Keithley 6512, Instruction Manual

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Operation
2-8
7 PREAMP OUT
The PREAMP OUT jack provides a guard output for voltage
and resistance measurements. This output can also be used as
an inverting output or with external feedback when measur-
ing current or charge. The PREAMP OUT has a maximum
output value of ±300V and uses a standard 5-way binding
post.
WARNING
Hazardous voltage may be present at
the PREAMP OUT, depending on the
input signal.
8 COM Terminal
The COM terminal is a 5-way binding post that provides a
low connection for both the 2V ANALOG OUTPUT and the
PREAMP OUT. This terminal is also used for input low con-
nection when in guarded mode; COM is internally connected
to input low through a 100
Ω
resistor.
CAUTION
Do not connect PREAMP OUT, COM,
or 2V ANALOG OUTPUT to earth
ground when floating the input.
9 2V ANALOG OUTPUT
The 2V ANALOG OUTPUT provides a scaled 0-2V output
from the electrometer (2V output for full range input). The
output uses a standard 5-way binding post and is inverting in
the volts and ohms modes.
10 METER COMPLETE OUTPUT
This BNC connector provides an output pulse when the
Model 6512 has completed a reading; it is useful for trigger-
ing other instrumentation.
11 EXTERNAL TRIGGER INPUT
This BNC connector can be used to apply external trigger
pulses to the Model 6512 to trigger the instrument to take one
or more readings, depending on the selected trigger mode.
2.7 Basic measurement techniques
The paragraphs below describe the basic procedures for us-
ing the Model 6512 to make voltage, resistance, charge, and
current measurements.
2.7.1 Warm-up period
The Model 6512 is usable immediately when it is first turned
on. However, the instrument must be allowed to warm up for
at least two hours to achieve rated accuracy.
NOTE
While rated accuracy is achieved after the
two-hour warm up period, input bias cur-
rent may require additional time to be re-
duced to its optimum level. Allow two
more hours for input bias current to settle
to less than 10fA and eight hours for set-
tling to less than 5fA. In sensitive applica-
tions, it is preferable for the unit to be left
on continuously.
2.7.2 Input connections
The rear panel INPUT connector is a Teflon-insulated, 3-lug
female triax connector intended for all input signals to the
Model 6512. As shown in Figure 2-3, the center terminal is
high, the inner ring or shield is low, and the outer shield is
connected to instrument chassis ground. In the guarded
mode, the inner shield is driven at guard potential, while the
outer shield is chassis ground.
NOTE
The INPUT connector must be kept clean to
maintain high input impedance. Place the
supplied rubber dust cap on the INPUT con-
nector when the instrument is not in use.