ADC Driver Output Stage With Fast Feedback for Transient Settling

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Solution Overview

Problem

Existing drivers for analog-to-digital converters (ADCs) face challenges in responding to output transients caused by in-rush currents, limiting the minimum sampling time and signal-to-noise ratio due to their limited bandwidth and inability to operate at high closed loop gains.

Innovation Solution

A driver with a fast feedback loop in the output stage that compensates for output transients before the overall feedback loop reacts, allowing for high closed loop gains and improved signal-to-noise ratio without distorting the analog signal, featuring a high-bandwidth output stage with unity gain and a compensation capacitor isolated from the driver output.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If the driver operates in high closed loop gain to improve signal to noise ratio, then the signal to noise ratio improves, but the bandwidth and ability to respond to transients decreases

Engineering Contradiction:

Improvesignal to noise ratioVSAvoidbandwidth

Core Design Contradiction:

Measurement precisionVSSpeed

Solution Approach 1:

The feedback system is divided into two separate loops: an overall feedback loop for maintaining accuracy and a fast feedback loop in the output stage for responding to transients. This segmentation allows each loop to be optimized independently - the overall loop can operate at high gain for noise reduction while the fast loop provides high bandwidth for transient response.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The output stage acts as an intermediary between the high-gain amplifier and the ADC capacitive load. It includes a fast feedback loop with minimal components that quickly corrects transients before they affect the ADC, while the overall feedback loop maintains accuracy. This intermediary structure resolves the conflict between high gain and high bandwidth requirements.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Device complexity

If the driver uses a single feedback loop for maintaining accuracy and load drive, then the circuit complexity is reduced, but the response to output transients is insufficient

Engineering Contradiction:

Improvefeedback loop structureVSAvoidresponse to transients

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

3Productivity

If the sampling time is reduced to increase productivity, then the conversion speed increases, but the accuracy decreases due to signal distortion from in-rush current

Engineering Contradiction:

Improvesampling rateVSAvoidconversion accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The fast feedback loop in the output stage proactively and continuously corrects transients caused by in-rush current before they can significantly distort the sampled signal. This preliminary correction action allows the ADC to accurately capture voltage even at reduced sampling times, enabling higher sampling rates without sacrificing accuracy.

Inventive Principle:

Principle #10Preliminary action

4Reliability

If additional unity gain buffers are added after the gain stage to settle ADC transients, then the transient response improves, but the power consumption increases

Engineering Contradiction:

Improvetransient settlingVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The output stage is designed to perform multiple functions: it provides the fast transient response typically requiring separate buffer stages, while also maintaining the high-gain amplification function. By making the output stage multi-functional with a fast feedback loop, additional unity gain buffers become unnecessary, reducing power consumption while maintaining transient settling performance.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS8866553B2Output stage with fast feedback for driving ADC

Publication Date: 2014.10.21 ANALOG DEVICES INT UNLTD CO

AI summary

A driver for an analog-to-digital converter (ADC) has an overall feedback loop between its input and its output for maintaining overall accuracy, and a much faster feedback loop in its output stage that quickly compensates for output transients before the overall feedback loop can substantially react to the transients. Output voltage transients are created by the intermittent capacitive load of the ADC. The fast feedback loop can be made very fast since there are only a few components in the fast feedback path. The fast reduction of the output transients enables a shorter sampling time, leading to more accurate analog-to-digital conversion. The overall gain of the driver can be set to be greater than unity while still providing good output transient suppression.