TOF Sensor Ambient Light Subtraction via Dual Floating Diffusions
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Solution Overview
Problem
Existing image sensors face challenges in effectively subtracting ambient light and separating illumination signals from fixed pattern noise, which hinders accurate object feature detection and depth measurement in three-dimensional spaces.
Innovation Solution
The implementation of AB Frame Correlated Data Sampling (AB Frame CDS) in time-of-flight image sensors, which involves two floating diffusions per photoelectric conversion device, allows for the subtraction of ambient light by performing two integrations: one with the illumination source off and one with it on, and further separates illumination signals from ambient light and fixed pattern noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ambient light subtraction is implemented using conventional methods, then some ambient light removal is achieved, but fixed pattern noise and illumination signal separation remain problematic
Solution Approach 1:
The pixel circuit is segmented into two separate floating diffusions (first and second floating diffusions), each handling different integration periods. This segmentation allows independent processing of ambient light and illumination signal components, enabling precise separation of these signals while maintaining high measurement accuracy for depth detection
Solution Approach 2:
The patent introduces an intermediary processing mechanism where the first floating diffusion integrates during a first period (capturing ambient light), the second floating diffusion integrates during a second period (capturing illumination signal), and a differential circuit acts as an intermediary to subtract these signals. This intermediary approach effectively separates the illumination signal from ambient light and fixed pattern noise
2Measurement precision
If multiple integrations are performed to separate illumination signal from ambient light, then measurement accuracy improves, but processing complexity increases
Solution Approach 1:
The patent merges multiple functions into the pixel circuit structure: both floating diffusions share common photoelectric conversion devices and readout circuitry. This merging approach performs multiple integrations and signal separations without proportionally increasing overall circuit complexity, as shared components reduce the total element count
Solution Approach 2:
The first and second floating diffusions are designed with multi-functionality, serving both as integration nodes for different time periods and as sources for differential processing. This universal design allows the same structural elements to perform multiple roles in signal separation, reducing the need for additional dedicated components
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 precise detection of object features by isolating the illumination signal, improving the accuracy of depth measurement and reducing noise, thereby enhancing the performance of image sensors in various applications, including photography and autonomous systems.
Implementation Method 1
charge is collected in a photoelectric conversion device of the pixel circuit as a result of impinging light
Data Source
AI summary
A time-of-flight image sensor (TOF) for imaging with ambient light subtraction. In one embodiment, the TOF image sensor includes a pixel array, a control circuit, and a signal processing circuit. The signal processing circuit reads out a first data signal from respective first floating diffusions and respective second floating diffusions during a first frame while a light generator is not emitting, reads out a second data signal from the respective first floating diffusions and the respective second floating diffusions during a second frame after the first integration and after a second integration while the light generator is emitting, generate a third data signal indicative of ambient light, generate a fourth data signal indicative of the ambient light and a light signal reflected off an object, and generate a fifth data signal indicative of the light signal by subtracting the third data signal from the fourth data signal.