Sigma-Delta Modulator Chopping to Suppress Fold-Back Tones

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Solution Overview

Problem

Sigma-delta ADCs face issues with fold-back of modulated offset into the signal band due to sample rate conversion and Sinc-type digital filters, resulting in large spurious tones at specific frequencies, which are not effectively addressed by conventional chopping techniques at higher frequencies that can introduce charge injection and thermal noise issues.

Innovation Solution

A chopping technique using a variable frequency clock to spread the offset across the signal spectrum, suppressing fold-back into the signal band by generating a variable frequency chop signal and applying it to the chop switches within the ADC circuit, thereby eliminating large spurious tones at a few single frequencies.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional chopping techniques at higher frequencies are used, then offset modulation can be pushed out of band, but charge injection and thermal noise issues are introduced

Engineering Contradiction:

Improveoffset suppressionVSAvoidcharge injection and thermal noise

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent employs periodic chopping action at a frequency lower than the signal band to modulate offset. The chop signal periodically switches the chopping switches, creating a periodic modulation effect that pushes offset to a lower frequency rather than higher frequency, thereby avoiding charge injection and thermal noise issues associated with high-frequency chopping

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The patent changes the frequency parameter of the chopping signal from conventional high frequency to a lower frequency that is still above the signal band. This parameter change allows offset suppression while avoiding the harmful effects of high-frequency chopping, specifically reducing charge injection and thermal noise

Inventive Principle:

Principle #35Parameter changes

2Extent of automation

If sample rate conversion and Sinc-type digital filters are used, then digital signal processing is achieved, but fold-back of modulated offset into the signal band occurs, creating large spurious tones

Engineering Contradiction:

Improvedigital signal processingVSAvoidfold-back and spurious tones

Core Design Contradiction:

Extent of automationVSObject-generated harmful factors

Solution Approach 1:

The patent introduces an intermediary approach by using a lower frequency chop signal that acts as a mediator between the offset modulation and the digital filtering process. This intermediary frequency choice prevents the fold-back effect that would otherwise occur with conventional high-frequency chopping, thereby eliminating large spurious tones in the signal band

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent converts the potential harm of offset fold-back into a benefit by deliberately choosing a chop frequency that, when modulated, does not fold back into the signal band after sample rate conversion. The offset modulation at this specific frequency becomes beneficial rather than harmful, as it avoids creating spurious tones

Inventive Principle:

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

Data Source

PatentUS11967972B2Spread spectrum chopping for sigma delta modulators

Publication Date: 2024.04.23 ANALOG DEVICES INT UNLTD CO

AI summary

Chopping techniques that suppress fold-back into the signal band and spreads the offset across the spectrum are described. By using various techniques, chopping may be performed with a variable frequency clock to spread the offset across the signal spectrum. Spreading the offset across the signal spectrum means that there are no longer large spurious tones at a few frequencies.