Image Sensor Dynamic Range Extension via Sneak Readouts
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Solution Overview
Problem
Image sensors face a limitation in dynamic range due to saturation of storage capacitors at stronger optical intensities, which prevents accurate measurement of optical intensity and results in an upper limit on the dynamic range of pixel cells.
Innovation Solution
Implementing sneak readouts of storage capacitor voltage levels between formal row select voltage sense readouts to monitor charge accumulation and adjust exposure or readout schemes to prevent saturation, allowing for multiple charge transfers and voltage measurements during image capture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If storage capacitors are used to accumulate charge from photodiodes during exposure, then charge integration is achieved, but saturation occurs at stronger optical intensities limiting dynamic range
Solution Approach 1:
The patent segments the charge accumulation process into multiple discrete transfer stages. Instead of a single continuous integration, charge is transferred from photodiodes to storage capacitors in multiple steps during the exposure period. This segmentation allows intermediate monitoring and prevents saturation by redistributing charge accumulation across time intervals, thereby extending the measurable dynamic range.
Solution Approach 2:
The patent implements preliminary charge transfer operations before the final readout. Multiple charge transfers occur during the exposure period, with each transfer being a preliminary step toward the final measurement. This preliminary action allows the system to prepare and stabilize charge levels before the critical measurement phase, preventing saturation and enabling extended dynamic range measurement.
2Adaptability or versatility
If multiple charge transfers are implemented during exposure, then dynamic range is extended, but device complexity increases
Solution Approach 1:
The patent merges the charge transfer function with the existing pixel readout architecture. Multiple charge transfers are integrated into the standard pixel operation sequence, sharing control signals and timing mechanisms with the formal row select readouts. This merging approach enables extended dynamic range functionality without proportionally increasing device complexity, as the additional transfers utilize existing circuit resources.
Solution Approach 2:
The storage capacitors and transfer mechanisms serve multiple functions: they act as both integration elements during exposure and as temporary storage during the extended readout sequence. The same hardware components perform charge transfer, charge storage, and measurement functions across multiple operational phases, reducing the need for dedicated additional circuitry and thereby limiting the increase in device complexity.
3Adaptability or versatility
If sneak readouts are performed between formal readouts, then saturation is prevented, but measurement precision may be affected
Solution Approach 1:
The sneak readouts provide feedback information about charge accumulation levels during the exposure period. By monitoring voltage levels at intermediate points, the system gains feedback on the integration progress, allowing for adjusted control of subsequent charge transfers. This feedback mechanism prevents saturation by enabling real-time monitoring and adaptive control, while the feedback data can be integrated into the final measurement to maintain precision.
Solution Approach 2:
The sneak readouts act as intermediary measurement steps between the charge accumulation phase and the final formal readout. These intermediate measurements do not replace the formal readout but complement it, providing additional data points that help characterize the charge accumulation process. The intermediary nature of these readouts allows saturation prevention while the combined use of sneak and formal readout data maintains overall 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
Extends the upper limit of the dynamic range by enabling accurate measurement of optical intensity even at higher intensities, preventing saturation and maintaining reliable time-of-flight information across all phases.
Implementation Method 1
a light sensitive photodiode 203 generates and collects negative charge (electrons) as a function of the intensity of the light that it receives over the exposure time
Implementation Method 2
a storage capacitor 201, reset transistor Q1, transfer gate transistor Q2, photodiode 203
Data Source
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AI summary
An apparatus is described that includes an image sensor having timing and control circuitry and threshold circuitry. The timing and control circuitry is to generate signals to cause multiple transfers of charge from a photo-diode to a storage capacitor within a pixel cell during an image capture sequence. The threshold circuitry is to track the storage capacitor's voltage over the course of the multiple transfers and recognize when the storage capacitor's voltage reaches a threshold.