MT-022 Datasheet, PDF (1/12 Pages) Analog Devices – ADC Architectures III: Sigma-Delta ADC Basics

MT-022
TUTORIAL
ADC Architectures III: Sigma-Delta ADC Basics
by Walt Kester
INTRODUCTION
The sigma-delta (Σ-Δ) ADC is the converter of choice for modern voiceband, audio, and high-
resolution precision industrial measurement applications. The highly digital architecture is
ideally suited for modern fine-line CMOS processes, thereby allowing easy addition of digital
functionality without significantly increasing the cost. Because of its widespread use, it is
important to understand the fundamental principles behind this converter architecture.
Due to the length of the topic, the discussion of Σ-Δ ADCs requires two tutorials, MT-022 and
MT-023. This first tutorial (MT-022) first discusses the history of Σ-Δ and the fundamental
concepts of oversampling, quantization noise shaping, digital filtering, and decimation. Tutorial
MT-023 discusses more advanced topics related to Σ-Δ, including idle tones, multi-bit Σ-Δ
ADCs, multistage noise shaping Σ-Δ ADCs (MASH), bandpass Σ-Δ ADCs, as well as some
example applications.
HISTORICAL PERSPECTIVE
The Σ-Δ ADC architecture had its origins in the early development phases of pulse code
modulation (PCM) systems—specifically, those related to transmission techniques called delta
modulation and differential PCM. (An excellent discussion of both the history and concepts of
the Σ-Δ ADC can be found by Max Hauser in Reference 1). Delta modulation was first invented
at the ITT Laboratories in France by E. M. Deloraine, S. Van Mierlo, and B. Derjavitch in 1946
(References 2, 3).
The principle was "rediscovered" several years later at the Phillips Laboratories in Holland,
whose engineers published the first extensive studies both of the single-bit and multi-bit concepts
in 1952 and 1953 (References 4, 5). In 1950, C. C. Cutler of Bell Telephone Labs in the U.S.
filed an important patent on differential PCM which covered the same essential concepts
(Reference 6).
The driving force behind delta modulation and differential PCM was to achieve higher
transmission efficiency by transmitting the changes (delta) in value between consecutive samples
rather than the actual samples themselves.
In delta modulation, the analog signal is quantized by a one-bit ADC (a comparator) as shown in
Figure 1A. The comparator output is converted back to an analog signal with a 1-bit DAC, and
subtracted from the input after passing through an integrator. The shape of the analog signal is
transmitted as follows: a "1" indicates that a positive excursion has occurred since the last
sample, and a "0" indicates that a negative excursion has occurred since the last sample.
Rev.A, 10/08, WK
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