ADC Circuit for Photodiode Saturation Detection and Dynamic Range Extension
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Solution Overview
Problem
Image sensors face challenges in accurately measuring light intensity due to noise sources like dark current and quantization errors, which limit their dynamic range and accuracy, especially in wearable VR/AR/MR systems that require wide dynamic range and high-speed image generation across varying light intensities.
Innovation Solution
The proposed solution involves a pixel cell structure with a photodiode, charge storage unit, and ADC circuit that operates in multiple measurement modes, including a first mode for medium light intensity and a second mode for low light intensity, using ramping voltages and count values to determine photodiode saturation and output accurate digital values, while mitigating the effects of dark current through voltage headroom and full well capacity considerations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single measurement mode is used, then the device complexity is reduced, but the measurement precision deteriorates due to inability to accurately measure across wide dynamic range
Solution Approach 1:
The system dynamically switches between first and second measurement modes based on incident light intensity conditions. The ADC circuit operates in first measurement mode when light intensity is within a certain range, and switches to second measurement mode when light intensity exceeds that range, enabling adaptive optimization of measurement precision across different lighting conditions without requiring complex manual configuration
Solution Approach 2:
The measurement process is segmented into two distinct measurement modes: first measurement mode for normal light intensity conditions and second measurement mode for high light intensity conditions. Each mode uses optimized measurement parameters and sequences tailored to its specific operating conditions, allowing high precision measurement across the full dynamic range by dividing the measurement space into manageable segments
2Adaptability or versatility
If multiple measurement modes are implemented, then the adaptability improves for different light intensities, but the device complexity increases
Solution Approach 1:
The ADC circuit is designed with multi-functionality to perform both first measurement mode and second measurement mode operations using the same hardware resources. The circuit can universally handle different measurement sequences and parameter configurations without requiring separate dedicated hardware for each measurement mode, achieving wide adaptability while controlling device complexity through resource sharing
3Measurement precision
If dark current is not compensated, then the device complexity is reduced, but the measurement precision deteriorates due to noise from dark current
Solution Approach 1:
The system performs preliminary dark current compensation by establishing a voltage headroom in the charge storage unit before actual light measurement. This pre-established headroom accounts for and isolates the effects of dark current and noise charge, allowing subsequent measurements to proceed with improved precision without requiring complex real-time dark current subtraction algorithms
Solution Approach 2:
The voltage headroom acts as an intermediary mechanism that mediates between the dark current noise and the light measurement signal. By creating this intermediate voltage buffer, the system separates the dark current effects from the photodiode signal, enabling accurate measurement while maintaining relatively simple circuit implementation without direct complex noise cancellation
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 extends the dynamic range of image sensors, improving accuracy and user experience in VR/AR/MR systems by accurately measuring light intensity across a wide range and reducing the impact of noise sources, enabling reliable operation in diverse lighting conditions.
Implementation Method 1
A typical image sensor includes a photodiode to sense incident light by converting photons into charge (e.g., electrons or holes)
Data Source
AI summary
In one example, an apparatus comprises: a photodiode, a charge storage unit, and an analog-to-digital converter (ADC) circuit. In a first mode, the ADC circuit can compare a first voltage representing a quantity of the overflow charge stored at the charge storage unit against a first ramping voltage to generate a first decision; and obtain, based on the first decision output, a first digital value. In a second mode, the ADC circuit can compare a second voltage representing a quantity of residual charge stored in the photodiode against a second ramping voltage to generate a second decision, and obtain, based on the second decision, a second digital value. The ADC circuit can determine, based on one of the first decision output or the second decision output, whether the photodiode saturates, and output one of the first digital value or the second digital value to represent an intensity of incident light.