Bootstrapped ADC Anti-Alias Filter Bypass for Charge Injection Control

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Solution Overview

Problem

Dynamic anti-alias filters in analog-to-digital converters face challenges due to charge injection and capacitive coupling errors caused by the bypass switch, leading to differential signal errors and increased power consumption.

Innovation Solution

Incorporating a boot-strapped bypass switch into the dynamic anti-alias filter, which makes charge injection and capacitive coupling independent of the input signal, allowing for common-mode rejection and reducing settling time requirements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If a dynamic anti-alias filter with bypass switch is used, then noise aliasing rejection is improved, but charge injection and capacitive coupling errors increase

Engineering Contradiction:

Improvenoise aliasingVSAvoidsignal accuracy

Core Design Contradiction:

Object-affected harmful factorsVSMeasurement precision

Solution Approach 1:

The harmful charge injection and capacitive coupling effects are extracted and separated from the signal path by routing them through a dedicated cancellation path, allowing the main signal path to remain clean while the harmful effects are processed separately and subtracted

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

A feedback loop is implemented where the charge injection and capacitive coupling effects are captured, processed through a cancellation path with matched components, and fed back to the ADC input to cancel the harmful effects in real-time

Inventive Principle:

Principle #23Feedback

2Object-affected harmful factors

If a dynamic anti-alias filter with bypass switch is used, then noise aliasing rejection is improved, but power consumption increases

Engineering Contradiction:

Improvenoise aliasingVSAvoidpower consumption

Core Design Contradiction:

Object-affected harmful factorsVSUse of energy by moving object

Solution Approach 1:

The bypass switch operates periodically rather than continuously, being activated only during specific phases when charge injection cancellation is needed, thereby reducing overall power consumption while maintaining noise aliasing rejection effectiveness

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The circuit recovers and reuses the charge that would otherwise be wasted during switch transitions by routing it through the cancellation path, reducing the need for continuous power consumption to maintain signal integrity

Inventive Principle:

Principle #34Discarding and recovering

3Loss of time

If a dynamic anti-alias filter with bypass switch is used, then settling time is reduced, but differential error increases

Engineering Contradiction:

Improvesettling timeVSAvoidsignal accuracy

Core Design Contradiction:

Loss of timeVSMeasurement precision

Solution Approach 1:

The charge injection and capacitive coupling effects are preliminarily captured and processed through the cancellation path before the main signal conversion, allowing the ADC to settle faster while the cancellation path prepares the compensating signal in advance

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

An intermediary cancellation path is introduced that processes the harmful effects separately and mediates their removal from the main signal path, allowing faster settling in the main path while maintaining accuracy through the intermediary correction

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS10298252B2Dynamic anti-alias filter for analog-to-digital converter front end

Publication Date: 2019.05.21 ANALOG DEVICES INC

AI summary

An analog front end system can include a filter bypass switch connected in a boot-strapped configuration to pull a control terminal of the filter bypass switch above or below a supply voltage. Using bootstrapped switches can allow both the charge injection and capacitive coupling of the bypass switches of a differential anti-alias filter (AAF) to be common mode. A differential input signal of the ADC is not affected by the charge injection and capacitive coupling of the bypass switches in the AAF filter to a first order.