of Microprocessors

266 MUSICAL ApPLICATIONS OF MICROPROCESSORS
presented by the DAC. Because the speed of most DACs is determined by the
output amplifier, it would be nice ifit could be eliminated. This, in fact, can
be done easily if the connection to the comparator is slightly altered. Rather
than using a voltage comparator with a voltage output DAC and an unknown
input voltage, what is desired is a current comparator that can be connected
directly to a current-output DAC. The unknown voltage can be converted to
a current by use of an accurate series resistor.
True current comparators with sufficient accuracy are not generally
available but the configuration shown in Fig. 7-33 is virtually as good and
uses the same LM31l-type voltage comparator. A mismatch between the
DAC current output and the unknown current will tend to pull the 311
inverting input away from ground, thus switching the comparator according
to the direerion of the mismatch. The two diodes across the comparator
inputs limit the voltage swing for large mismatches, thereby maintaining
high speed (small voltage swing minimizes effect of stray capacitances) and
keeping the current output DAC happy.
Figure 7-34 shows a complete 12-bit ADC circuit that is easy and
inexpensive to build and is very fast. A DAC-312 current output OAC is
combined with a DM2504 successive approximation register (made by
National Semiconductor and others) and the comparator circuit just described
to yield a conversion time of only 6 f-Lsec. The 2-MHz clock
sequences the successive approximation at the rate of one trial per clock cycle.
A conversion is started by a negative-going pulse at the START input and
completion is signaled by EOC going low. The analog input impedance is
5K but varies somewhat during the conversion and should be driven by a
high-speed op-amp output. If a 12 f-Lsec conversion time is acceptable, a
74C905 CMOS successive approximation register can be used in place of the
DM2504 and the clock slowed to 1 MHz.
Complete Hybrid module ADC circuits using the successive approximation
technique have been available for many years, but their cost has
always been much higher than an equivalent circuit constructed from a DAC,
a comparator, and a successive approximation register. Until recently, all
monolithic IC ADCs were of the much slower dual-slope type. Even now, 8
bits is the highest resolution offered in an integrated circuit AOC.
UNKNOWN
INPUT
VOLTAGE
R
+5V
I kfi
TO
>----.. ~~~~~S~~iTION
LOGIC
Fig. 7-33. Analog circuitry for a high-speed ADC