Modulated Pixel Imaging Array Noise Reduction
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Solution Overview
Problem
CMOS imaging arrays face challenges with high noise levels, particularly 1/f noise and common mode noise, which affect low-light sensitivity and increase costs, making it difficult to design smaller pixels that can operate effectively at low light levels without significant increases in cost or complexity.
Innovation Solution
The implementation of a modulated pixel design with a sense amplifier that includes high pass and low pass filters, mixers, and a differential amplifier to reduce 1/f noise and common mode noise, along with a common mode noise rejection pixel and a controller to manage pixel oscillator signals and reset gates, allowing for reduced noise without increasing the size of the imaging array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the size of photodiodes is increased to overcome readout circuitry noise at low light levels, then low-light sensitivity is improved, but the area of the imaging array and cost increase
Solution Approach 1:
The patent applies periodic modulation by switching the pixel oscillator between two frequencies (first and second frequencies) during different time intervals. This periodic frequency switching modulates the pixel signals, allowing them to be differentiated from the 1/f noise spectrum which is concentrated at low frequencies. The modulation technique enables noise reduction without requiring larger photodiode area.
2Object-affected harmful factors
If complex circuitry such as better regulated power supplies or power regulators is provided to reduce common mode noise, then common mode noise is reduced, but the cost of the imaging array increases
Solution Approach 1:
The patent extracts and separates the common mode noise component from the pixel signals by utilizing a differential amplifier configuration. The differential amplifier compares signals from adjacent pixels and extracts only the differential component, effectively rejecting common mode noise. This approach eliminates the need for complex power regulation circuitry while achieving common mode noise reduction.
3Reliability
If the size of photodiodes is increased to overcome readout circuitry noise, then noise performance is improved, but manufacturing cost increases
Solution Approach 1:
The patent changes the frequency parameter of the pixel signals by modulating them with a pixel oscillator that switches between two frequencies. This frequency modulation shifts the signal spectrum away from the 1/f noise region, improving noise performance without changing the physical dimensions of the photodiodes. Consequently, the imaging array area and manufacturing cost remain unchanged.
4Measurement precision
If larger pixels are designed to operate at low light levels, then low-light sensitivity is improved, but the minimum light intensity threshold increases
Solution Approach 1:
The patent employs periodic frequency modulation of the pixel signals using a pixel oscillator that alternates between two frequencies. This modulation technique shifts the signal spectrum to frequencies where 1/f noise is minimal, thereby reducing the noise floor and enabling detection of lower light intensities without requiring larger pixel areas.
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 1/f noise and common mode noise, improving low-light sensitivity while maintaining cost-effectiveness by not requiring significant increases in silicon area or additional circuitry, thus enabling more efficient operation at low light levels.
Implementation Method 1
Each pixel includes a photodiode and a readout circuit. The signal from the photodiode is proportional to the amount of light that is intercepted by the photodiode during the image exposure period.
Implementation Method 2
a light pixel mixer that mixes a signal on the light pixel node with a pixel oscillator signal having a first frequency
Implementation Method 3
The sense amplifier also includes a first high pass filter connected to the first input node and attenuating signals with frequencies less than a cutoff frequency
Implementation Method 4
The first mixed signal is input to a low pass filter that blocks signals having frequencies greater than a cutoff frequency
Implementation Method 5
The sense amplifier utilizes a differential amplifier to generate a signal indicative of a difference between the first and second mixed signals
Data Source
AI summary
An imaging array and a method for operating the same are disclosed. The imaging array includes a plurality of light pixels and a sense amplifier. Each light pixel includes a photodetector that generates and couples a signal indicative of a light exposure to a light pixel node, a readout circuit, and a mixer that mixes a signal on the light pixel node with a pixel oscillator signal. The sense amplifier includes an input node that receives a signal from each light pixel, one light pixel at a time. The sense amplifier also includes a high pass filter that attenuates signals with frequencies less than a cutoff frequency and a mixer that demodulates the signal from the filter to provide a signal that is related to the potential on the light pixel node of the light pixel connected to the first input node.