Sigma-Delta Quantizer DEM Layout for Low-Latency Loop Stability
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Solution Overview
Problem
Conventional continuous-time sigma-delta modulators face a tradeoff between loop stability and latency due to the need for dynamic element matching (DEM) module operation within the loop, leading to excess loop delay and instability, especially when using non-return-to-zero (NRZ) DAC converters.
Innovation Solution
The DEM module is moved out of the loop, and a digital multiplexer is introduced to shuffle comparator outputs, allowing the DEM module to operate independently, with a return-to-zero (RZ) DAC converter used to reduce latency by processing DEM outputs in the next cycle, and a DEM group with pre-calculated results is employed to minimize latency further.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the DEM module operates within the loop in conventional continuous-time sigma-delta modulators, then dynamic element matching can be performed, but excess loop delay occurs leading to loop instability
Solution Approach 1:
The DEM module is extracted from the feedback loop and placed outside it. The patent implements this by separating the DEM functionality into an independent module that receives quantizer outputs and provides DAC control signals without being part of the critical feedback path, thereby eliminating the loop delay caused by DEM operations while maintaining the matching functionality
Solution Approach 2:
The patent pre-calculates and stores DEM lookup tables containing predetermined DEM codes for all possible quantizer output combinations. This preliminary action allows the system to quickly retrieve appropriate DEM codes without performing complex real-time calculations within the loop, reducing the time required for DEM operations and preventing loop instability
2Productivity
If a time slot is reserved for the DEM module within each clock cycle, then dynamic element matching can be performed, but the available time for other operations is reduced and latency increases
Solution Approach 1:
The patent implements periodic updating of DEM codes where the DEM module updates its control signals at specific intervals rather than continuously within each clock cycle. The lookup tables are pre-computed and stored, allowing the system to periodically retrieve and apply DEM codes without occupying time slots in every cycle, thus improving time availability while maintaining DEM functionality
Solution Approach 2:
The patent creates lookup tables that are copies of pre-computed DEM code mappings for all possible quantizer outputs. Instead of performing real-time DEM calculations that would require dedicated time slots, the system copies the appropriate pre-computed codes from the lookup tables based on current quantizer outputs, significantly reducing the time required for DEM operations
Data Source
AI summary
In a continuous-time sigma-delta modulator, by using dynamic element matching (DEM) with respect to comparators of a quantizer, or by generating a plurality of candidate DEM results in advance for selecting an approximate DEM result, a time slot for DEM operations in each cycle of a sampling signal is significantly increased without being rushed.