Page 359 - Op Amps Design, Applications, and Troubleshooting

CHAPTER EIGHT

Digital-to-Analog and

Analog-to-Digital Conversion

The world of electronics can be neatly divided into two general classes based on
the nature of the signal or circuit: digital and analog. Digital signals, devices, and
circuits operate in one of two states at all times. These states may be high/low,
on/off, up/down, 0 volts/5 volts, -5 milliamps/+5 milliamps, or any other set of
two-valued terms.
Analog signals, devices, and circuits, on the other hand, operate on a continu-
ous range with an infinite number of values represented within a given range. An
analog voltage, for example, may be 1.5 volts or 1.6 volte, but it can also be an infi-
nite number of values between these two numbers, such as 1.55 volts or 1.590754
volts.
A technician or engineer generally must be capable of working with both
analog and digital devices and systems. This text will avoid revealing a prejudice
toward one type or another, and the reader is encouraged to avoid developing
such a prejudice. It is true that digital devices and techniques are steadily taking
over operations and functions previously implemented by analog systems. But
equally true is the fact that the world in which we live is inherently analog. Tem-
perature, pressure, weight, speed, light intensity and color, volume, and all other
similar quantities are analog in that they vary continuously and have an infinite
number of possible values.
This chapter will focus on the circuits that interface analog with digital sys-
tems. Analog-to-digital (A/D) converters accept an analog signal at their input
and produce a corresponding digital signal at the output. This output can then be
processed and interpreted by a digital circuit (typically a microprocessor system).
A digital-to-analog (D/A) converter, on the other hand, is used to convert the dig-
ital output from a microprocessor or other digital device into an equivalent analog
signal. The analog signal is frequently used to control a real-world quantity (e.g.,
temperature or pressure).
The intent of this chapter, then, is to provide the reader with the concepts
and terminology associated with A/D and D/A conversion. Additionally, several
representative circuits will be presented that utilize operational amplifiers. An

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