of Microprocessors

252 MUSICAL ApPLICATIONS OF MICROPROCESSORS
million) unit goes for nearly $1,000. With this kind of precision, one no
longer casually connects ordinary op-amps with solder on ordinary circuit
boards. The various gain and offset errors of the amplifiers and thermocouple
voltages in the wiring can easily wipe out the advantage of 16- and IS-bit
accuracy. Fortunately, 12-bit accuracy is generally sufficient for producing
control voltages in a computer-controlled analog synthesizer system.
Many DACs are now available with input registers to hold the binary
value being converted. These are often promoted as being "microprocessorcompatible,"
since they can be connected right across the system's data bus
and strobed when the DAC is addressed by the program. Unfortunately, their
extra cost is often much more than equivalent external latches would be.
Also, at the 12-bit level, it can be very difficult to keep noise from the data
bus, which is flipping about wildly all the time, out of the analog circuitry
connected to the same Ie. Noise isolation is much easier with external
latches.
Multiplexing DACs
Even though DACs have decreased dramatically in size and cost from
earlier units, it is still far too expensive to use a 12-bit DAC every time a
computer-generated control voltage is needed. Fortunately, it is possible to
multiplex the output of a single fast converter and make it look like several
identical but slower converters.
Figure 7-23 shows the fundamental concept of DAC multiplexing.
Each multiplexed output consists of an analog switch, storage capacitor, and
a voltage-follower op-amp. The idea is to store the output voltage for each
channel in the capacitor rather than a separate register-DAC combination.
For example, when output 1 must be updated the single DAC is given the
corresponding digital input, a delay for DAC settling is taken, and SI is
closed momentarily to update the charge on C1. Following this, other channels
could be updated with no effect on output 1. If the switch, capacitor,
DIGITAL -:
INPUT
•-
VOLTAGE
OUTPUT
DAC
SI
>--__--~OUTN
-:J cN
Fig. 7-23. DAC multiplexing
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