TOF Sensor Pixel In-Pixel Subtraction Circuitry
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Solution Overview
Problem
Conventional time-of-flight (TOF) sensor pixels face challenges with capacitor saturation and reduced signal-to-noise ratio due to large background light contributions, leading to increased system complexity and cost from the need for separate capacitor voltages and additional processing steps.
Innovation Solution
A TOF system and sensor pixel design where the photocurrent is reversed through a capacitor at twice the clock frequency during integration, resulting in a differential photocharge that isolates the time-of-flight signal from background light contributions, thereby reducing saturation and simplifying processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separate first and second capacitors are used to integrate photocurrent during high and low portions of clock cycle, then time-of-flight signal can be captured, but capacitor saturation occurs due to large background light contribution and signal-to-noise ratio decreases
Solution Approach 1:
The patent merges the function of separate first and second capacitors into a single capacitor that performs both integrations. The capacitor integrates photocurrent during the high portion of the clock cycle, then reverses the flow direction to integrate during the low portion, eliminating the need for separate capacitors and reducing saturation risk while maintaining measurement precision
Solution Approach 2:
The patent inverts the flow direction of photocurrent through the capacitor during the low portion of the clock cycle. By reversing the flow direction, the capacitor can accumulate differential photocharge (A-B) rather than accumulating background light contributions, preventing saturation while preserving the time-of-flight signal
2Measurement precision
If two separate capacitor voltages are stored and processed to correct for distance aliasing, then measurement accuracy is improved, but system complexity and processing cost increase
Solution Approach 1:
The patent combines the storage and processing of multiple capacitor voltages into a single capacitor voltage that contains differential information. The single capacitor voltage (V1) is sufficient for distance measurement after performing in-pixel subtraction, reducing memory requirements and processing complexity while maintaining accuracy
Solution Approach 2:
The patent extracts the time-of-flight signal from the photocurrent by performing in-pixel subtraction of background light contributions. This extraction is achieved through the capacitor flow reversal mechanism, which isolates the differential photocharge (A-B) from the total photocharge, simplifying subsequent processing while preserving measurement accuracy
3Measurement precision
If in-pixel subtraction is performed by reversing photocurrent flow through capacitor at twice the clock frequency, then background light contribution is isolated and signal-to-noise ratio is enhanced, but device complexity increases
Solution Approach 1:
The patent employs periodic action by reversing the photocurrent flow direction through the capacitor at twice the clock frequency. This periodic reversal occurs during each clock cycle, alternating the flow direction to achieve in-pixel subtraction and isolate background light contributions, thereby enhancing the signal-to-noise ratio through regular, predictable circuit operation
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 the signal-to-noise ratio, reduces capacitor saturation, and simplifies the TOF sensor pixel design by performing in-pixel subtraction, decreasing system complexity and cost while maintaining accurate distance determination.
Implementation Method 1
The photodetector detects the light pulses emitted by the light source in response to a clock signal, as well as background light, to provide a photocurrent
Implementation Method 2
The first capacitor integrates the photocurrent during the high portion of each clock cycle, over an integration period, to provide a first photocharge
Data Source
AI summary
A time-of-flight (TOF) sensor pixel is provided that performs in-pixel subtraction. The TOF sensor pixel includes a photodetector, a capacitor, and circuitry. The photodetector detects light pulses emitted at a clock frequency, after a time of flight, to provide a photocurrent. The capacitor integrates the photocurrent over an integration period, while the circuitry reverses a flow direction of the photocurrent through the capacitor at twice the clock frequency. At the end of the integration period, the capacitor provides a differential photocharge, corresponding to a capacitor voltage. The capacitor voltage is related to the time of flight of the light pulses and may be used to determine a distance to a target.