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INSTRUCTION MANUAL
Digital
Multimeter
Models
172A.
173A
THEORY OF OPERATION
I)
The dc attenuator
is
shown on the
simplified
schematic
(Figure
5-3).
On the
300mV
and 3V ranges,
K202 is
de-energized
as shown
and no attenuation
occurs.
&bove
r3 volts.
K202 will
energize.
This
means
that
the
output
on the
3OV and 300V ranges
will
be l/l00
of
the
input
as derived
from
the
following
equation.
I ODK
I
150K + 9.75M
+ IOOK
=
100
2)
For
1200 voltrange,
switch
Q206 will
also
be on,
which
makes
tie
output
l/l000
of
the
input.
Thus,
for
all
dc ranges,
a full
scale
dc voltage
input
will
result
in either
a ?3 volt
or
?300 millivolt
output
from
the
attenuator,
and
the
A/D converter
gain
(buffer)
will
be XI or
XI0
accordinqly.
Table
5-2
lists
the
respective
gains
for
dc
voltage
(and
ac voltage)
ranges.
d.
AC/DC Converter.
TABLE 5-2.
Gain
Chart
For
AC/DC Voltase
RANGE CONTROL L I NE8
RLD
+a v
+a v
0 v
0 v
0 v
1
H
-12
v
-12
v
-12
v
-12
v
+8 v
I)
The basic
transfer
function
of
the
AC/DC converter
is a shown
on the
simplified
schematic
(Figure
5-4).
The resistor
values
were
ielected
SO
the
IVacrmsin
= -IVdcOU-
Above
3 volts,
feedback
resistance
(Rf)
is
reduced
to keep
the
output
always
less
than
-3Vdc.
The dc output
is a half-wave
rectified
sine
wave,
and
the
converter
is
average
responding,
calibrated
to the
rms value
of
a sine
wave.
2)
Cl blocks
dc inputs,
and the
dc offset
voltage
of
the
amplifier
is autozeroed
oi
C4,
R and C provide
some output
filtering,
along
with
the
active
filter
(to
be descri!
later.)
3)
In actual
circuit
operation
(as
shown
on Schematic
29145E,
Sheet
1) feedback
resistance
of
lJ201
is controlled
by K201 and K101.
With
both
relays
de-energized
as
shown,
the
overall
gain
of
the
ac/dc
converter
is
unity
(i.e.
I VACrms
input
= -1Vdc
1
With
only
K20l
energized,
gain
is
ilOO,
and with
both
relays
energized
gain
is
+lOOO.
The
relays
are
controlled
by the
relay
drivers
shown
in
the
lower
left
corner
Of
the
schematic.
The relay
drivers
are,
in
turn,
controlled
by signals
front
the
RLD and H
lines
from
LSI circuit
UVOI.
See Table
5-2
for
gain
chart
of
the
ac voltage
ranges.
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