of Microprocessors

246 MUSICAL ApPLICATIONS OF MICROPROCESSORS
5 B7
6 B6
7 B5
BINARY
INPUT
8 B4
1408
TTL 9 B3
LEVELS
10 B2
II BI
RI
12 BO lQAC
+Vref
5 kfi 2 mAI4 o TO
-2 mA
(+10 V)
Iref+
lout 4
15
Iref-
+5V -= 13 Vee
-15V VEE
100 pF
16 C eomp
2 GND
OFFSET"="
R2
10 kfi
SELECT TO
MINIMIZE RINGING
R3
GAIN
+9.92
-10.00
Fig. 7-H3. Application of 1408~type
IC DAC
Like most inexpensive monolithic DACs, the 1408 is a bare-bones
device incorporating little more than the analog switches and R-2R ladder
network. An external reference source and output amplifier are required for a
complete DAC. The 1408 is basically a current-activated device; the reference
is a current and the output is a current. It is also a multiplying DAC.
The output current, Iottt, is equal to the reference current, Iref, times the
binary input expressed as an unsigned binary fraction between 0 and 0.994.
The reference current may range from 0 to about 4 mA, although below 0.5
mA linearity errors increase such that monotonicity cannot be guaranteed.
The reference current must always be positive and the output current is
actually negative, meaning that the DAC output sinks current. Standard
power supply voltages of +5 V and -15 V are used.
Figure 7-18 shows a typical hookup for the 1408. Although other
methods of supplying the reference current exist, the one shown is the
simplest. The Ire/+ input sinks the reference current at a voltage equal to the
voltage at Iref-. For a reference voltage of + 10 V with respect to ground, Ire/­
is tied to ground and Iref+ receives a reference current of 2 mA (the optimum
value) through a 5K resistor (R 1) tied to the reference voltage. A compensation
capacitor of 100 pF is required to frequency compensate the internal
reference current circuitry.
An output voltage range of - 10 to + 10 is developed from the 0 to - 2
mA output current with a current-to-voltage converter. A + 1.0 mA current
from the reference voltage source through R2 offsets -1.0 mA of the DAC
output, giving a net current swing into the amplifier of -1.0 to + 1.0 mAo
This current is then converted into a -10-V to + 10-V swing as a
function ofR3. The current output settling time is about 0.3 JLsec, which is
extended to approximately 3 JLsec by the amplifier. R2 and R3 may be made