Chopper-Stabilized LOG-RMS Converter for Low-Frequency Accuracy

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Solution Overview

Problem

Existing sigma-delta modulators face challenges in converting root-mean-square (RMS) signal values to direct current (DC) signals with high accuracy, particularly due to sensitivity to DC offsets and limited input-referred dynamic range, especially at low frequencies.

Innovation Solution

A sigma-delta difference-of-squares LOG-RMS to digital converter is developed by merging a sigma-delta modulator with an analog LOG-RMS to DC converter, utilizing chopper-stabilization and multiple integrators in the loop filter for frequency compensation, which reduces sensitivity to DC offsets and extends the input-referred dynamic range.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a sigma-delta modulator is used for RMS to DC conversion, then high resolution analog-to-digital conversion is achieved, but sensitivity to DC offsets and limited input-referred dynamic range occur, especially at low frequencies

Engineering Contradiction:

Improveconversion accuracyVSAvoidsensitivity to DC offsets

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

Chopper stabilization is applied to preemptively counteract DC offsets and low-frequency errors before they degrade conversion accuracy. The chopper modulates the input signal to a higher frequency where offset effects are minimized, then demodulates the result to recover the accurate signal with offsets suppressed.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The chopper stabilization employs periodic modulation and demodulation cycles to continuously suppress DC offsets. By periodically switching the chopper switches, the system rhythmically transfers the signal to offset-free frequency regions and back, maintaining reliable conversion throughout operation.

Inventive Principle:

Principle #19Periodic action

2Measurement precision

If a sigma-delta modulator is used for RMS to DC conversion, then high resolution analog-to-digital conversion is achieved, but the input-referred dynamic range is limited, especially at low frequencies

Engineering Contradiction:

Improveconversion accuracyVSAvoidinput-referred dynamic range

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

Chopper stabilization preemptively extends the usable dynamic range by suppressing offset effects before they limit the input signal range. This preliminary correction allows the converter to accurately process a broader spectrum of input amplitudes, particularly at low frequencies where offsets would otherwise constrain the dynamic range.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The periodic chopper modulation continuously expands the effective dynamic range by rhythmically operating in frequency regions where offset limitations are minimized. This periodic frequency translation enables the system to maintain adaptability across varying input conditions that would otherwise fall outside the reliable operating range.

Inventive Principle:

Principle #19Periodic action

3Adaptability or versatility

If chopper stabilization is applied to reduce sensitivity to DC offsets, then input-referred dynamic range is extended, but device complexity increases

Engineering Contradiction:

Improveinput-referred dynamic rangeVSAvoidconverter architecture

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The chopper stabilization functionality is merged with the existing sigma-delta modulator architecture rather than being implemented as a separate system. The chopper switches are integrated into the signal path of the modulator, combining offset suppression with high-resolution conversion in a unified structure that minimizes overall complexity.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The chopper stabilization circuit performs multiple functions simultaneously: it suppresses DC offsets, extends dynamic range, and maintains compatibility with the sigma-delta modulation process. This multi-functionality reduces the need for additional dedicated circuits, thereby limiting the increase in device complexity while achieving extended adaptability.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS8665128B2Sigma-delta difference-of-squares log-RMS to DC converter with forward path multiplier and chopper stabilization

Publication Date: 2014.03.04 NAT SEMICON CORP

AI summary

A sigma-delta (ΣΔ) difference-of-squares LOG-RMS to digital converter for true RMS detection by merging a ΣΔ modulator with an analog LOG-RMS to DC converter based on a difference-of-squares. Chopper-stabilization, implemented through commutators running at two different frequencies, can be employed to reduce sensitivity to DC offsets and low-frequency errors, resulting in an extension of the useful input-referred dynamic range. High-order ΣΔ LOG-RMS converters can be implemented with a loop filter containing multiple integrators and feedforward and/or feedback paths for frequency compensation. The resulting implementations are ΣΔ difference-of-squares LOG-RMS to DC converters with a natural digital output and a logarithmically compressed dynamic range.