Dual-Mode Pixel Circuit for Depth Sensor Light Saturation
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Solution Overview
Problem
Conventional depth sensors face challenges in accurately determining distance under high ambient light conditions due to charge saturation in the floating diffusion node, which prevents the generation of accurate analog signals reflecting the intensity of reflected light.
Innovation Solution
A pixel circuit operating in two modes based on ambient light intensity, where in high light conditions, a portion of the charge is discharged to the supply voltage to prevent saturation, and in low light conditions, charge is continuously accumulated without discharge, allowing for accurate signal generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the floating diffusion node accumulates charge from ambient light, then the node can capture light intensity information, but the node becomes charge saturated under high ambient light conditions, preventing accurate distance measurement
Solution Approach 1:
The pixel circuit is divided into two separate nodes: a first node for accumulating charge from incident light during the integration period, and a second floating diffusion node for generating the output signal. This segmentation allows the first node to capture ambient light without saturating the signal-generating second node, resolving the contradiction between capturing light intensity information and preventing charge saturation for accurate measurement.
Solution Approach 2:
The harmful effect of ambient light charge saturation is extracted and isolated in the first node, which is then discharged to a reference potential before the integration period begins. This prevents the ambient light charge from interfering with the reflected light signal measurement in the second node, maintaining both measurement accuracy and signal generation reliability.
2Device complexity
If the pixel circuit uses a single node for charge accumulation, then the circuit structure remains simple, but the circuit cannot accurately discriminate reflected light from ambient light under high ambient light conditions
Solution Approach 1:
The pixel circuit is divided into two separate nodes: a first node for accumulating charge from incident light during the integration period, and a second floating diffusion node for generating the output signal. This segmentation allows the first node to capture ambient light without saturating the signal-generating second node, resolving the contradiction between capturing light intensity information and preventing charge saturation for accurate measurement.
Solution Approach 2:
The pixel circuit operates in periodic cycles: during the integration period, the first node accumulates charge from both ambient and reflected light; then the first node is discharged to reference potential, and the second node generates the output signal. This periodic operation allows the circuit to handle high ambient light conditions while maintaining the ability to detect reflected light signals.
3Duration of action of stationary object
If the floating diffusion node holds charge throughout the detection period, then the circuit can maintain continuous signal levels, but the node becomes charge saturated and cannot generate accurate analog signals
Solution Approach 1:
The pixel circuit is divided into two separate nodes: a first node for accumulating charge from incident light during the integration period, and a second floating diffusion node for generating the output signal. This segmentation allows the first node to capture ambient light without saturating the signal-generating second node, resolving the contradiction between capturing light intensity information and preventing charge saturation for accurate measurement.
Solution Approach 2:
Before the integration period begins, the first node is discharged to a reference potential to ensure it starts in a known state. This preliminary action prevents residual charge from affecting the next measurement cycle, allowing the first node to hold charge throughout the detection period when needed while maintaining signal accuracy.
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
Enables the depth sensor to faithfully generate analog signals representing distance to an object under varying ambient light conditions, improving accuracy and reliability.
Implementation Method 1
a light receiving unit configured to generate charge in response to incident light
Data Source
AI summary
A pixel circuit for a depth sensor operating in a detection period and an output period in either a first operating mode (high incident light intensity) or a second operating mode (low incident light intensity). The pixel circuit includes a light receiving unit generating charge in response to the incident light, a signal generation unit accumulating charge in a FDN in response to a transmission signal, reset signal and selection signal during the detection period, and generating an analog signal having a level corresponding to a voltage apparent at the FDN during the output period, and a refresh transistor coupled between a supply voltage and the light receiving unit and discharging charge to the supply voltage in response to a refresh signal.