CMOS Image Pickup Circuit Noise Cancellation via Equipotential Signals
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Solution Overview
Problem
CMOS sensors face issues with noise in images due to variations in CDS circuits, increased circuit area, and susceptibility to switching noise, particularly from power supply voltage fluctuations.
Innovation Solution
An image pickup circuit with amplifying, ramp signal generating, and comparing means, where the reference potentials of the pixel and ramp signals are at the same level, allowing for noise reduction by canceling out noise sources common to both signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If CDS circuits are used to process image signals, then noise removal capability is improved, but stripe-shaped fixed pattern noise occurs due to variations in CDS circuits across pixel columns
Solution Approach 1:
The patent uses a capacitor to store a copy of the reset signal level, which is then subtracted from the pixel signal to remove noise. This copying approach avoids the variations inherent in active CDS circuits while maintaining noise removal capability.
Solution Approach 2:
The patent replaces the mechanical/electronic CDS circuit with a simpler capacitive storage and subtraction mechanism. Instead of using active CDS circuitry that varies across pixel columns, the invention uses passive capacitors to store signal levels and perform subtraction through circuit topology.
2Measurement precision
If CDS circuits are implemented in each pixel column, then noise processing is improved, but circuit area increases due to capacitive elements required for signal retention
Solution Approach 1:
The patent stores a copy of the reset signal level in a capacitor rather than using complex CDS circuitry. This capacitive copying approach significantly reduces the circuit area while maintaining the ability to perform noise removal through signal subtraction.
3Speed
If rapid horizontal scanning of analog signals is performed by shift register, then processing speed is improved, but susceptibility to switching noise increases
Solution Approach 1:
The patent replaces the shift register-based rapid scanning mechanism with a parallel readout architecture where pixel signals are read out simultaneously through word lines and bit lines. This eliminates the rapid switching operations that generate switching noise while maintaining processing speed through parallelism.
4Ease of operation
If reference potentials of pixel signal and ramp signal are at different levels, then signal processing is simplified, but noise cancellation effectiveness is reduced
Solution Approach 1:
The patent ensures that the reference potentials of the pixel signal and ramp signal are at the same level by using a common reference potential line. This equipotential configuration enables effective noise cancellation through differential signaling while maintaining signal processing simplicity through the unified reference structure.
Data Source
AI summary
Disclosed herein is an image pickup circuit including: amplifying means for amplifying a charge corresponding to an amount of light received by a photodetector, and outputting a pixel signal; ramp signal generating means for generating a ramp signal whose voltage drops with a fixed slope from a predetermined initial voltage; and comparing means for comparing the pixel signal output by the amplifying means with the ramp signal output by the ramp signal generating means. A reference potential of the pixel signal output by the amplifying means and a reference potential of the ramp signal output by the ramp signal generating means are at a same level.