Delta-Sigma ADC Offset Removal for Image Sensor CDS Signals
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Solution Overview
Problem
Conventional analog-digital converters (ADCs) used in image sensors struggle to effectively remove offset voltage generated during correlated double sampling (CDS) from amplifiers, which affects the accuracy of image signal processing.
Innovation Solution
An ADC configuration that includes a CDS circuit to generate correlated double sampled reset and image signals, and a delta-sigma (ΔΣ) ADC to output a difference between digital codes generated from these signals, thereby isolating and removing the offset component from the amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ADC is used for analog-digital conversion in image sensors, then the device complexity is reduced, but the measurement precision deteriorates due to inability to effectively remove offset voltage from amplifier
Solution Approach 1:
The ADC operation is segmented into distinct phases: reset phase for capturing offset voltage and signal phase for capturing image signal. This temporal segmentation allows separate measurement of offset and signal components, improving measurement precision without requiring additional hardware complexity
Solution Approach 2:
The reset phase performs preliminary action by capturing the offset voltage before the actual image signal conversion. This preliminary measurement of offset voltage enables its subsequent subtraction from the signal phase measurement, improving precision while using the same ADC hardware
2Measurement precision
If offset voltage removal is implemented in conventional ADC, then the measurement precision is improved, but the device complexity increases due to additional circuit requirements
Solution Approach 1:
The single ADC is designed to perform multiple functions: offset voltage measurement during reset phase and image signal conversion during signal phase. This multi-functionality enables offset removal without requiring separate dedicated circuits, maintaining device simplicity while improving precision
Solution Approach 2:
The ADC conversion parameters are dynamically changed between phases: during reset phase it converts offset voltage, during signal phase it converts image signal. This parameter change approach allows the same hardware to handle different measurement tasks, avoiding additional circuit complexity
3Measurement precision
If dual-phase conversion is used to remove offset, then the measurement precision is improved, but the productivity decreases due to extended conversion time
Solution Approach 1:
The ADC operates in periodic alternating phases between reset phase and signal phase conversions. This periodic action structure efficiently utilizes the ADC resource by continuously switching between offset measurement and signal conversion, improving precision while maintaining throughput through systematic time-multiplexing
Data Source
AI summary
An analog-digital converter (ADC) includes a correlated double sampling (CDS) circuit configured to perform CDS on each of a reset signal and an image signal output from a pixel to generate a correlated double sampled reset signal and a correlated double sampled image signal, respectively. A delta sigma (ΔΣ) ADC, also included in the ADC, is configured to output a difference between a first digital code that is generated by performing ΔΣ analog-digital conversion on the correlated double sampled reset signal and a second digital code that is generated by performing ΔΣ analog-digital conversion on the correlated double sampled image signal.