MT-002 Datasheet, PDF (1/12 Pages) Analog Devices – What the Nyquist Criterion Means to Your Sampled Data System Design

MT-002
TUTORIAL
What the Nyquist Criterion Means to Your
Sampled Data System Design
INTRODUCTION
by Walt Kester
A quick reading of Harry Nyquist's classic Bell System Technical Journal article of 1924
(Reference 1) does not reveal the true significance of the criterion which bears his name. Nyquist
was working on the transmission of telegraph signals over a channel that was bandwidth limited.
A thorough understanding of the modern interpretation of Nyquist's criterion is mandatory when
dealing with sampled data systems. This tutorial explains in easy to understand terms how the
Nyquist criterion applies to baseband sampling , undersampling, and oversampling applications.
A block diagram of a typical real-time sampled data system is shown in Figure 1. Prior to the
actual analog-to-digital conversion, the analog signal usually passes through some sort of signal
conditioning circuitry which performs such functions as amplification, attenuation, and filtering.
The lowpass/bandpass filter is required to remove unwanted signals outside the bandwidth of
interest and prevent aliasing.
f_s
f_s
f_a
LPF
OR
BPF
N-BIT
ADC
DSP
N-BIT
DAC
LPF
OR
BPF
AMPLITUDE
QUANTIZATION
DISCRETE
TIME SAMPLING
f_a
t_s=
1
f_s
t
Figure 1: Typical Sampled Data System
The system shown in Figure 1 is a real-time system, i.e., the signal to the ADC is continuously
sampled at a rate equal to f_s, and the ADC presents a new sample to the DSP at this rate. In order
to maintain real-time operation, the DSP must perform all its required computation within the
sampling interval, 1/f_s, and present an output sample to the DAC before arrival of the next
sample from the ADC. An example of a typical DSP function would be a digital filter.
Rev.A, 10/08, WK
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