TOF Pixel Circuit Reduces Complexity via Comparator Extraction
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Solution Overview
Problem
Conventional time of flight imaging technologies face limitations in flexibility of sensing distance and increased pixel complexity due to the need for comparators in continuous wave TOF cameras.
Innovation Solution
A pixel design for TOF cameras that includes a constant current source, integrating capacitor, sampling transistor, photodiode, sampling capacitor, amplifier, and reset transistor, along with supporting circuitry for converting signals into image data, allowing for improved distance sensing and reduced pixel complexity by synchronizing voltage integration with illumination pulses and transferring voltage to a sampling capacitor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If continuous wave TOF cameras use comparators for distance measurement, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent extracts the comparator function from the pixel circuit and implements it externally in the readout integrated circuit (ROIC). The pixel only contains the photodiode, integrating capacitor, and transfer transistor, while the comparator is located in the ROIC that processes signals from multiple pixels. This extraction reduces in-pixel complexity while maintaining measurement precision through external comparison operations.
Solution Approach 2:
The patent merges the signal processing functions (integration, transfer, and comparison) into a unified readout architecture where multiple pixels share common readout circuitry. The integrating capacitors from multiple pixels are sequentially connected to a single comparator in the ROIC, allowing shared resource utilization and reduced overall system complexity while maintaining precision for each pixel.
2Measurement precision
If conventional TOF pixels integrate multiple functions, then measurement capability is improved, but adaptability decreases
Solution Approach 1:
The patent segments the TOF measurement system into distinct functional modules: photodetection (photodiode), signal integration (integrating capacitor), signal transfer (transfer transistor), and comparison (external comparator). This segmentation allows independent optimization and configuration of each module, enabling flexible adaptation to different sensing distance requirements by adjusting integration time, capacitor values, or readout timing without redesigning the entire pixel.
Solution Approach 2:
The patent implements dynamic control of the integration process through externally controllable transfer transistors and timing signals. The integration time can be dynamically adjusted by controlling when the transfer transistor connects the integrating capacitor to the readout circuit, allowing the system to adapt to varying sensing distances and lighting conditions while maintaining measurement precision.
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 solution enhances the flexibility of sensing distance and reduces pixel complexity, providing superior time of flight imaging capabilities compared to traditional systems.
Implementation Method 1
A photodiode is electrically connected between a base terminal of the sampling transistor and the ground for switching electrical connectivity through the sampling transistor in response to radiation incident on the photodiode
Data Source
Figure 1~2
AI summary
A pixel includes a constant current source electrically connected to a first node. An integrating capacitor is electrically connected between the first node and a ground. A sampling transistor is electrically connected between the first node and a second node. A photodiode is electrically connected between a base terminal of the sampling transistor and the ground for switching electrical connectivity through the sampling transistor in response to radiation incident on the photodiode.