MT-002 Datasheet, PDF (8/12 Pages) Analog Devices – What the Nyquist Criterion Means to Your Sampled Data System Design

MT-002
antialiasing filter is shown in Figure 7. Notice that this filter is specified to achieve at least 80 dB
attenuation between f_c and 1.2f_c. The corresponding passband ripple, return loss, delay, and
phase response are also shown in Figure 7.
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Figure 7: Characteristics of 11-Pole Elliptical Filter (TTE, Inc., LE1182-Series)
From this discussion, we can see how the sharpness of the antialiasing transition band can be
traded off against the ADC sampling frequency. Choosing a higher sampling rate (oversampling)
reduces the requirement on transition band sharpness (hence, the filter complexity) at the
expense of using a faster ADC and processing data at a faster rate. This is illustrated in Figure
6B which shows the effects of increasing the sampling frequency by a factor of K, while
maintaining the same analog corner frequency, f_a, and the same dynamic range, DR,
requirement. The wider transition band (f_a to Kf_s – f_a) makes this filter easier to design than for
the case of Figure 6A.
The antialiasing filter design process is started by choosing an initial sampling rate of 2.5 to 4
times f_a. Determine the filter specifications based on the required dynamic range and see if such
a filter is realizable within the constraints of the system cost and performance. If not, consider a
higher sampling rate which may require using a faster ADC. It should be mentioned that sigma-
delta ADCs are inherently highly oversampled converters, and the resulting relaxation in the
analog anti-aliasing filter requirements is therefore an added benefit of this architecture.
The antialiasing filter requirements can also be relaxed somewhat if it is certain that there will
never be a full-scale signal at the stopband frequency f_s – f_a. In many applications, it is
improbable that full-scale signals will occur at this frequency. If the maximum signal at the
frequency f_s – f_a will never exceed X dB below full-scale, then the filter stopband attenuation
requirement can be reduced by that same amount. The new requirement for stopband attenuation
at f_s – f_a based on this knowledge of the signal is now only DR – X dB. When making this type
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