Imaging Pixel Circuit Charge Generation Mode Noise Reduction
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Solution Overview
Problem
Conventional imagers using photodiodes in charge depletion mode face issues such as shot noise, dynamic range limitations, non-linearity due to changing photodiode responsivity with bias voltage, and kTC noise from bias restoration, which affect image quality and signal accuracy.
Innovation Solution
The implementation of pixel circuits that operate in a 'charge generation' mode with a constant bias voltage across the photodiode, using a charge-to-voltage converter to accumulate and convert charges into output voltage, reducing leakage current and eliminating the need for bias restoration, thereby maintaining linearity and minimizing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photodiodes are operated in charge depletion mode with reverse bias, then light sensitivity and signal generation are improved, but photodiode leakage current depletes stored reverse bias causing shot noise and dynamic range limits
Solution Approach 1:
The patent changes the operating mode from charge depletion to charge generation by applying constant voltage bias, fundamentally altering the photodiode's electrical characteristics to eliminate leakage current effects while maintaining light sensitivity
Solution Approach 2:
The patent uses a separate capacitor to store and represent the photodiode signal charge, decoupling the signal storage from the photodiode's bias voltage, thereby preventing leakage current from affecting the stored signal
2Quantity of substance
If photodiode reverse bias is used for signal accumulation, then signal charge is generated, but photodiode responsivity changes with bias voltage causing non-linearity
Solution Approach 1:
The patent maintains constant bias voltage across the photodiode during signal accumulation, preventing responsivity changes and ensuring linear signal generation throughout the integration period
Solution Approach 2:
The patent transfers the signal charge to a separate capacitor that maintains a fixed voltage-charge relationship, ensuring linear signal representation independent of photodiode bias variations
3Stability of the object's composition
If bias restoration action is performed on photodiode capacitance, then reverse bias is restored to pre-exposure level, but kTC noise (reset noise) is introduced
Solution Approach 1:
The patent performs reset operation on the signal capacitor rather than the photodiode capacitance, copying the reset function to a component where it does not generate harmful noise in the signal path
Solution Approach 2:
The patent extracts the signal charge from the photodiode to a separate capacitor, separating the reset operation from the photodiode and eliminating the source of kTC noise generation
4Measurement precision
If accumulated signal charge is represented by voltage across photodiode capacitance, then signal is read out, but capacitance being a function of voltage introduces non-linearity
Solution Approach 1:
The patent copies the signal charge to a separate capacitor with fixed capacitance value, decoupling the signal representation from the voltage-dependent photodiode capacitance and ensuring linear voltage-charge conversion
Solution Approach 2:
The patent extracts the signal charge from the photodiode capacitance to a separate readout capacitor, removing the source of non-linear capacitance variation from the signal path
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 reduces shot noise, extends operating time, and maintains signal linearity, improving the overall image quality and reducing power consumption by minimizing kTC noise and non-linearity issues.
Implementation Method 1
Conventional imagers use photodiodes that are light sensitive electronic elements which convert incident light to either current or voltage
Data Source
AI summary
An example imaging system includes a digital conversion circuit and a plurality of pixel circuits each having a photodiode, a biasing circuit, a charge-to-voltage converter, and a switch. The photodiode is configured to generate charges in response to light or radiation. The biasing circuit includes an operational amplifier having an input signal port for receiving a bias reference signal which controls a bias current flowing through an internal circuit of the operational amplifier. The charge-to-voltage converter is configured to accumulate the charges drained by the biasing circuit and convert the accumulated charges into a corresponding output voltage. The switch configured to selectively couple the charge-to-voltage converter to at least one data line. The digital conversion circuit is configured to generate a digital correlated signal sample for each pixel circuit using a difference between a digital signal sample and a digital reset level sample.