse lighting conditions.
Implementation Method 1
Each pixel includes a sensing element (e.g., a photodiode) that is capable of converting a portion of an optical image into an electronic (e.g., voltage) signal
Data Source
AI summary
A CMOS image sensor in which each pixel includes a conventional pinned diode (photodiode), a Wide Dynamic Range (WDR) detection (e.g., a simplified time-to-saturation (TTS)) circuit, a correlated double sampling (CDS) circuit, and a single output chain that is shared by both the CDS and WDR circuits. The pinned diode is used in the conversion of photons into charge in each pixel. In one embodiment, light received by the photodiode is processed using a TTS operation during the CDS integration phase, and the resulting TTS output signal is used to determine whether the photodiode is saturated. When the photodiode is saturated, the TTS output signal is processed to determine the amount of light received by the photodiode. When the photodiode is not saturated, the amount of light received by the photodiode is determined using signals generated by the readout phase of the CDS operation.