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Figure 2-6.
CHASSIS
GROUND
-
JUMPER WIRE
LOCATION
Model
845AR
on
the
desired range.
This
reading
must
then be subtracted
from
all
subsequent
voltage
measure-
ments.
A
thorough understanding
of
these
effects
can
lead
to
reducing or eliminating
them
completely.
2-23.
THERMOELECTRIC VOLTAGES
2-
24.
If
a
circuit is
composed
of
two
dissimilar metals,
a
net voltage
will result
if
the
two
dissimilar junctions
are
maintained
at different
temperatures.
These
ther-
moelectric
voltages, also
known
as
thermals,
thermo-
couple
voltages,
or
Seebeek
voltages,
can be reduced
by using metals having low thermoelectric
potentials,
and
keeping
ail
junctions
at
the
same
temperature.
The
terminals
of
the
Model
84SAB
are
made
of
pure
copper,
gold-flashed to
prevent
tarnish.
For
lowest thermal
voltages,
all
connections
to the
Model
845AR
should
foe
made
with,
pure copper
wire.
Silver plated
copper or
solder coated
copper
also
produce
satisfactory
results.
Tinned
copper
is
less satisfactory
than
silver
-plated
or
copper
coated copper,
Nickel
and
nickel-based
alloys
are
not
suitable for
connections
to the
instrument.
Excellent results
can
be obtained using ordinary
TV
twin
lead,
or
even
lamp
cord
if
high
insulation
re-
sistance
is
not required.
If
shielding
is
necessary, use
a length
of flat
braid over the
cable.
2-25.
HIGH
SOURCE IMPEDANCE
2-26.
Due
to
the
very high
input resistance
and
ex-
treme
sensitivity
of the
Model 845AR,
it.
is
charge,
sensitive.
Thus,
a
person's
body
potential,
an
electro-
static voltage,
can cause
charge
redistribution
at
the
input
to
the
instrument and result
in
meter
needle
de-
flection
as a
hand approaches
the input terminals.
Careful
shielding will
eliminate
this
problem.
Also,
due
to
charges
that
may
be deposited on
the-
input
termi-
nals
when
the
OPR-
ZERO
switch
is set to
ZERO,
an
appreciable transient
will
result
when
the switch
is set
to
OPR
if
nothing
is
connected
to the
input
terminals.
Turning
the
switch
back and
forth
will
dissipate
this
charge, eliminating
the
problem.
With a
high
source
impedances,
the
response
of
the
instrument
is
unavoid-
ably slow due
to the
low
pass
filter
used
to
suppress
superimposed
noise.
However,
the design
of
the
low
pass
filter
is
such
that
common
mode
rejection
is
ex-
tremely
high
while
the
response time
for the
normally
encountered low source
impedances
is
very
fast,
2-4
2-27.
OVERLOAD
VOLTAGES
2-28.
The
instrument,
is
designed
to
withstand
up
to
1100
volts
dc or
1100
volts
peak ac
continuously applied
between any two
of
the
three input
terminals or between
cabinet
ground
and any
of
the
three input terminals
regardless
of the setting
of
the
RANGE
or
OPR-ZERO
switch.
However,
repeated or continuous overloads
above 200
volts in the
ranges below
3
millivolts will
result in dissipation
in
protective,
low -pass
-filter
resistor
RUG.
This
will
result
in
thermal
voltages
which
may
take several
minutes
to
subside
after the
overload
is
removed.
2™
29.
GUARDING
2™
30.
The
instrument has an inner chassis connected
to the
GUARD
terminal on
the front panel.
Ordinarily,
this
GUARD
terminal
is
strapped
to the
COMMON
termi-
nal,,
When
connected
in this
way
tSie
inner chassis
serves as a
shield.
This
greatly
improves
the
leakage
resistance
to
ground and
the
common mode
rejection.
However,
since
the
inner chassis
is
available
at
the
GUARD
terminal,
it
may
be driven
at
the
same
voltage
as
the
COMMON
terminal.
This
further
increases
the
leakage resistance and
common
mode
rejection
by
about
ten times.
The
voltage
used
to
drive the
GUARD
termi-
nal
should be
obtained
from
a separate
source
or by
means
of
a voltage divider connected
directly
across
the
source
so
that the
leakage currents
do
not
cause
voltage
drops across impedances
in the circuit
under
measurement.
2-31.
CREASING INPUT RESISTANCE
2-82,
in the 1
microvolt
to
i
millivolt
ranges, a
JS'
megohm
resistor
is
connected
directly
across
the input
of
the
instrument.
The
input
resistance
may
be in-
creased on
these ranges
by disconnecting
the
><3
meg-
ohm
resistor
where
it
attaches
to
the
RANGE
switch.
However,
the input resistance
will
no longer
be
well
defined.
Typical
input
resistances with
the
3$
megohm
resistor
removed
are as
follows:
t
Range
Input
Resistance
.1
uv
300
megohms
3
uv
1,000
megohms
.10
uv
3,
000
megohms
iv
to I
mv
10,000
megohms
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