of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 251
pitfall in the device's application is that its output capacitance is nearly 200
pF as the result of large junction analog switches needed for 12-bit
performance. Although the current settling time is specified at 1.°J-Lsec, and
most FET amplifiers have a 2-J-Lsec settling time, it is difficult in practice to
get an overall voltage output settling time under 6 J-Lsec because of the
output capacitance.
The DAC-312 is another 12-bit device that uses the segmentation
technique to provide low cost (under $12) and 12-bit differential (lO-bit
integral) linearity in a monolithic Ie. It uses current segmentation with a 3­
bit segment generator (eight segments) and a 9-bit interpolator. It is
connected just like the DAC-OS described earlier, but the output current (up
to 4 rnA) is four times the reference current. Being a bipolar transistor
device, its current settling time is very fast at 250 nsec. Figure 7-22 shows a
circuit using a DAC-312 and an LM31S high-speed op-amp to produce a
voltage outpur settling time in the I-J-Lsec range.
Hybrid modules are also common at the 12-bit level, and, while they
tend to cost more than the monolithic ICs do, they are usually complete
voltage output units that require no additional components. The Hybrid
Systems DAC349 is an example that is packaged in a standard 24-pin
ceramic DIP and looks like any other Ie. For under $30, it includes a
precision-regulated voltage reference and an output amplifier. Varying
connection patterns among several of its pins select unipolar or bipolar
output and 5- or lO-V output ranges. Perhaps the only disadvantage of this
unit is the slow internal amplifier, which can take as long as 20 J-Lsec to settle
after a full-scale output change.
Even higher resolution is available at a proportionate increase in cost.
Fourteen-bit linearity (16-bit resolution) costs about $50, true 16-bit
linearity is around $200, and an unbelievable IS-bit (1 part in a quarter
MSB
I
BII
2 BID DAC-312
3 B9 2-10 pF
12 BIT
DIGITAL
INPUT 10 82 ..
II
2.5 K
81
LS8 12
80
19 2
lout I
+V,ef 10 K 14
lref t
V out
(+10 v.I 15 3 -IOTO +10
I,ef-
=10K
IB
+ 5 OR 20 lout 2
+ 15
Vee
17
-15 Vee
0.01 16
C +15
}JF 13 eomp
=
Vlth
Fig. 7-22. Application of DAC-312 12-Bit DAC