Multiple-Slope Column ADC Calibration for Faster Image Sensor Readout

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Solution Overview

Problem

Single-ramp single-slope (SRSS) column parallel analog-to-digital conversion in image sensors results in relatively slow conversion times, especially at high resolutions, which is a drawback in applications requiring fast processing speeds.

Innovation Solution

Implementing a multiple slope voltage ramp (MS) approach with calibration circuitry to adjust digital image data, using a resistive ladder and voltage sources to generate calibration voltages, and comparing outputs from MS and SS voltage ramps to account for non-idealities and improve image quality.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If single-ramp single-slope (SRSS) column parallel A/D conversion is used, then the architecture is simple and reliable, but the conversion time is slow especially at high resolutions

Engineering Contradiction:

Improveconversion reliabilityVSAvoidconversion time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The voltage ramp is segmented into multiple slopes rather than using a single ramp. The A/D conversion process is divided into multiple phases with different ramp slopes, allowing faster conversion while maintaining accuracy through calibration.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The ramp voltage parameters are changed by implementing multiple slopes with different rates. The calibration process adjusts digital values to account for non-idealities introduced by the multiple slope approach, resolving the contradiction between speed and accuracy.

Inventive Principle:

Principle #35Parameter changes

2Productivity

If multiple slope voltage ramp (MS) approach is implemented to reduce conversion time, then readout speed increases, but non-idealities and noise may increase

Engineering Contradiction:

Improvereadout speedVSAvoiddigital value accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

A calibration process is implemented that measures the actual behavior of the multiple slope ramp and uses this feedback to adjust digital values. This compensates for non-idealities and ensures accuracy matches single-slope conversions.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The multiple slope ramp introduces intentional asymmetry in the voltage progression, with different slopes for different portions of the conversion range. Calibration corrects for this asymmetry to maintain measurement precision.

Inventive Principle:

Principle #4Asymmetry

Data Source

PatentUS8941527B2Calibration in multiple slope column parallel analog-to-digital conversion for image sensors

Publication Date: 2015.01.27 OMNIVISION TECHNOLOGIES INC

AI summary

A method of an aspect includes acquiring analog image data with a pixel array, and reading out the analog image data from the pixel array. The analog image data is converted to digital image data by performing an analog-to-digital (A/D) conversion using a multiple slope voltage ramp. At least some of the digital image data is adjusted with calibration data. Other methods, apparatus, and systems, are also disclosed.