CMOS Image Sensor Offset Cancellation for High-Speed Photon Detection
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Solution Overview
Problem
Existing CMOS image sensors face challenges in detecting extremely small pixel signals, such as those encountered during low-illuminance photon counting, due to high offset noise levels that require extensive reference signal sweeping, leading to reduced frame rates and increased noise.
Innovation Solution
The implementation of a CMOS image sensor with a sense circuit section that includes a comparator for each pixel, where an independent offset bias is applied to cancel out comparator offsets, allowing for digital decision-making by comparing a step-changing reference signal with the pixel signal, thereby reducing noise and increasing detection speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional comparator is used without offset cancellation, then the circuit is simple and fast, but the offset noise dominates and prevents detection of extremely small pixel signals
Solution Approach 1:
The comparator circuit is segmented into multiple independent components: a first comparator for offset cancellation, a second comparator for signal detection, and associated circuitry. This segmentation allows each component to be optimized for its specific function, enabling precise detection of small signals while maintaining manageable circuit complexity through functional decomposition.
Solution Approach 2:
The first comparator performs preliminary offset cancellation before the main signal detection occurs. By pre-subtracting the offset component from the pixel signal, the system prepares the signal in advance for accurate detection by the second comparator, enabling detection of extremely small signals that would otherwise be buried in offset noise.
2Measurement precision
If a lengthy reference signal sweep is performed to cancel out comparator offsets, then offset cancellation is achieved, but detection speed decreases and frame rates are reduced
Solution Approach 1:
The mechanical sweeping process is replaced with an electrical subtraction method. Instead of mechanically sweeping through reference signals to find the offset cancellation point, the system electrically subtracts the offset component measured by the first comparator from the pixel signal, enabling rapid offset cancellation without time-consuming sweeps and maintaining high detection speed.
3Measurement precision
If the aperture ratio of pixels is increased to improve light gathering, then more photons are detected, but the sense circuit and counter circuit occupy more area
Solution Approach 1:
The sense circuit and counter circuit are moved from the same plane as the pixel array to a different dimension or layer. This spatial separation allows the pixel aperture to be maximized for light gathering while the supporting circuits are arranged in a separate area, enabling both large aperture ratios and functional completeness without proportionally increasing total chip 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 enables high-precision detection of extremely small signals with reduced noise and increased frame rates, enhancing the performance of image pickup devices for high-performance shooting.
Implementation Method 1
Each pixel in a CMOS image sensor converts incident light into electrons by a photodiode as a photoelectric conversion device
Data Source
AI summary
Provided are an image pickup device and a camera system that are capable of detecting an extremely small signal from a pixel or one photon signal with low noise and high precision at high speed, and are capable of performing various kinds of high-performance shooting by increasing a frame rate with use of this. Each sense circuit includes a comparator configured to compare an output signal from a pixel with a reference signal, and when signal detection is performed, a charge allowing a first pixel signal output from a selected pixel to be cancelled out is held in one or both of input sections of the comparator, an independent offset bias for each comparator is applied to one of the input sections of the comparator to cancel out an offset of the comparator, and a digital decision on intensity of light incident on the pixel is performed by comparing a reference signal changing in steps with a second pixel signal output from the selected pixel.