Image Sensor Comparator Linearity Test Using Shared Ramp Signals
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Solution Overview
Problem
Existing image sensors face challenges in accurately measuring the linearity of comparators and reducing variation between ramp signals, which affects the quality of analog-to-digital conversion and image data generation.
Innovation Solution
An image sensor configuration that includes a pixel array, a ramp signal generator, a signal selector, and a comparison circuit, which alternately outputs ramp signals and pixel signals to measure linearity and compensate for signal deviations, using internal ramp signal generators to adjust slope and voltage levels for precise comparison.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ADC circuits are used without dedicated test functionality, then device complexity is reduced, but measurement precision of comparator linearity cannot be achieved
Solution Approach 1:
The comparison circuit is designed to perform dual functions: normal analog-to-digital conversion during imaging mode and comparator linearity measurement during test mode. The signal selector enables the same hardware resources to be shared between these two functions, eliminating the need for separate dedicated measurement circuits and thus resolving the contradiction between measurement precision and device complexity.
Solution Approach 2:
The test signal is designed to apply predetermined voltage levels to the comparison circuit before actual imaging operations. By performing preliminary measurements of comparator linearity using the same hardware, the system establishes baseline characteristics without requiring additional measurement equipment, thereby achieving high measurement precision while maintaining simple device architecture.
2Measurement precision
If multiple ramp signal generators are used to improve measurement accuracy, then measurement precision improves, but device complexity increases
Solution Approach 1:
Instead of implementing multiple independent ramp signal generators, the invention creates a copied version of the ramp signal by applying predetermined voltage levels to the output of a single ramp signal generator. This copying approach through voltage level manipulation achieves the same measurement objective as multiple generators would provide, while maintaining simple device structure and avoiding the complexity of multiple signal generation circuits.
3Measurement precision
If dedicated test circuits are added for linearity measurement, then measurement precision improves, but manufacturing precision requirements increase
Solution Approach 1:
The comparison circuit measures its own characteristics by comparing internal signals against predetermined reference levels. This self-service measurement approach uses the existing circuit components to evaluate their own performance, eliminating the need for external precision measurement equipment and reducing the impact of manufacturing variations on measurement accuracy.
Solution Approach 2:
The invention changes the operating parameters of the comparison circuit by applying different predetermined voltage levels during test mode versus normal operation. By manipulating voltage levels rather than adding physical measurement infrastructure, the system achieves high measurement precision while being tolerant of manufacturing variations in the circuit fabrication process.
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 configuration enables accurate measurement of comparator linearity and reduction of signal variations, leading to improved image data quality by compensating for deviations and ensuring consistent ramp signal performance.
Implementation Method 1
each pixel including a photodiode PD
Data Source
AI summary
An image sensor is provided. The image sensor according to an example embodiment includes a pixel array including a plurality of pixels, a ramp signal generator configured to generate a ramp signal that increases or decreases with a constant slope, a signal selector configured to output the ramp signal and one of a pixel signal and a test signal output from the pixel array, and a comparison circuit configured to receive the ramp signal and the test signal from the signal selector during a first period, output a first comparison result of the ramp signal and the test signal, receive the pixel signal and the ramp signal from the signal selector during a second period after the first period, and output a second comparison result of the pixel signal and the ramp signal.