of Microprocessors

Digital-to-Analog and
tlnalog-to-Digital Converters
The primary interface element between the digital logic found in a microcomputer
system and the analog modules ofa voltase-controlled system are
digital-to-analog converters (DACs) and analog-to-digital converters
(ADCs). For synthesizer control, only DACs are needed to convert numbers
from the computer into control voltages. However, ADCs are used in some of
the human interface techniques to be described later. Fortunately, the two
devices are very closely related, and, in fact, most ADC circuits utilize a
DAC as a key element.
The purpose of these "data-conversion" devices is to translate between
the electrical quantities of current or voltage and digital quantities. For
analog synthesizer control with a microprocessor, voltage is the preferred
analog representation and twos-complement fixed-point binary is the preferred
numerical representation. We can further specify that analog voltages
in the range of - 10 V to + 10 V, and logic voltages compatible with TTL
should be acceptable to the converter.
A number of terms are used to describe and specify data-conversion
devices. Most all of them are equally applicable to DACs and ADCs so the
discussion will focus on DACs. Although converters that work with binarycoded
decimal numbers are available, their variety is extremely limited.
Also, since BCD arithmetic is inconsistent with maximum utilization of
microprocessor speed, the discussion will be restricted to binary converters.
Resolution
Data Conversion Terminology
The most important and most quoted converter specification is its
resolution measured in terms of bits. Resolution is essentially a measure of the
number of different voltage levels that a DAC can produce. A 3-bit DAC, for
example, accepts 3-bit binary numbers as input and can produce no more
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