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THEORY OF OPERATION
INSTRUCTION MANUAL
Dlgltal
Multimater
Models 17ZA. 17JA
PRECISION
DIVIDER
~602
FIGURE 5-16.
Simplified
Schematic
of
Reference
Supply.
9.
Reference
SUDPlY.
Although
not
actually
a part
of
the
A/D converter
circuitry,
the
reference
supply
(shown
in
Figure
5-16)
furnishes
the
stable
known
voltages
required
to
auto
calibrate
the
A/D converter.
These
voltages
are
also
used
in
the
ohms converter
circuitry.
The heart
of
the
reference
supply
is
a stable,
low
temperature
coefficient
zener
diode.
It
is
driven
from
a constant
current
(nominally
7.5mA)
developed
by ampli-
fier
~601.
With
the
zener
current
well
regulated,
it
is
immune
to power
supply
variations
The output
divider
resistors
ar.e part
of
a precision
resistor
network
which
provides
very
stable
reference
voltages.
R606 and the
input
resistors
of
the
divider
are
specially
selected
(depending
on the
zener
voltage)
to provide
a nominal
200uA
of
current
to
the
divider.
The zener
voltage
is
between
6.15
and 6.5
volts,
with
the
resistors
matched
accordingly.
h.
Digital
Section.
The digital
section
is
composed
almost
entirely
of
the
custom
LSI
circuit
U901.
It
keeps
track
of
the
flinction
and
range,
allows
auto
or manual
ranging
generates
the
display
information,
controls
the
A/O converter
timing,
provides
ranging
lines
for
the
signal
conditioning
circuitry,
and provides
information
for
the
optional
digital
interfaces,
as well
as controlling
them.
5-20
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