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OPERATION
MODEL 630 POTENTIOMETRIC ELECTROMETER
b.
To find
the loading
effect
the Model 630 will
have on a circuit,
use Equation
1 to
compute its
effective
input
resistance.
At null,
where V = 0, the input
resistance
is
infinite.
Off null,
the Model 630 input
resistance
is usually
much greater
than the
source
resistance,
and loading
will
not be enough to affect
the measurement
accuracy.
The
graph
in Figure
6 shows the Model 630 effective
input
resistance
for
the l-millivolt
to
l-volt
null
ranges
with
the DETECTOR INPUT Switch
set to OPEN.
Note that
the input
resis-
tance
for
a reading
1.0%
off
null
is
1015 ohms.
NOTE
For a ful.ler
treatment
of paragraphs
2-6 and 2-7,
send for
Keithley
Instruments
Product
Note,
"The Effective
Input
Resistance
of Potentiometric
Voltmeters."
0.6
zo.5
0
:,o.,
"
0.3
0.2
0.L
18
FIGURE 6.
Model 630 Effective
Input
Res
~llrluw
/.
F,rrecr
or
xxlL^ce
KeSlStnnCc
on
tance.
The graph
shows the effective
input
Model 630 Null
Sensitivity.
The graphs
are
resistance
for
the l-millivolt
to l-volt
for
the two null
detector
input
resistance
ranges
when the DETECTOR INPUT Switch
is
settings.
Ideally,
V = Vs - Ed as defined
set to OPEN.
Percent
off
null
is
160 V/Ed
in equation
2.
The graphs
indicate
the per-
as defined
in Equation
1.
cent
of the actual
difference
between
the
source
and reference
voltages
the meter
will
indicate.
10
0266R
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