Nifty System Tricks You Can Play With A SigmaDelta Converter

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Nifty System Tricks You Can PlayWith A
Sigma-Delta Converter

• David A. Sobel
• Prof. Robert W. Brodersen
• Berkeley Wireless Research Center
• Dept. of EECS
• UC-Berkeley

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Intro to SD Modulation

• 1st-order SD block diagram

assume quantizer is additive white noise source
PE D2/12

• Linearized, Discrete-Time model

ideal DT integrator
ideal 1-bit DAC
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Intro to SD Modulation

• Feedback analysis

white noise source

• Quantization noise spectrum
• Noise is shaped to high-frequency
• Digital LPF can filter out HF noise
• Residual LF content has much less quantization
  noise
• Performance improves with oversampling

Quantization Noise
Digital LPF
Desired Signal
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Motivation for SD Converter

• Process-related arguments
• DSM-CMOS technology
• Provides faster, yet crappier analog circuits.
  (Gain, linearity, matching, etc.)
• SD modulators
• Requires faster, yet crappier analog circuits.
• Match made in heaven?
• System-related arguments
• ?D is a mostly digital converter
• Opportunities for system/circuit co-design.

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CDMA and SD modulation

• Difficult to implement
• CDMA signal is already wideband.
• Speed limitations prevent high oversampling
  ratios.
• Typical CDMA/SD converter architecture

Notation fu information BW N spreading gain M
oversampling ratio Si desired user PN sequence

• Despreading process reduces quantization noise by
  only 3dB/octave.

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Code-based Noise Shaping

• Equivalence of FDMA and CDMA systems
• Both systems divide bandwidth into N subsets.
• Only difference is basis of N-dimensional space.
• SD modulators shape noise out of desired
  subset
• FDMA systems shape quantization noise into other
  frequency band.
• Can we extend this to CDMA?
• First concept
• Despreading process reduces quantization noise by
  much more than 3dB/octave.
• Achieves effective oversampling of MN.
• Analog PN-sequence.

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Summary

• Exploration of system/circuit co-design
• Analog baseband domain is on the border.
• Analyze issues relating to both circuits and
  system. Exploit characteristics of one in other.
• SD modulators are appropriate vehicle for
  co-design.
• Mostly digital nature suitable for system
  tricks.
• Increasing process speed relaxes bandwidth
  restrictions of SD approach.