of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 245
BIPOLAR
OUTPUT
"MATCH TO 1/2 LSB OR BETTER
v-
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SIGN BIT
CONTROL
Fig. 7-17. Sign-bit amplifier
Conversion of twos complement, which would still be the preferred
internal computer code, to sign magnitude is fairly simple. The function
needed would pass the twos-complement value bits unaltered if the sign bit
were zero and invert them if it were one. The conditional inversion is easily
accomplished with exclusive-or gates in the interface or can be done by
software.
Some Commercial DACs
For most applications requiring resolutions greater than 5 or 6 bits, the
expense and difficulty of finding precision resistors outweighs the cost of a
commercial prepackaged DAC. Most modern 8- and lO-bit units are
monolithic and packaged in standard 16-pin IC packages. Twelve-bit and
higher resolutions until recently were invariably hybrid devices usually
packaged in small epoxy modules. Now several 12-bit monolithic units are
on the market at prices under $20 for the commercial temperature grade.
Segmented DACs up to 16 bits are also available as monolithic ICs, but none
have true 16-bit integral linearity and accuracy. In this section, the most
widely used inexpensive devices in the 8-, 10-, and 12-bit resolution range
will be briefly described. There are, of course, a large number of similar
devices on the market that will perform just as well in computer-controlled
synthesizer applications.
1408 and DAC-08 Type for 8 Bits
One of the earliest 8-bit monolithic DACs was developed by Motorola
and bears the generic type number 1408/1508. The 1508 is an expensive
military temperature range device, but the 1408 performs nearly as well in
room temperature environments and costs less than $5. The 1408 is offered
in 6-,7-, and 8-bit linearity grades, although all have 8 bits of resolution.
The linearity grade is indicated by an "-X" following the type number. For
this discussion, use of the 8-bit grade is assumed.