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L303.4.V4
41
Drexel University
Electrical and Computer Engr. Dept.
Electrical Engineering Laboratory III, ECE L303
E. L. Gerber
ANALOGTODIGITAL
CONVERSION
Object
The object of this experiment is to learn the elements of an analogtodigital
converter and to utilize an analogtodigital chip in the Laboratory.
Introduction
An analog signal is one that can assume any voltage value as a function of time,
e.g., sine waves, speech waveforms, etc. In contrast, a digital binary signal can assume
only two values called “1” and “0”; for transistortransistorlogic (TTL) the 1 state is
approximately +5 V and the 0 state is 0 V.
If we wish to transfer analog signals from one point to another with minimum
interference from noise, or if we need to change the form or characteristics of the
signal, we need to convert the analog signal to a digital signal. The conversion process
consists of taking sample points on the analog waveform (see Fig. 1) and converting
each point a digital word. The more points we take, the closer the sampled waveform
follows the original waveform. A series of binary words will represent the analog signal
over a time interval.
Theory
In the analog waveform shown in the Figure 1, the signal has a maximum value
of +3.1 V.
Let us assume that after processing and transmission, we would like the
analog signal to be restored to within 0.1 V of its original value. We can determine the
required resolution by dividing the 3.1 V into 0.1V increments. With a signal range of
0 to +3.1 V, we have 32 steps of 0.1 V each, i.e., 0, 0.1, 0.2
…
3.1V, which gives a
resolution of 1/32 = 1/2
n
. Also the resolution can be expressed in terms of the fullscale
voltage, that is, V
FS
/(2
n
 1) volts per bit. In this example we have 3.1/(32  1) = 0.1
volts/bit.
To represent these 32 steps as a digital word requires five bits (2
5
= 32). With five
bits we can get 32 combinations of ones and zeros.
Zero volts can be represented by
the binarycoded decimal count of 00000, and 0.1 V by 00001, and so on. At each
particular sample time, a digital word consisting of five bits is generated for each
particular analog value. This process must be repeated many times.
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Figure 1.
Sampling an Analog Waveform
Analogtodigital (A/D) converters are integrated circuit devices that perform
the described conversion process.
There are four basic types of A/D converters:
1. Successive approximation
2. Flash
3. Dual slope
4. Tracking
The successive approximation A/D converter is used where reasonably fast
conversion is required. The flashtype A/D converter is used for fastchanging analog
input signals.
Dual slope conversion is slow, but offers excellent rejection of power
supply noise.
Finally, the tracking converter is faster than the dual slope, but slower
than the successive approximation or flash types. We will investigate the successive
approximation converter in this experiment.
• Successive Approximation A/D Converter
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 Fall '07
 GERBER

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