Programming instructions

Chapter 5 Things You Should Know about Analog Input
LabVIEW Data Acquisition Basics Manual 5-2
© National Instruments Corporation
You might be saying to yourself, “I know that I have a thermocouple and
that the primary information (temperature) is contained in the level of the
analog voltage. Now I am ready to go hunting!” Well, you are
almost
ready
to hunt, but you first must figure out a few more signal characteristics
before you can begin. For example, to what is your signal referenced? How
fast does the signal vary with time? The rate you sample determines how
often the A/D conversions take place. A fast sampling rate acquires more
points in a given time, and therefore can often form a better representation
of the original signal than a slow sampling rate. According to the Nyquist
Theorem, you must sample at a rate greater than twice the maximum
frequency component in that signal to get accurate frequency information
about that signal. The frequency at one half the sampling frequency is
referred to as the
Nyquist frequency
. For more information on the Nyquist
frequency, refer to the section
Sampling Considerations
in Chapter 11,
Introduction to Analysis in LabVIEW
of the
LabVIEW User Manual
.
What Is Your Signal Referenced To?
Signals come in two forms:
referenced
and
non-referenced
signal sources.
More often, referenced sources are said to be
grounded
signals, and
non-referenced sources are called
floating
signals.
Grounded Signal Sources
Grounded signal sources have voltage signals that are referenced to a
system ground, such as earth or a building ground. Devices that plug into a
building ground through wall outlets, such as signal generators and power
supplies, are the most common examples of grounded signal sources, as
shown in Figure 5-2.
Figure 5-2.
Grounded Signal Sources
Vs
Ground
+
_