CMOS Pixel Analog Digital Readout Dynamic Range
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Solution Overview
Problem
CMOS imaging sensors face limitations in achieving high dynamic range within a single scene, as existing pixel designs either compromise low-light performance or suffer from reduced signal-to-noise ratio and increased noise levels when attempting to enhance dynamic range.
Innovation Solution
A CMOS pixel design that combines analog and digital readouts, utilizing a comparator to test accumulated charge at exponentially increasing exposure times, with a one-bit digital output indicating when the charge exceeds a threshold, allowing for analog readout at the end of the integration period, thereby increasing dynamic range without in-pixel A/D conversion and maintaining low-light sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If in-pixel analog-to-digital conversion is implemented to increase dynamic range, then dynamic range is improved, but signal-to-noise ratio deteriorates due to quantization noise
Solution Approach 1:
The patent divides the readout process into multiple exponential time intervals, with each interval producing one bit of digital output. This segmentation avoids single-step analog-to-digital conversion and its associated quantization noise, while still achieving extended dynamic range through multi-bit sequential readout.
Solution Approach 2:
The patent performs preliminary integration of photocurrent during defined exposure intervals before readout. By accumulating charge in advance during multiple time intervals and then reading out sequentially, the system captures dynamic range information without requiring in-pixel analog-to-digital conversion that would introduce quantization noise.
2Measurement precision
If exposure time is extended to capture dark objects, then low-light sensitivity is improved, but saturation occurs for bright objects
Solution Approach 1:
The patent implements dynamic exposure timing where different regions or pixels can have different integration time intervals based on local illumination conditions. Bright regions use shorter intervals to avoid saturation while dark regions utilize full extended intervals, allowing simultaneous capture of both bright and dark objects without compromise.
Solution Approach 2:
The patent uses periodic sampling at multiple exponential time intervals to read out pixel data. This periodic readout at different exposure times allows the system to capture both early-time (bright object) and late-time (dark object) information, effectively handling the dynamic range challenge without extending single exposure time indefinitely.
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 enhances the dynamic range of the pixel while maintaining low-light performance, reducing noise and power dissipation, and avoiding the limitations of previous techniques that relied on precision comparators and in-pixel A/D conversion.
Implementation Method 1
Each pixel contains a photo-detector plus control and multiplexing circuitry. Each pixel generates an output signal that is proportional to the accumulated radiation incident on the photo-detector during a defined integration period.
Implementation Method 2
The pixel comprises a photo-detector, an integration capacitor Cint, a source follower device M1, a pre-charge device M2, and a row-select device M3.
Data Source
AI summary
An improved CMOS pixel with a combination of analog and digital readouts to provide a large pixel dynamic range without compromising low-light performance using a comparator to test the value of an accumulated charge at a series of exponentially increasing exposure times. The test is used to stop the integration of photocurrent once the accumulated analog voltage has reached a predetermined threshold. A one-bit output value of the test is read out of the pixel (digitally) at each of the exponentially increasing exposure periods. At the end of the integration period, the analog value stored on the integration capacitor is read out using conventional CMOS active pixel readout circuits.