Analog Dithering Circuit for Vertical Fixed Pattern Noise

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Solution Overview

Problem

Existing image sensors face challenges in completely eliminating vertical fixed pattern noise, which causes unwanted vertical stripes and banding in images due to variations in column-centric circuits' responsivity.

Innovation Solution

An analog dithering circuit is introduced that adds a random offset voltage to the reference voltage used in the readout process, behaving as random noise to mitigate the vertical fixed pattern noise by comparing it with the image charge voltage during analog-to-digital conversion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If traditional readout circuits are used without dithering, then device complexity is low, but vertical fixed pattern noise appears causing stripes and banding

Engineering Contradiction:

Improvevertical fixed pattern noiseVSAvoidreadout circuit complexity

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

An analog dithering circuit is introduced as an intermediary component between the pixel array and readout circuits. This circuit adds random noise to the reference voltage input of the ramp generator, which masks the vertical fixed pattern noise caused by column-centric circuit variations without requiring fundamental changes to the existing readout architecture

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention modifies the reference voltage parameter by adding a random offset voltage component to it. This parameter change transforms the deterministic reference voltage into a stochastic one, which effectively randomizes the quantization error and eliminates the systematic vertical stripe patterns caused by column circuit mismatches

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If higher resolution is pursued, then image quality improves, but vertical fixed pattern noise becomes more prominent

Engineering Contradiction:

Improveimage resolutionVSAvoidvertical stripe visibility

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The invention converts the harmful vertical fixed pattern noise into beneficial random noise by adding analog dithering. The random offset voltage transforms systematic column variations into stochastic variations, which appear as natural image noise rather than artificial stripes, thereby improving perceived image quality at high resolutions

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Use of energy by moving object

If column-centric circuits are miniaturized to reduce power consumption, then power efficiency improves, but variations in responsivity increase causing more noise

Engineering Contradiction:

Improvepower consumptionVSAvoidcolumn circuit responsivity variations

Core Design Contradiction:

Use of energy by moving objectVSObject-affected harmful factors

Solution Approach 1:

The analog dithering circuit performs preliminary anti-action by pre-randomizing the reference voltage before it enters the comparison process. This preemptive randomization counteracts the harmful effects of column circuit responsivity variations, allowing miniaturized low-power circuits to operate without introducing visible vertical fixed pattern noise

Inventive Principle:

Principle #9Preliminary anti-action

Data Source

PatentUS9848152B1Analog dithering to reduce vertical fixed pattern noise in image sensors

Publication Date: 2017.12.19 OMNIVISION TECHNOLOGIES INC

AI summary

Apparatuses and methods for reducing vertical fixed pattern noise in imaging systems are disclosed herein. An example apparatus may include an analog dithering circuit coupled to randomly add an offset voltage to a first reference voltage in response to a random binary signal during an analog to digital conversion operation, and a ramp generator circuit coupled to receive the first reference voltage, and provide a second reference voltage in response, wherein the randomly added offset voltage to the first reference is also present in the second reference voltage.