Keithley 630 Instruction Manual page 12

MODEL 630 POTENTIOMETRIC ELECTROMETER OPERATION
2-5. RECOFDER OUTPUT.
a. Recommended recorder for use with the Model 630 is the Keithley Model 370. Any re-
corder used must be able to float 500 volts off ground and its input must be fully isolated
(1010 ohm minimum leakage resistance to ground). The Model 370 meets these requirements.
b. before attaching the recorder, set all Reference Voltage Dials to zero. Disconnect
the unknown voltage and short the Model 630 high input to the low. Set the NULL Switch
to 10 MV.
Connect the recorder to the OUTPUT Terminals on the Model 630 rear panel. The
370 Recorder mates directly with the Model 630 using the 3701 Cable.
c. Set the Reference Voltage Dials to 10 millivolts to apply an accurate lo-millivolt
potential to the null detector on the lo-millivolt null range.
This will provide a full-
scale recorder output of 1 volt at 1 milliampere. On the l-millivolt null range, the re-
corder may load the Model 630 output. On this range, output resistance is 100 ohms.
d.
To obtain accurate results and/or
to
prevent ,damage to the instruments, the recorder
must be able to float off-ground with the Model 630. Leakage and pickup between the two
instruments should also be minimized.
The Model 370 meets these requirements.
NOTE
On the l-millivolt range there is a 10% loading error if the recorder draws
1 ma of current. Thus the output for full scale deflection will be .90 volt,
1 ma.
1. Make sure neither recorder terminal is grounded. Use a 3-wire grounded power line
for the recorder.
If a 2-wire line is used, connect the recorder chassis and the Model
630 chassis with a separate lead.
2. Minimize all sources of leakage between the output terminals, the recorder and
ground. Use polystyrene or Teflon-insulated cable where possible. If the connecting
wires are shielded, connect the shield to the LO Post.
3. Avoid long leads between the Model 630 and the recorder.
Do not short either Model 630 output terminal to the case; this may damage the
Kelvin-Varley divider.
2-6.
EFFECTIVE INPUT RESISTANCE.
a. The Model 630 input resistance varies. It can be set either 1013 or 1010 ohms for
the l-volt and lower ranges, and it is 1010 ohms for ranges above 1 volt. These, however,
are not the Model 630's effective input resistances.
Its input resistance is considerably
higher due to the potentiometric principle of operation. When reference voltage (Refer-
ence Voltage Dial setting) is much greater than meter readings the value is
Ri* = Rn(
Ed )
Equation 1
v
Where Rin is the effective input resistance of the Model 630;
Ed is the setting of the Reference Voltage Dials in volts;
'Rn is the input resistance of the null detector in ohms;
V is the null detector meter reading in volts.
1067R
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