Correlated Double Sampled Pixel Amplifier Noise Reduction
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Solution Overview
Problem
Conventional photodiode-based imagers face challenges in minimizing noise and offset variations, which affect the accuracy of light intensity representation due to parasitic capacitance and 1/f noise in photodiode junctions.
Innovation Solution
The implementation of a correlated double sampled (CDS) pixel architecture, which includes an image sensing device, an inverting amplifier, a feedback capacitor, and switches to control sample timing, effectively reduces noise and offset variations by correlating offset and signal voltages, thereby enhancing signal power gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional photodiode-based imager architecture is used, then device complexity is reduced, but noise and offset variations increase
Solution Approach 1:
The pixel architecture is segmented into distinct functional blocks: photodiode for charge generation, sense amplifier for signal amplification, and CDS circuit for noise reduction. This segmentation allows each component to be optimized independently, with the CDS circuit specifically targeting noise and offset variation reduction through correlated sampling of signal and reference levels.
Solution Approach 2:
The CDS circuit acts as an intermediary between the photodiode and output, introducing reference sampling and correlation processing to eliminate noise and offset variations. The reference signal serves as a mediator that captures the noise and offset components, which are then subtracted from the signal path to achieve noise reduction.
2Measurement precision
If CDS pixel architecture is implemented, then noise and offset variations are minimized, but device complexity increases
Solution Approach 1:
The CDS functionality is merged directly into the pixel structure, combining the photodiode, sense amplifier, and CDS circuit into a single integrated pixel unit. This merging eliminates the need for separate CDS circuits outside the pixel array, reducing overall system complexity while maintaining noise reduction benefits within each pixel.
Solution Approach 2:
The sense amplifier is designed to perform multiple functions: signal amplification, reference sampling, and CDS processing. This multi-functionality reduces the need for separate dedicated circuits for each function, thereby reducing device complexity while achieving noise minimization through correlated double sampling.
3Measurement precision
If parasitic capacitance in photodiode junction is present, then device complexity is reduced, but measurement precision deteriorates
Solution Approach 1:
The CDS circuit performs preliminary sampling of the reference level (including parasitic capacitance effects) before the actual signal measurement. By capturing the parasitic capacitance influence in the reference sample and subtracting it during correlation processing, the harmful effects of parasitic capacitance are eliminated from the final measurement.
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 improves the accuracy of light intensity representation by minimizing noise and offset variations, leading to increased signal power gain and improved image sensing performance.
Implementation Method 1
When a photon 13 hits the PD 12, it is converted into some number of electrons, each with a charge of one electron volt.
Implementation Method 2
The first capacitor is configured as a feedback to the inverting input of the inverting amplifier, where the first capacitor is configured as a switching capacitor and is configured to integrate an image signal received by the image sensing device.
Implementation Method 3
The PN junction (also known in the art as a p-n junction) of the photodiode (PD) has a parasitic capacitance, which varies with the reverse bias voltage across the PD.
Data Source
AI summary
A correlated double sampled (CDS) pixel is provided. The CDS pixel comprises an image sensing device, an inverting amplifier, a capacitor, and first and second switches. The image sensing device generates charge based on image content. The inverting amplifier is in operable communication with the image sensing device. The capacitor is configured as a feedback to the inverting amplifier, wherein the first capacitor configured as a switching capacitor and configured to integrate an image signal received by the image sensing device. The first switch is in operable communication with the inverting amplifier and is configured to control sample timing of a correlated offset signal. The second switch is in operable communication with the image sensing device and is configured to control sample timing of the image signal.