Guest editorial: Delta–sigma modulators and noise shaping: current status, recent developments and future trends
✍ Scribed by Feely, Orla; Schreier, Richard
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 40 KB
- Volume
- 25
- Category
- Article
- ISSN
- 0098-9886
No coin nor oath required. For personal study only.
✦ Synopsis
The past decade has witnessed an explosive growth in the applications of delta-sigma (or sigma-delta-the two terms are interchangeable) modulation and related techniques. The most obvious manifestation of this to the general public has been in digital audio, where in the mid-1980s the ever-increasing number of bits of resolution claimed by competing manufacturers was suddenly, and puzzlingly, interrupted by the appearance of 'one-bit' products. The parallel appearance of the word 'oversampling' provided the clue as to why this apparently backward step was actually progressive-although delta-sigma converters use low-resolution samples to reproduce a signal, they provide many more per second than conventional converters. The output of the delta-sigma modulator can then be filtered and decimated to retrieve a high-resolution signal.
Although the concepts underlying delta-sigma modulation were first published in the early 1960s, it was with the arrival of VLSI technology in the 1980s that data converters based on these principles became commercially feasible. These converters relax constraints on analogue component accuracy by increasing demands on sampling speed and digital processing-a trade-off that is very well suited to the characteristics of today's CMOS technology. As a result, delta-sigma data converters have become the converters of choice in a wide variety of applications, notably digital telephony, digital audio and instrumentation.
Progress in this area has been dramatic and can be loosely grouped into three overlapping areas.
The area of circuit design has seen the development of a variety of modulator topologies to meet a growing range of applications, supported by the development of new CAD tools. The original delta-sigma modulators consisted of a filter and a one-bit quantizer connected in a feedback loop in such a way as to shape the quantization noise away from low frequencies. Among the important extensions to this basic topology we now have multiloop systems, which employ multiple feedback loops around a single quantizer; multistage systems, which employ a cascade of basic modulators; multibit systems, in which the quantizer is no longer restricted to one bit (recent and highly promising systems of this type employ noise shaping in a second manner, this time to the DAC errors within the loop); and bandpass systems, which operate on highfrequency narrowband signals.
The second of our loose groupings deals with the theory of delta-sigma systems. The most common form of analysis replaces the quantizer with a noise source and then treats the modulator as a two-input linear system. This technique yields much useful insight, but it fails to take into account the true non-linear nature of the system and thus fails to account for such important effects as tonal behaviour and modulator instability. In a sense, delta-sigma systems are particularly simple non-linear systems-typically of low order and with the non-linearity often just a single discontinuitybut underneath this facade of simplicity can lie fiendishly complex dynamics. Many contributions have been made in the arena of delta-sigma theory, but many opportunities still exist for the interested researcher.