Shared Pixel Circuit Readout for Low Power Image Sensors
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Solution Overview
Problem
Conventional image sensors with shared pixel architectures require multiple power-consuming conversions for each photodiode, leading to high dynamic power consumption, and removing the reset conversion increases read noise intolerably.
Innovation Solution
Implementing a readout process where one photodiode's signal value is used as another's reset value, reducing the number of conversions needed from 2*m to (m+1), thereby minimizing power consumption while maintaining noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two separate conversions are performed for each photodiode to remove kTC noise, then read noise is reduced, but dynamic power consumption increases significantly
Solution Approach 1:
The patent merges the reset sampling and signal sampling operations by using the signal value from one photodiode as the reset value for the next photodiode. This combines what were previously separate conversions into a unified process, reducing the total number of conversions from 2m to m+1 while maintaining noise reduction through correlated double sampling
Solution Approach 2:
The patent makes the signal value serve multiple functions: it serves as both the measured output for one photodiode and as the reset reference value for the next photodiode. This multi-functionality eliminates redundant operations and reduces power consumption while preserving the noise cancellation capability
2Use of energy by moving object
If the reset conversion is removed to reduce power consumption, then dynamic power decreases, but read noise increases intolerably
Solution Approach 1:
The patent performs preliminary sampling of the floating diffusion node value before charge transfer, storing it as the reset value. This preliminary action enables subsequent signal measurements to be referenced against a pre-captured baseline, allowing noise cancellation without requiring a separate reset conversion for each photodiode
Solution Approach 2:
The patent introduces an intermediary mechanism where the signal value from one photodiode acts as the reset reference for the next photodiode. This intermediary approach maintains the correlated double sampling functionality while reducing the total number of conversions required
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 significantly reduces power consumption by half for shared pixel circuits, making it suitable for low-power image sensor applications without compromising noise reduction.
Implementation Method 1
Each image pixel in the array includes a photodiode that is coupled to a floating diffusion region via a transfer gate
Data Source
AI summary
An image sensor may include a shared pixel circuit having multiple photodiodes coupled to a common floating diffusion node via respective charge transfer gates. First, the pixel circuit may be reset, and a sample-and-hold reset (SHR) value may be read out. Charge from a first of the photodiodes may be transferred to the floating diffusion node, and a first sample-and-hold signal (SHS) value may be read out. A first correlated double sampling (CDS) value is obtained by computing the difference between the SHR value and the first SHS value. Without resetting again, charge from a second of the photodiodes may be transferred to the floating diffusion node, and a second SHS value may be read out. A second CDS value is obtained by computing the difference between the first and second SHS values. Reading out the shared pixel circuit in this way substantially reduces power consumption.