of Microprocessors

DIGITAL-TO-ANALOG AND ANALOG-TO-DIGITAL CONVERTERS 261
reference voltage is connected to the integrator. Being of opposite polarity,
the integrator starts to recharge toward zero again. The time necessary to
reach and cross zero is monitored by the microcomputer as before. Ifa similar
loop is used in each phase, the converted value will be in fractional terms of
the reference voltage and the value of the capacitor is no longer critical.
Accuracy in excess of 12 bits is readily obtained with this dual-slope
circuit, although long conversion times would still be necessary with the
microcomputer doing the timing and control. Every additional bit of resolution
would double the time required for conversion. Dual slope takes about
twice as long as the single-slope or single-shot method because both the
reference and the unknown are, in effect, digitized. A hardware counter
could be used instead to increase the speed, but these techniques still remain
relatively slow.
Integrated circuit dual-slope ADCs are available with resolutions as
high as 20,000 steps. Unfortunately, most of these are intended for digital
panel meters and therefore have decimal outputs. The MM5863 from National
Semiconductor, however, provides 12-bit binary output and conversion
times as short as 20 msec when used with an LF11300 "analog front
end" circuit. This is adequately fast for digitizing most slowly changing
voltages.
Linear Search and Tracking ADC
The better techniques for analog-to-digital conversion work by comparing
the unknown analog voltage with the output of a DAC using the basic
configuration in Fig. 7-29. In practice, the DAC output is systematically
varied until it matches the unknown as closely as possible as determined by
the comparator. When this is accomplished, the input to the DAC is the
digital equivalent of the unknown voltage. The key to high speed is an
efficient search algorithm and a fast DAC and comparator.
The simplest search method is the linear search. The DAC is set to zero
(or negative full scale ifit is bipolar) and then incremented one step at a time
UNKNOWN
INPUT VOLTAGE
OAC
'f tf:ltt===t>---- CONVERTED
VALUE
Fig. 7-29. Analog-to-digital conversion using a DAC