Column Parallel A/D Conversion Circuit: Principles, Architecture, and Design Optimization
Overview
In modern digital imaging systems, image sensor chips serve as the core components responsible for converting optical signals into processable digital signals. With the continuous expansion of high-resolution and high frame rate imaging applications, such as smartphone cameras, autonomous driving vision systems, medical imaging equipment, etc., the performance requirements for image sensors are becoming increasingly stringent. Among them, the analog-to-digital converter (A/D or ADC) serves as a "bridge" connecting analog pixel output and digital image processing, and its performance directly determines image quality, dynamic range, power consumption, and system bandwidth. To meet the requirements of high speed, low noise, and low power consumption, column parallel A/D conversion circuit (Column Parallel ADC) has become a key technology in the design of mainstream CMOS image sensor (CIS) chips. This article will systematically explain the working principle, typical architecture, key technical challenges, and design optimization strategies of column parallel A/D conversion circuits. Combined with the current development of patent technology (such as CN114189637A), it will explore their innovative applications in image sensors and further analyze their technological evolution direction and potential breakthroughs.
The basic principle of column parallel A/D conversion circuit
Typical Architecture and Technology Selection
Key technical challenges and solutions
Innovative Design Trends and Future Directions
Architecture type | feature | Applicable scenarios | technical details |
Single Slope ADC | The structure is simple, the area is small, and the power consumption is low, but the conversion speed is limited by the slope of the slope | Medium low speed, high-resolution sensor | By comparing the ramp voltage with the input signal and counting the number of comparisons, a digital code is obtained; Suitable for low-power application scenarios. |
Sequential approximation register ADC (SAR ADC) | Fast conversion speed, moderate power consumption, suitable for medium to high precision conversion | High speed, medium high resolution applications | By gradually approximating the input signal through binary division, one bit is determined for each clock cycle, which is faster than single slope, but the circuit complexity is slightly higher. |
Two Step ADC | High speed and precision, reducing the number of comparators through coarse and fine tuning | High end imaging, scientific grade sensors | Two step conversion: coarse quantization (such as high 4 bits) and fine quantization (low N bits), reducing the number of comparators required for high resolution. |
ΔΣ ADC | High resolution, low noise, but limited bandwidth | Low speed and high dynamic range imaging | Improved resolution through oversampling and noise shaping techniques, suitable for noise sensitive applications such as astronomical observations. |
Column parallel A/D conversion circuit is the core supporting technology of modern high-performance image sensor chips, and its design directly determines the speed, dynamic range, noise performance, and power consumption performance of the imaging system. By reasonably selecting ADC architecture (such as hybrid SAR and single slope), optimizing conversion algorithms (such as segmented approximation and oversampling), implementing inter column calibration and power management, high-precision, high-speed, and low-noise image acquisition can be achieved under limited chip area and power budget.
