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INSTRUCTION MANUAL
Digital
Multimeter
Models
172A.
173A
SECTION 5.
THEORY OF OPERATION
THEORY OF OPERATION
5-I.
GENERAL.
This
section
contains
circuit
descriptions
for
the
Model
172A/173A,
Model
1728
Battery
Pack option
and Model
1722
Digital
Interface
option.
(It
should
be noted
that
the
Theory
of
Operation
for
the
Model
1723,
IEEE Standard
488 Bus
Interface
option
is
provided
under
separate
cover,
see Model
1723
instruction
Manual.)
The descriptions
contained
in
this
section
are
broken
down
into
the
following
major
categories.
a.
Overall
Operation.
b.
Input
Signal
Conditioning.
c.
A/D Converter
and Digital
Circuits.
d.
Display
Board.
e.
Power
Supply.
f.
Battery
Pack
(Model
1728).
9.
Digital
Interface
(Model
1722).
5-2.
OVERALL OPERATION.
The overall
signal
flow
of
the
Model
172A/l73A
DHM is shown
in
Figure
5-l.
a.
Input
Signal
Conditioning.
An input
to
the
DMM (a resistance,
a dc voltage
or
current,
or an ac voltage
or
current)
is applied
to
the
appropriate
signal
conditioning
circuit.
The output
from
the
signal
conditioning
circuit
is a filtered
dc voltage
in
thr
range
of
either
0 to +3 volts
or 0 to 1300 millivolts,
which
is directly
prdportional
to
the
input
signal
within
the
measurement
range.
b.
A/D Converter
and Diqital
Circuits.
The A/D converter
changes
the
voltage
out
of
the
signal
conditioning
circuitry
into
digital
pulses
by the
charge
balance
technique,
and these
pul~as
are
accumulated
by the
digital
circuitry
during
the
measurement
conver-
sion
period.
At
the
end of
this
period,
the
digital
circuitry
uses
the
total
count
to
determine
the
correct
polarity
and value
of
the
digitized
input
signal,
which
is
then
displayed
with
the
appropriate
polarity,
range,
decimal
point
location
and function
on
the
LED display.
The A/D converter
auto
calibrates
and autozeros
itself,
as well
as the
signal
conditioning
circuitry,
to obtain
a high
measurement
accuracy.
c.
Digital
Interface
Options.
The display
information
of
the
Model
172A/l73A
can aIs<
be made available
as an output
by using
either
the
Model
I722
Digital
Interface
or
the
Model
1723
IEEE Standard
488 Bus
Interface.
Both
of
these
optically
isolated
options
car
also
be used
to program
range,
trigger
a conversion,
or
hold
the
reading
of
the
Model
172A/l73A.
d.
Power
Supply.
The power
supply
furnishes
regulated
dc voltages
for
all
of
the
DHH
circuitry,
and receives
its
power
from
either
the
ac line
or
the
optional
Model
1728
Rechargeable
Battery
Pack.
5-3.
INPUT SIGNAL CONDITIONING.
Most
of
the
input
signal
conditioning
circuitry
is
contained
on the
main
printed
circuit
board
(mother
board)
~~-466
and shown
on schematic
29145E
(Sheet
I).
These
circuits
consist
of
input
switching,
an ohms converter,
a dc
attenuator,
an ac/dc
converter,
a clamp
and an active
filter.
The current
to voltage
(I/V)
converter
circuitry
is
located
on the
current
board
~~-406
for
the
Model
l73A
and
mounted
on a shield
for
Model
172A.
Both
circuits
are
shown on Schematic
27478Q.
5-
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