MT-022 Datasheet, PDF (10/12 Pages) Analog Devices – ADC Architectures III: Sigma-Delta ADC Basics

MT-022
Figure 9 shows the relationship between the order of the Σ-Δ modulator and the amount of
oversampling necessary to achieve a particular SNR. For instance, if the oversampling ratio is
64, an ideal second-order system is capable of providing an SNR of about 80 dB. This implies
approximately 13 effective number of bits (ENOB). Although the filtering done by the digital
filter and decimator can be done to any degree of precision desirable, it would be pointless to
carry more than 13 binary bits to the outside world. Additional bits would carry no useful signal
information, and would be buried in the quantization noise unless post-filtering techniques were
employed. Additional resolution can be obtained from the 1-bit system by increasing the
oversampling ratio and/or by using a higher-order modulator. Other methods are often used to
achieve higher resolution, such as the multi-bit Σ-Δ architecture, and are discussed in Tutorial
MT-023.
120
THIRD-ORDER LOOP*
21dB / OCTAVE
100
80
SNR
(dB)
60
40
20
SECOND-ORDER LOOP
15dB / OCTAVE
FIRST-ORDER LOOP
9dB / OCTAVE
* > 2nd ORDER LOOPS DO NOT
OBEY LINEAR MODEL
0
4
8
16
32
64
128
256
OVERSAMPLING RATIO, K
Figure 9: SNR Versus Oversampling Ratio for
First, Second, and Third-Order Loops
SUMMARY
This tutorial has covered the basics of Σ-Δ ADCs from a historical perspective including the
important concepts of oversampling, digital Filtering, noise shaping, and decimation. Tutorial
MT-023 covers some of the more advanced concepts and applications of Σ-Δ ADCs, such as idle
tones, multi-bit Σ-Δ, MASH, and bandpass Σ-Δ.
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