Charge-Transfer Pixel DAC for Low-Noise Image Sensor Conversion

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Solution Overview

Problem

Existing digital to analog converters (DACs) in image sensors suffer from high noise due to inner resistivity and capacitance, leading to increased current consumption and significant chip area occupation.

Innovation Solution

Implementing DACs using imaging pixels within the pixel array, where each pixel includes a transfer gate coupling a memory node and a capacitive sensing node, with a control circuit to store and transfer electrical charge to generate an output voltage, reducing noise and current consumption.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional DAC architectures are used, then conversion function is achieved, but noise increases due to inner resistivity and capacitance

Engineering Contradiction:

Improvenoise performanceVSAvoidinner resistivity and capacitance noise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent replaces conventional voltage-based DAC operation with a charge-based operation system. Memory nodes store electrical charge instead of voltage, and transfer gates move charge packets to the sensing node. This charge-domain operation eliminates the noise generated by inner resistivity and capacitance inherent in voltage-based DACs, achieving superior noise performance while maintaining the DAC conversion function.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Reliability

If noise reduction measures are implemented in known DAC architectures, then noise decreases, but current consumption increases

Engineering Contradiction:

Improvenoise performanceVSAvoidcurrent consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent employs charge-based operation where memory nodes store and transfer discrete charge packets through transfer gates controlled by digital input signals. This approach achieves noise reduction without requiring additional current-consuming noise filtering circuits or low-noise amplifiers, thereby maintaining low current consumption while improving noise performance.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Productivity

If conventional DAC solutions are implemented, then conversion function is provided, but chip area occupation increases significantly

Engineering Contradiction:

ImproveDAC conversion capabilityVSAvoidchip area

Core Design Contradiction:

ProductivityVSArea of stationary object

Solution Approach 1:

The patent implements a multi-functional pixel structure that serves both as an imaging pixel and as a DAC unit. The same pixel components (memory node, transfer gate, sensing node) are used for both photodetection and digital-to-analog conversion functions. This eliminates the need for separate dedicated DAC circuitry, significantly reducing chip area occupation while maintaining full DAC conversion capability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Measurement precision

If charge transfer is controlled by transfer gates, then precise voltage generation is achieved, but device complexity increases

Engineering Contradiction:

Improveoutput voltage precisionVSAvoidtransfer gate control circuitry

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent uses identical pixel structures and transfer gate mechanisms for both imaging and DAC functions. The transfer gates are controlled by digital input signals that directly modulate the charge transfer, leveraging the inherent precision of the pixel's charge-to-voltage conversion. This homogeneous design achieves precise output voltage generation without requiring complex dedicated control circuitry, as the pixel itself provides the precision mechanism.

Inventive Principle:

Principle #33Homogeneity

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

The proposed DAC design achieves low noise and current consumption while maintaining a compact chip area, improving differential linearity and power supply rejection ratio, and allowing for efficient voltage ramp generation.

Implementation Method 1

controlling, based on a digital input signal, the first transfer gates of the first and second pixels to transfer the charge stored at the memory nodes to the sensing node

Methodology Applied

Scientific EffectCharge transfer: Electrical Accumulator

Implementation Method 2

storing electrical charge at each of the memory nodes by activating the reset transistor to apply a reference voltage to the memory node of each of the first and second pixels

Methodology Applied

Scientific EffectCapacitive charging: Capacitance

Data Source

PatentUS12531571B2Digital to analog converter

Publication Date: 2026.01.20 STMICROELECTRONICS (CROLLES 2) SAS

AI summary

The present disclosure relates to a DAC that includes: a first pixel including a first transfer gate coupling a memory node of the first pixel and a capacitive sensing node (SN); a second pixel comprising a first transfer gate coupling a memory node of the second pixel and the capacitive SN; a reset transistor coupling the sensing node to a first voltage supply rail; and a control circuit configured to store electrical charge by activating the reset transistor to apply a reference voltage to the memory node of each of the first and second pixels; and generate a voltage of the DAC at the sensing node by deactivating the reset transistor and controlling the first transfer gates of the first and second pixels to transfer the charge stored.