of Microprocessors

248 MUSICAL ApPLICATIONS OF MICROPROCESSORS
other. For example, for a digital input of 01000000, IoutI will be one-quarter
of the reference current and IOllt2 will the three-quarters of the reference
current. Thus, the full reference current magnitude is effectively divided
between the two outputs according to the digital code. Availability of the
two outputs is an advantage in a bipolar output DAC as shown in Fig.
7- ~9A. Here the Iout1 output is connected to the op-amp summing junction
as before, but the Iollt2 output is connected to the noninverting input of the
same amplifier. The op-amp thus effectively subtracts IOllt1 from IOllt2 and
converts the difference into a voltage. The parts values given produce an
output range of - 10 to + 10 V and a settling time under 2 fLsec. Note that
all of the resistors are the same value and if matched to within 0.2%, no
adjustments are needed.
Another advantage of the DAC-08 is that the outputs have a large
voltage compliance between about + 18 and - 10 V, which permits use of a
simple resistor current-to-voltage converter in some applications. Figure
7-19B shows such a circuit, which also produces a - 10 to + 10 V output
swing into an open circuit. Note that the unused outpur terminal is
connected to ground. The output impedance is 10K ohms and can drive
finite load resistances with no loss in linearity, although the voltage swing, of
course, will be reduced.
7530 Type for 10 Bits
For several years, 8-bit linearity was the best that could be done with
bipolar monolithic circuitry. However, ion implantation coupled with CMOS
switch technology has made possible an inexpensive ($10) 1O-bit DAC with
an unusual but very flexible structure. The generic number is 7530 and the
originator is Analog Devices, although it is available from other manufacturers.
Like the 1408, reduced linearity devices are available so check
carefully that the grade being ordered is rated for lO-bit linearity.
As can be seen in Fig. 7-20, the 7530 consists of nothing except
precision resistors and CMOS analog switches! The reference is a voltage that
can range anywhere between - 10 V and + 10 V. The reference is applied
directly to the R-2R ladder network, which provides an equivalent 10K pure
resistive load to ground. The analog switches steer weighted current from the
ladder into the two output buses, which must be held at near ground potential.
A single supply voltage is required, which may range between + 5 V
and + 15 V. Since the power supply operates only the switches, its value and
regulation has virtually no effect on the analog output. A clever feature of
this design is that all parts of the analog switches are always at ground
potential considerably simplifying the internal switch drivers and allowing
the single low-voltage power supply. The logic inputs are TTl level compatible
regardless of supply voltage and draw essentially zero input current. A
precision feedback resistor is provided for an external current-to-voltage
converter.