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limiting resistor be used to prevent exceeding the maximum
2.5.5 Grounding
Considerations
overload input limit of the Model 455 in the event the item
under test is shorted.
Input LO (outer ring of input connector) should be connected
2.5.4 Zero Time Stability
to a potential at the source which
is within
30V RMS of
power line ground of the Model 455 or the equipment near it.
The typical zero time stability of the Model 455 is less than
one count per week, exclusive of temperature effects.
Connection
to power line ground is not made internally
in
order to avoid possible ground loops.
SHORT
CIRCUIT
TEFLON
CURRENT
-
LIMITER
INSULATOR
VVV'I
MODEL
485
Figure 2-4. Test Fixture Guarding
2.5 APPLICATIONS
The following
applications
can be automated
to various
degrees with the use of programmable equipment over the
IEEE-455 bus. The Model 455 becomes fully programmable
with the addition of the Model 4553 interface. Programmable
sources are available from Keithley and other manufacturers.
The documentation
that is provided with the Model 4553 in-
cludes example programs using various controllers.
The Model 455 may be used in conjunction
with the Model
6104. The Model 6104 is a guarded and shielded test box for
2-terminal or 3-terminal
connections.
The Model 6104 pro-
vides
excellent
electrostatic
shielding
and high
isolation
resistance for the component
under test. Clips plug into
banana jacks allowing the user to fashion modified connec-
tions to suit the component
to be tested.
The Model 6104 is useable with the Model 455 picoammeter
and Keithley voltage supplies, such as the Model 230. When
connected together as a system, using the cable and connec-
tor options, they form a complete test facility for measuring
leakage current and other low current parameters for various
test applications.
A possible test set up is hewn in Figure 2-5.
The measurement sample would be connected
between Jl
and J7 or J6.
EXT. SOURCE
MODEL
BLhK
[
j4
45
j
CASE
Figure 2-5. Leakage Test Sat Up Using Modal 6104
2.9.1 Op Amp Input Offset Currents
As the parameters of new BIFET, BIMOS, and FET op amps
approach
the
parameters
of the
ideal
op amp,
such
characteristics
as the input
offset
current
are breaking
through the 1pA level. Use the Model 455 to measure these
low level currents as shown in Figure 2-6. Note the connec-
tion of Model 455 input low.
Figure 2-6. OP Amp Input Offset Currents
Measurement
2-5
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