SAR ADC Readout for 1.5-Conversion Digital CDS Pixels
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Solution Overview
Problem
Current CMOS image sensors require two analog-to-digital conversions per pixel for Digital Correlated Double Sampling (DCDS), leading to increased energy consumption and complexity in signal processing.
Innovation Solution
A method that reduces the number of analog-to-digital conversions to 1.5 per pixel by directly generating the DCDS result from the analog-to-digital conversion process, eliminating the need for arithmetic subtraction between signal and reset conversions, using a two-pixel-column wide SAR ADC with differential inputs and charge redistribution DACs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two analog-to-digital conversions are performed per pixel for Digital Correlated Double Sampling, then noise reduction and image quality are improved, but energy consumption and device complexity increase
Solution Approach 1:
The patent combines the reset conversion and signal conversion operations into a unified ADC process. By using a two-pixel-column wide SAR ADC with differential inputs, the system performs correlated double sampling with only 1.5 conversions per pixel instead of two, merging the conversion operations to reduce energy consumption while maintaining noise reduction capability.
Solution Approach 2:
The ADC structure is designed to handle multiple functions: it performs both reset sampling and signal sampling, and simultaneously executes correlated double sampling operations. The differential input architecture allows the same ADC resource to serve multiple pixel columns, increasing utilization efficiency and reducing per-pixel energy consumption.
2Measurement precision
If two analog-to-digital conversions are performed per pixel for Digital Correlated Double Sampling, then noise reduction is achieved, but the complexity of signal processing increases
Solution Approach 1:
The patent merges the CDS subtraction operation with the ADC conversion process itself. The differential input SAR ADC directly computes the difference between signal and reset values during conversion, eliminating the need for separate arithmetic subtraction circuits and reducing signal processing complexity while maintaining noise reduction.
3Use of energy by moving object
If 1.5 analog-to-digital conversions per pixel are used, then energy consumption and complexity are reduced, but the requirement for differential ADC architecture increases
Solution Approach 1:
The patent segments the ADC functionality into two independent one-pixel-column wide SAR ADCs that operate in parallel with differential inputs. This segmentation allows each ADC unit to handle half the workload while sharing common resources, reducing per-pixel energy consumption without requiring a completely new complex architecture.
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 energy consumption per pixel and simplifies signal processing, while maintaining image quality by directly providing the DCDS result without additional arithmetic operations, thereby improving power efficiency and reducing noise in image data.
Implementation Method 1
two-pixel-column wide SAR ADC with differential inputs
Implementation Method 2
charge redistribution DACs
Implementation Method 3
array of pixels which are photosensitive to incident light from a scene
Data Source
AI summary
A CMOS image sensor comprises an array of pixels. A column of the pixel array is coupled to a readout column. The readout column is couple to a readout circuitry (RC) that reads out image data from the pixel array. The RC comprises a sampling switch which is coupled to a 1-column successive approximation register (SAR) analog-to-digital converter (ADC). The 1-column SAR ADC comprises a differential comparator, a local SAR control, and a digital-to-analog converter (DAC). The sampling switch is coupled between a readout column and a non-inverting input of the differential comparator. An image readout method reads one pixel with two conversions through the RC. The RC is operated by the local SAR control to set the DAC based on comparator output, and upon which a reset digital value is obtained and stored. An overall reduced algorithm calculation is achieved herein.