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845AR
Figure
2-4.
BRIDGE
DETECTOR
-
FLOATING
NULL
DETECTOR
2-13.
MEASURING VOLTAGES
WITH
A
STANDARD
CELL
2-14.
The
Model
845AR may
be used
with a voltage
divider
and a standard
cell to calculate
unknown
volt-
ages
with
a
high
degree
of
accuracy.
Connect
the
equip-
ment
as
illustrated in
Figure
2-5.
Perform
the
pre-
liminary operation as
outlined
in
paragraph
2-5 and
proceed
as
follows:
a.
Place
the
Model
845AR
controls as
follows:
POWER
ON
OPR/ZERO
OPR
RANGE
as desired
b.
Adjust
the voltage
divider for zero or
null
deflection
on
the
Model
845AR
meter
while placing
the
RANGE
switch
to
successively
more
sensitive ranges.
Figure
2-5.
STANDARD
CELL
VOLTAG
E
MEASUREMENTS
c.
Calculate
the
unknown
voltage by
dividing
the
standard
cell
voltage
by
the final division ratio
of
the divider.
2-15.
USE
OF
ISOLATED
OUTPUT
2-16.
DC
ISOLATION
AMPLIFIER
2-17.
The
Model
845AR
may
be used
as a dc
isolation
amplifier having a
voltage gain
of
up
to
120
db,
de-
pending on
the settings
of
the
RANGE
switch and
the
OUTPUT LEVEL
control.
To
compute
the
maximum
voltage gain
on any
range
of
the
Model
845AR, use
the
following formula:
1 volt
(maximum
Voltage gain
in
db
=
20 log
^
isolated
output)
Range
(in
volts)
2-18.
RECORDER OUTPUT
2-19.
The Model
845AR
ISOLATED
OUTPUT
may
be
used
to
provide
an
output voltage, adjustable
from
zero
to
one
volt for
a
full-scale
meter
deflection for
use
with
a
recorder.
Since
the output
is
isolated
from
the input,
floating
measurements
can be
made
without
the
use
of
a
floating
recorder.
To
use
the adjustable
recorder
out-
put,
proceed
as
follows:
a.
Connect
the
recorder
to the
ISOLATED OUTPUT
terminals.
The
lower
ISOLATED
OUTPUT
terminal
is
connected
to
chassis ground.
If
a ground
reference
is
undesirable,
remove
the
jumper
wire above
R202 on
the
power
supply
circuit
board.
Refer
to
Figure
2-6
for
jumper
wire
location.
b.
Turn
the
recorder
on.
c.
Proceed
as outlined
in
paragraph
2-9
or 2-11, as
desired.
d.
Adjust
the
ISOLATED
OUTPUT LEVEL
control
for the desired output
to the
recorder.
This
control
has
a log taper so that
smooth
control
is
possible
at
both high and low
settings.
"Hotel
The
ISOLATED
OUTPUT
current
capability
is
100
microamperes
with
a
5
kilohm source
impedance.
2-20.
OPERATING NOTES
2-21.
SPURIOUS
VOLTAGES
AND
CURRENTS
2-22.
Voltage
measurements
at the
microvolt
level
involve
the
persistant
problems
of
thermoelectric
ef-
fects.
These
effects
may
be
compensated
for by
tempo-
rarily
disconnecting
the voltage
from
the circuit
under
measurement
and
noting the
meter
deflection
of
the
2-3
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