Circuit Techniques to Improve Low-Light Characteristics and High-Accuracy Evaluation System for CMOS Image Sensor

Area of Science:

• Image Sensors
• Integrated Circuits
• Analog-to-Digital Converters

Background:

• Surveillance cameras require optimized performance in low-light conditions.
• Simplifying Analog-to-Digital Converters (ADCs) can introduce RUSH current issues.
• Area reduction is a key design priority for modern imaging systems.

Purpose of the Study:

• To propose novel column-ADC techniques for enhancing low-light characteristics in surveillance cameras.
• To address and mitigate RUSH current issues arising from ADC comparator simplification.
• To develop a high-accuracy evaluation system for low-light optical and electrical signal measurements.

Main Methods:

• Implemented RUSH current compensation to reduce transient current fluctuations during AD conversion.
• Utilized timing shift ADCs to decrease the number of simultaneously operating ADCs, minimizing noise.
• Fabricated test chips using a 55 nm CIS process for experimental validation.

Main Results:

• Achieved a 63% reduction in low-light linearity degradation.
• Reduced column interference (RUSH current) by 50%.
• Demonstrated an inter-sample variation of 0.05 LSB for column interference using the high-accuracy evaluation system.

Conclusions:

• The proposed techniques effectively improve low-light performance in surveillance cameras by reducing operating noise.
• The developed ADC architecture offers a figure-of-merit (FoM) of 0.32 e-·pJ/step, proving its utility.
• The high-accuracy evaluation system validates the effectiveness of the proposed solutions for small signal measurements in low-light scenarios.