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INSTRUCTION MANUAL
Digital MultImeter
Models
172A.
173A
THEORY OF OPERATION
-
5-4.
A/D CONVERTER AND DIGITAL
CIRCUITS.
(Schematic
29145E.
Sheet
2).
a.
Introduction.
A simplified
block
diagram
of
the
A/O Converter
and Digital
Circuits
is shown
in
Figure
5-11.
The A/O converter
generates
a digital
pulse
train
which
over
a
specified
time
period
is directly
proportional
to
the
dc voltage
applied.
It
has
the
capability
to measure
its
own offsets
and gain
errors
as well
as those
of
circuitry
in
front
of
it
(i.e.,
ac/dc
converter,
dc attenuator
and ohms converter).
The converter
then
cancels
the
effect
of
these
errors
by storing
appropriate
correction
voltages
in auto-
zero
and auto-calibrate
storage.
A conversion
cycle
consists
of
four
different
operating
modes,
as shown in Figure
5-12.
During
each
mode,
a different
input
is
connected
to
the
A/O buffer.
The difference
voltage
between
ACAL and AZ-1
is
the
calibration
voltage,
while
the
difference
between
signal
integrate
and AZ-2
is
the
measured
voltage.
The
converter
uses
the
charge
balancing
conversion
technique,
and polarity
detection
is
accomplished
digitally.
Almost
all
of
the
digital
tasks
are
handled
by a custom
large
scale
integration
(LSI)
circuit.
Note:
details
of
the
operation
of
the
LSI
chip
will
not
be given
in
the
A/D converter
discussion.
However,
various
of
its
outputs
which
are
necessary
for
the
operation
of
the
remainder
of
the
A/O converter
will
be referred
to
when necessary.
b.
Charqe-Balancinq
Technique.
The charge-balancing
converter
of
the
Model
172A/l73A
is
built
around
an active
integrating
circuit
and a comparator
circuit
which
at
first
glance
may appear
similar
to a dual-slope
converter.
However,
its
operation
is quite
different,
In
the
charge
balance
converter,
the
net
charge
for
an integrating
cycle
(charge/discharge
cycle)
is
zero,
and the
digital
output
from
the
converter
is
a function
of
the
number
of
integrating
cycles
that
occur
during
the
measurement
conversion
period
(signal
integrate).
The number
of
integrating
cycles
in turn,
depend
upon
the
level
of
the
input
signal.
In the
Model
172A/173A,
the
input
to the
A/O converter
is offset
to
-
handle
bipolar
input
signals
and the
number
of
integrating
cycles
(output
counts~f
in-
creases
as the
input
voltage
increases
in the
positive
direction.
For example:
on the
3VOC range,
a -3 volt
input
would
produce
zero
counts,
zero
volts
(30,000
counts)
and
+3v
(60,000
counts).
It
can be seen
from
the
example
that
.a minus
sign
would
have
to be
displayed
from
0 to 30,000
counts
and that
30,000
counts
would
have
to be subtracted
in
order
to
display
zero
with
a zero
volt
input.
Both
of
these
tasks
are
handled
by the
digital
circuits.
ONE A/O CONVERSION
4
c
SIGNAL
INTEGRATE
200ms
L
A
J
ERROR
SIGNAL
CORRECTION
MEASUREMENT
FIGURE
5-12.
A/O Converter
System
Timing.
5-15
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