Active Pixel Sensor Dynamic Range Extension via Charge Clipping
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Solution Overview
Problem
Active pixel image sensors using MOS technology face challenges in extending high-end dynamic range without increasing pixel size, particularly in high ambient light conditions, while maintaining low-end dynamic range and avoiding noise introduced by reset transistors.
Innovation Solution
The method involves transferring charge from the photodiode to a memory node via a first transfer transistor, with intermediate voltage pulses and clipping thresholds set by a second transfer transistor, allowing for a change in the response curve slope beyond a luminous flux threshold, enabling extended dynamic range without modifying pixel structure or size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the integration period is extended to capture more charge for high dynamic range, then the sensor can handle higher light levels, but the pixel saturates and loses sensitivity to low light levels
Solution Approach 1:
The integration period is divided into multiple segments with different transfer timings. The first transfer occurs at an intermediate voltage during the integration period, while the second transfer occurs at the end. This segmentation allows the photodiode to accumulate charge in stages, extending the linear dynamic range while preserving sensitivity to low light levels throughout the integration period.
Solution Approach 2:
The transfer transistor gate voltage is dynamically changed during the integration period. It transitions from an intermediate voltage (enabling first transfer) to a final voltage (enabling second transfer). This dynamic voltage adjustment allows the system to adapt the charge transfer timing to match varying light conditions, extending dynamic range without sacrificing low-light performance.
2Measurement precision
If a memory node is added between the photodiode and readout node to enable correlated double sampling, then noise is reduced, but the pixel area increases
Solution Approach 1:
The memory node serves multiple functions: it stores charge from the photodiode, enables correlated double sampling for noise reduction, and works with the dual-transfer mechanism to extend dynamic range. By making the memory node multi-functional, the design achieves noise reduction without requiring additional dedicated structures that would increase pixel area.
Solution Approach 2:
The memory node is merged with the existing pixel structure, sharing space with other pixel components. The dual-transfer mechanism merges the charge accumulation and transfer functions into a coordinated sequence, allowing the memory node to be efficiently utilized for both noise reduction and dynamic range extension without proportionally increasing pixel area.
3Quantity of substance
If the photodiode area is increased to capture more charge, then the dynamic range is extended, but the pixel size increases
Solution Approach 1:
Instead of changing the physical size of the photodiode, the system changes operational parameters: the transfer transistor gate voltage is modified to create intermediate and final voltage states. This parameter change enables the existing photodiode charge capacity to be utilized more effectively through staged transfer, extending dynamic range without increasing pixel size.
Solution Approach 2:
The charge transfer occurs in periodic stages rather than as a single event. The first transfer at intermediate voltage and the second transfer at final voltage create a periodic action pattern that maximizes the utilization of photodiode charge capacity over time, effectively increasing charge capacity without increasing photodiode area.
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 dynamic range of the sensor to handle high light levels without saturating, while maintaining sensitivity to low light levels, by adjusting the integration period and clipping thresholds based on ambient light measurements, thereby optimizing image capture.
Implementation Method 1
a photodiode that converts received photons to electric charge
Data Source
AI summary
In an active pixel sensor comprising a photodiode Dp, a memory node MN and a readout node SN, the memory node being provided to contain the charge generated by the photodiode at the end of an integration period allowing an integration in global shutter mode and a correlated double sampling, it is envisaged to carry out, in each integration period, at least one transfer ② of charge from the photodiode to the memory node followed by clipping ③ of the amount of charge contained in the memory node at an intermediate voltage t1 after the start of the integration period but before a last transfer of charge ④ to the memory node at the end of the integration period. The pixels are subsequently read out, row by row, by correlated double sampling CDS. The one or more intermediate transfers, with clipping, to the memory node during the integration period allow the dynamic range of the sensor to be extended to high levels of ambient light while retaining good sensitivity to low levels of ambient light.