Keithley 610C, Instruction Manual

Page: 12 / 44

SECTION 2.
2-1.
INPllT
CONNECTIONS.
8.
The Model 610C INPUT Receptacle is a Teflon­
insulated UHF-type connector. A shield cap is provid­
ed. The Ground Post7 below the Receptacle, is connect­
ed to Chassis ground.
b. When working with a high impedance source, care­
fully shield the input connection and the source since
any variation.in the electrostatic field near the in­
put will cause definite meter disturbances.
c. Use high resistance, low-loss materials -- such
as Teflon (recommended), polyethylene or polystyrene
-- for insulation. The insulation leakage resistance
of text fixtures and leads should be several orders of
magnitude higher than the internal resistance of the
source. Excessive leakage reduces the accuracy of
reading from high impedance sources. Coaxial cables
used should be a low noise type which employs a graph­
ite or other conductive coating between the dielectric
and the surrounding shield braid.
NOTE
Clean� dry connections and cables are very
important to maintain the value of atl in­
sulation materials. Even the best insula­
tion will be compromised by dirt, dust,
solder flux, films of oil or water vapor.
A good cleaning agent is methyl alcohol,
which dissolves most common dirt without
chemically attacking the insulation.
Dry
the cables or connections after washing
with alcohol or use dry nitrogen if avail­
able. Or, if it is available, Freon is an
excellent cleaning agent.
d. When working with a high impedance source, any
change in the shunt capacitance of the input circuit
will cause disturbances in the reading.
Make
the mea­
suring setup as rigid as pOSSible, and tie down con­
necting cables to prevent their movement. A continu­
ous vibration may appear at the output as a sinusoidal
signal, and other precautions may be necessary to iso­
late the instrument and the connecting cable from the
vibration.
e. For low impedance measurements - below 108 ohms
or above 10-8 ampere - unshielded leads and a binding
post adapter may be used.
f. When the Model 610C is used on the most sensi­
tive current range with the FEEDBACK Switch at FAST,
some insulators - such as Teflon - may produce ran-
10m
signals which show up as erratic meter deflections.
·".nsulation used in the Model 6l0C is carefully select­
zd to minimize these spurious signals.
4
OPERATION
g. If a well shielded chamber and a well made high
impedance transfer switch is available, it is advan­
tageous to connect the Model 610C to the ctrcuit only
when a reading is being made.
In some cases, the off­
set current can charge the external test circuitry.
One example of this occurs when measuring a capaci­
tor's leakage resistance by observing the decay of the
terminal voltage. If the leakage current is less than
the offset current (less than 5 x 10-15 ampere), there
may be no decay of the terminal voltage when the Elec­
crometer is left connected across the capacitor's ter­
minals.
IMPORTANT
Make certain that the OUTPUT SWITCH 5108
(lMA-3V)
is set to the
"3V"
position un­
less a
1mA
recorder is connected at JlOl.
2 -2
•
PRELIMINARY PROCEDURES.
a. Check the
117-234V
Switch and the Fuse for the
proper ac line voltage.
b. Set the controls as follows:
ZERO CHECK Button
Range Switch
Multiplier Switch
FEEDBACK Switch
METER Switch
lmA-3V
Switc.h
LOCK
VOLTS
1
NORMAL
POWER OFF
3V
c. Connect the power cord and turn the METER Switch
to CENTER ZERO. Within ten seconds, the meter pointer
should come to the center position. If not, adjust
the meter zero with the MEDIUM and FINE ZERO Controls.
Normally, there is no need to use the COARSE ZERO
Switch.
d. After a few moments increase the voltage sensi­
tivity by advancing the Multiplier Switch to
.3,
.1,
etc. Continue zeroing with the FINE ZERO Control.
e. After long periods of storage or after an over­
load, the Model 610C may drift excessively. The input
transistors are insensitive to mechanical shock; how­
ever,
a
severe input overload may cause a zero offset.
This is corrected with the Zero controls. Drifting,
though, can occur for several hours.
f. Although the offset current of the Electrometer
is much below that found in conventional voltmeters,
it can be observed on the meter. The current charges
the input capacitance, and the Electrometer appears to
drift when the input is open.
Use the ZERO CHECK But­
ton to discharge�the charge build-up.