SPAD Pixel Circuit for Low-Noise Time-of-Flight Imaging
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Solution Overview
Problem
Traditional image sensors struggle with low-light performance and noise interference, particularly in 3D imaging applications, due to issues like ambient light and dark count noise, which affect the accuracy and efficiency of time-of-flight measurements.
Innovation Solution
The use of Single Photon Avalanche Diode (SPAD) image sensors, which incorporate a color filter array and Time-of-Flight (ToF) principles, allows for efficient 2D and 3D imaging by selectively capturing depth data for targeted areas, reducing noise interference through adaptive histogram analysis and multi-event sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional image sensors are used, then manufacturing simplicity is maintained, but low-light performance and measurement precision deteriorate due to noise interference
Solution Approach 1:
The patent changes the operating parameters of the photodiode by applying a high reverse bias voltage above the breakdown voltage, enabling the avalanche effect. This parameter change allows single photon detection and significantly improves time-of-flight measurement accuracy in low-light conditions while the harmful noise factors remain manageable through software processing
Solution Approach 2:
The patent replaces traditional mechanical noise filtering methods with electronic avalanche multiplication. The avalanche effect provides inherent gain that amplifies weak photon signals before they are affected by noise, substituting mechanical filtering approaches with an electronic amplification mechanism that improves measurement precision
2Reliability
If SPAD sensors operate in Geiger mode for single photon detection, then sensitivity is improved, but device complexity increases due to additional control circuits
Solution Approach 1:
The patent merges the avalanche photodiode functionality with standard CMOS pixel circuit structures. The SPAD is integrated into existing pixel circuits that include transistors and capacitors, combining multiple functions (photon detection, avalanche multiplication, charge storage, and signal readout) into a unified pixel structure that reduces overall device complexity
Solution Approach 2:
The pixel circuit is designed to perform multiple functions: operating the SPAD in Geiger mode for single photon detection, storing avalanche-generated charges, filtering noise through correlated dual-triplet processing, and outputting signals for both 2D and 3D imaging. This multi-functionality reduces the need for separate dedicated circuits for each function
3Measurement precision
If full-frame 3D imaging is performed, then measurement coverage is improved, but productivity decreases due to processing time requirements
Solution Approach 1:
The patent segments the image processing into two independent paths: a 2D imaging path that processes all pixels simultaneously for fast frame rates, and a 3D imaging path that processes only selected pixels (e.g., every other pixel or specific regions) for depth measurement. This segmentation allows full-frame coverage for 2D imaging while maintaining high frame rates, with 3D data available from the segmented subset
4Measurement precision
If noise filtering algorithms are applied to SPAD data, then measurement precision is improved, but processing time increases
Solution Approach 1:
The patent performs noise filtering actions during the photon detection and charge accumulation phase rather than after image capture. Correlated dual-triplet processing is applied in real-time as photons are detected, preliminarily separating signal from noise before the data is even read out. This preliminary action reduces the processing burden afterward and maintains high frame rates
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
SPAD sensors provide high sensitivity and precision in low-light conditions, enabling fast and accurate 2D and 3D imaging with reduced noise, suitable for applications like autonomous vehicles and medical imaging.
Implementation Method 1
Image sensors convert optical images to digital data that may be represented as digital images. An image sensor typically includes an array of pixel sensors, which are unit devices for the conversion of an optical image into electrical signals.
Implementation Method 2
An avalanche process can be triggered when a reverse biased p-n junction receives additional carriers, such as carriers generated by incident radiation. For example, in order to detect radiations with low intensities, the p-n junction is biased above its breakdown voltage, thereby allowing a single photon-generated carrier to trigger an avalanche current that can be detected.
Data Source
AI summary
A pixel of an image sensor includes a single photon avalanche detector (SPAD) and a counter circuit configured to selectively count pulses output by the SPAD based on detected photons in response to a recharge signal. A row select circuit is configured to selectively connect the SPAD to a time-to-digital (TDC) circuit in response to the recharge signal and a row select signal.