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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