Analog Devices AN530 Application Notes

AN-530

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APPLICATION NOTE

Narrow or Wide Bandwidth

5B Series Input Modules:

Which Is the Right Choice?

Two bandwidth versions of the 5B Series input modules
are available: 4 Hz (narrow) and 10,000 Hz (wide). The
version that is appropriate for your application depends
on two factors. First, the frequency content of the input
signal must be understood. Second, the sampling rate
of the A/D subsystem plays an important role.

The bandwidth of the 5B module is determined by inter­nal low-pass filters. Ideally, they pass information
through from input to output without any attenuation
for frequency components below the cutoff frequency
(either 4 Hz or 10,000 Hz), and they completely block fre­quency components above the cutoff. Practically, they
neither perfectly pass the lower frequencies nor totally
reject the higher.

Many signals measured by a data acquisition system
are slowly varying with time or “low frequency.” Ex­amples are temperatures, flows and liquid levels. Their
measurements typically do not move through cycles
lasting less than one second. If the signal moved
through a cycle with a period of one second, it would
have a frequency component of 1 Hz (1 cycle per sec­ond). A low bandwidth 5B module with a 4 Hz cutoff
would be ideal for this measurement. Because it rejects
information above 4 Hz, it will eliminate interference
from outside sources. A typical problem in precision
measurement systems is 60 Hz noise from ac power
lines. 60 Hz noise is reduced by a 4 Hz bandwidth mod­ule by a factor of 1000:1.

Some signals measured by a data acquisition system
vary rapidly with time. Some examples are:

• Pressure inside a plastic injection molding machine

• Strain on an automobile chassis during a crash test

• Vibrations from a machine bearing shortly before

failure

• Force on a machine tool during a milling operation

The measurements from these signals can go through
hundreds or thousands of cycles per second. If these
signals were conditioned by a narrow bandwidth mod­ule, all of the useful information would be filtered out.
The wide bandwidth 5B modules will pass through all
information with frequencies below 10,000 Hz (cycles
per second) and reject frequencies above 10,000 Hz.
They are recommended for these applications.

The sample rate of the A/D subsystem must also be
taken into consideration when choosing 5B modules. An
A/D converter approximates an analog signal by taking
samples at discrete points in time. These samples are
reconstructed by the software and used to display the
data or to control a process. The faithfulness of the re­construction is enhanced when more samples are taken
in a given time period and when the fineness of the mea­surement is increased (resolution). The sampled data
will actually contain errors if a certain minimum sam­pling rate is not maintained. The minimum rate is deter­mined by the Nyquist Theorem, which states that the
minimum rate must be at least two times the highest fre­quency in the signal. If this rate is not maintained, a type
of error known as aliasing will occur. Low-pass filters on
the input signals to restrict the frequencies to below
one-half the A/D sample rate are known as antialiasing
filters. The 5B modules perform this filtering in addition
to the other signal conditioning functions.

Practically, because the filters are not perfect, some fre­quencies above the cutoff get through. To avoid
aliasing, many data acquisition engineers program their
A/D subsystems to sample at rates of 5 or 10 times the
filter cutoff. This would mean sample rates of 20 to 40
for narrow bandwidth and 50,000 to 100,000 for wide
bandwidth modules.

E3280–5–3/98

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