TOF Macro-Pixel CMOS Readout for Higher Distance Resolution
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Solution Overview
Problem
Current time-of-flight distance sensors using CMOS image sensors face limitations in distance resolution due to large pixel sizes, which result in slower charge transfer and increased power consumption, especially when averaging multiple pixels for improved accuracy.
Innovation Solution
The use of an array of pinned photodiodes with a macro-pixel structure, including a second transistor and switched capacitor circuits for correlated double sampling, allows for efficient charge transfer and noise reduction, enabling better distance resolution with reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If large pixel sizes are used in CMOS image sensors for TOF distance sensors, then a significant number of photo-electrons can be accumulated, but charge transfer becomes slower and distance resolution worsens
Solution Approach 1:
The invention divides the pixel array into multiple groups, where each group shares a common readout node. This segmentation allows for efficient charge transfer from multiple pixels through a single node, maintaining fast transfer speeds while enabling the accumulation of sufficient photo-electrons across the group, thus resolving the contradiction between accumulating enough photo-electrons and maintaining fast charge transfer for good distance resolution
Solution Approach 2:
Multiple pixels are combined to share a common readout node, allowing the system to accumulate photo-electrons from multiple pixels while transferring charge through a single efficient path. This merging approach enables sufficient signal accumulation without the transfer speed penalties of individual large-pixel transfers, maintaining both photo-electron accumulation and distance resolution
2Measurement precision
If multiple pixels are averaged to improve distance accuracy, then measurement precision improves, but device complexity increases, readout speed decreases, and power consumption increases
Solution Approach 1:
Multiple pixels are merged into groups that share common readout nodes and processing circuits. This merging enables the system to achieve improved distance accuracy through multi-pixel averaging while reducing overall device complexity by eliminating redundant readout circuitry for each individual pixel, thus resolving the contradiction between measurement precision and device complexity
3Measurement precision
If multiple pixels are averaged to improve distance accuracy, then measurement precision improves, but power consumption increases
Solution Approach 1:
Multiple pixels share common readout nodes and processing circuits, reducing the total number of active components required for multi-pixel averaging. This merging approach enables improved distance accuracy while minimizing power consumption by eliminating redundant circuitry, thus resolving the contradiction between measurement precision and power consumption
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 enhances distance resolution while maintaining low power consumption, suitable for portable devices, by integrating photoelectrons in pinned photodiodes and using macro-pixels to combine signals effectively, overcoming the limitations of traditional CMOS image sensors.
Implementation Method 1
the conversion of light into an electric signal is performed by an array of pinned photodetectors (PPD)
Data Source
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AI summary
A Time-of-flight optical device and a 3D optical detector comprising a CMOS integrated circuit with an array of photosensitive pixels that are, at least in part, interconnected to form macro-pixels (180). Each macro-pixel (180) groups a plurality of individual pixels contributing their photocarriers to a common sense node SN through a plurality of transistors in parallel. Preferably the integrated circuit includes switched capacitor circuits arranged to combine the potential of the sense nodes SN of a plurality of macro-pixels, and/or to perform correlated double samplings in an energy efficient way. Each pixel has now an additional sink gate (194) between the pinned photodetector, PPD, potential well and a positive voltage source. By this additional sink gate (194), the storage well of the PPDs can be emptied without transferring the charge to the sense node. The value of the transfer gate voltage V_TG may be adapted such that the potential barrier is not lowered all the way down, but decreased to a value VB<VP. In this manner, the potential well of the PPDs is emptied only in part. This amounts to subtracting a constant value from VTRAN and can be used to zero a background illumination value. Importantly, the charge left in the potential well is discharged by the sink transistor (194) before the next integration and does not affect successive cycles.