Chopper-Stabilized RMS-DC Converter Offset Elimination
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Solution Overview
Problem
RMS power detectors face a tradeoff between dynamic range and bandwidth, with existing solutions either reducing maximum operating frequency or requiring large device sizes and trimming to achieve a reasonable dynamic range, which limits their effectiveness at high frequencies.
Innovation Solution
A chopper-stabilized square cell architecture that alternates input and reference signals between two square cells to eliminate DC offsets, allowing for high-bandwidth operation with reduced device size and minimizing power consumption by only having a single square cell operate at high frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dynamic range enhancement techniques are applied to improve the dynamic range of RMS-DC converters, then the dynamic range increases, but the maximum operating frequency significantly decreases
Solution Approach 1:
The patent applies chopping (periodic switching) to alternately connect the square cell to the input signal and reference signal. This periodic action modulates the DC offset to a higher frequency where it can be filtered out, thereby improving dynamic range without limiting the maximum operating frequency of the RMS-DC converter
2Measurement precision
If device sizes are enlarged to reduce DC offsets and improve dynamic range, then the dynamic range improves, but the bandwidth of the square cell is reduced
Solution Approach 1:
The patent changes the operating parameters by applying chopping at a specific frequency and using appropriate filtering. This allows the use of smaller device sizes while maintaining both high dynamic range and high bandwidth, as the chopping technique actively suppresses offsets rather than relying on large device dimensions
3Speed
If trimming is applied to reduce offsets without increasing device sizes, then bandwidth is maintained, but trimming cannot remove all offset components especially over full operating temperature range
Solution Approach 1:
The patent implements a feedback mechanism where the output of the square cell is fed back through a low-pass filter and subtracted from the input signal. This active feedback loop continuously compensates for DC offsets and their temperature drift, achieving high dynamic range without requiring large device sizes or complex trimming procedures
4Use of energy by moving object
If only a single square cell operates at high frequencies to reduce power consumption, then power consumption decreases, but DC offsets from device mismatches limit dynamic range
Solution Approach 1:
The patent extracts the DC offset component from the square cell output by modulating it to a higher frequency through chopping and then separating it using filtering. This allows a single square cell to operate at high frequency with low power consumption while the extracted and filtered offset does not degrade the dynamic range
Data Source
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Figure 3
Figure 4a~4h
AI summary
An RMS-DC converter includes a chopper-stabilized square cell that eliminates offset, thus enabling high-bandwidth operation. The chopper-stabilized offset requires only a small portion of the circuitry (i.e., a single component square cell) which operates at high frequencies, and is amenable to using high-bandwidth component square cells. Using the chopping technique minimizes required device sizes without compromising an acceptable square cell dynamic range, thereby maximizing the square cell bandwidth. The RMS-DC converter consumes less power than conventional RMS-to-DC converters that requires a high-frequency variable gain amplifier.