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Microcontroller Programming and Interfacing Texas Instruments MSP430 by Daniel J. Pack, Steven F. Barrett

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8.3. ANALOG-TO-DIGITAL CONVERSION 243
0 2 4 6 8 10
−60
−40
−20
0
20
40
60
80
Time (sec)
Magnitude
Sample Analog Signal
Figure 8.1: Sample analog signal.
8.3.1 SAMPLING
The sampling process is the way for a digital system to capture an analog signal at a particular time.
One can consider the sampling process as similar to taking snap shots of changing scenery using an
electro-optic camera.
Suppose we want to capture the movement of a baseball pitcher as he throws a ball toward
home plate. Also let us assume the only means for you to capture the motion of the pitcher is a
camera. Suppose it takes two seconds for the pitcher to throw a baseball. If you take a picture at the
start of the pitching and another one two seconds later, you have missed most of the action and will
not be able to accurately represent the motion of the pitcher.
The inverse of the period between taking pictures in this example is the sampling frequency
with the unit of Hertz, Hz. Since there is a two second interval between samples, the sampling rate
is 1/2 = 0.5 Hz. As you can imagine, the faster you take the pictures the more accurately you can
re-create the pitcher’s motion by sequencing photos. The above example illustrates the primary issue
of the sampling process, that of the sampling frequency. A correct sampling frequency depends on
the characteristics of the analog signal. If the analog signal changes quickly, the sampling frequency
must be high while if the signal does not change rapidly the sampling frequency can be slow and
still capture the essence of the incoming signal. In another analogy of sampling rate, suppose you are

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