Floating Diffusion Capacitance Measurement Circuit
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Solution Overview
Problem
CMOS image sensors face challenges in accurately determining the capacitance of the floating diffusion area, which affects conversion gain, due to sensitivity to process variations, driving voltage, and temperature changes.
Innovation Solution
A circuit and method for reading out voltage variations in the floating diffusion area using a reference capacitor and a read-out block with transistors to initialize and output electrical signals corresponding to initial and varied voltages, allowing for capacitance calculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance measurement methods are used, then the measurement process is simple, but the measurement precision is low due to sensitivity to process variation, driving voltage, and temperature
Solution Approach 1:
The patent applies preliminary action by initializing the floating diffusion area to a known voltage state before measurement. The reset transistor sets the initial voltage condition, ensuring that subsequent voltage variations are solely due to charge transfer and not residual charges from previous operations. This preliminary initialization step enables accurate capacitance measurement by eliminating uncertainty from prior states.
Solution Approach 2:
The patent introduces a reference capacitor as an intermediary element to transfer a known charge amount to the floating diffusion area. This reference capacitor serves as a mediator between the control circuit and the floating diffusion area, providing a standardized charge transfer mechanism that enables precise capacitance calculation through voltage variation measurement.
2Measurement precision
If the floating diffusion area capacitance is not accurately determined, then the circuit design is simpler, but the conversion gain accuracy deteriorates
Solution Approach 1:
The patent replaces direct physical capacitance measurement with an electrical measurement approach. Instead of using mechanical or direct electrical probes to measure capacitance, the system uses voltage variation measurement combined with known charge transfer from a reference capacitor to calculate capacitance. This substitution enables accurate capacitance determination through electrical signals that can be precisely controlled and measured using standard CMOS circuitry.
Solution Approach 2:
The patent implements feedback by measuring the voltage variation in the floating diffusion area after charge transfer and using this information to calculate the capacitance value. The measured voltage variation feeds back into the conversion gain calculation, allowing the system to compensate for process variations and ensure accurate conversion gain despite manufacturing tolerances.
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
Enables evaluation of product quality and compensation for process, voltage, and temperature variations, improving the accuracy of capacitance determination and conversion gain in CMOS image sensors.
Implementation Method 1
a reference capacitor which is suitable for causing a voltage variation of the floating diffusion area based on a charge transfer signal
Data Source
AI summary
A circuit for reading-out a voltage variation of a floating diffusion area includes a reference capacitor suitable for causing a voltage variation of the floating diffusion area based on a charge transfer signal; and a read-out block including the floating diffusion area, and suitable for initializing the floating diffusion area, performing a read-out on an initial voltage of the floating diffusion area and performing a read-out on a varied voltage of the floating diffusion area caused by the reference capacitor.