Keithley 153, Instruction Manual, Page: 15 / 51

MODEL 153
OPERATING INSTRUCTlO:�S
d.
Use high resistance,
low-noise
materials - such as Teflon (recoltlllended),
polyethy­
lene or pol�'styrene
-
for insulation. The insulation leakage resistance of l(:ads
sh:l'Jld
be greater than
500 megohms
to maintain the Model
153
input
resistance.
ExcessivE:
lea;:­
age reduces the
accuracy
of readings from high
impedance
sources. Voltage breai:d:>\,'n of
the cable must also be high:
1000
volts center conductor to inner shield;
500
volts bE:­
tween
shields. The Model
1534
C�ble meets these
requirements. Triaxial
cables
USE:d
s
ho
u
l
d
be
a
low-noise type which
employ
a graphite
or
other
conductive coating bet�een
the dielectric and
the surrounding
shield braid.
e.
Any change in
the
capacitance of the measuring circuit to
ground will
cause
extran­
eous disturbances.
For i
n s
t a
n c e , cable flexture changes the cable capacitance and thus
aff ects
meter
readings.
Make
the measuring setup
as
rigid
as
possible and
tie do�n
connec­
ting cables to prevent their movement. If a continuous vibration
is
present, it may
ap­
pear at the output
as
a sinusoidal
s
i g
n
a l and
other
precautions may be necessary
to
iso­
late
the instrument and the connecting cable from the vibration.
f. For
lm\l impedance
measurements,
unshie lded l eads and the
Mode
1
6012
Adapter
may
be used. Since
the
circuit low and ground are connected with the Adapter, do
not
use
it
for off-ground measurements.
NOTE
Keithley Instruments, Inc., has several booklets available on low voltage measure­
ments and low current
high
r e s
i
s
t
an c
e
measurements.
A
list is available from
Keithley Instruments, Inc., or its representative.
2-9.
ACCURACY CONSIDERATIONS. For
sensitive
measurements -- 100
m
i
l
l i volt
s
and below
-­
other considerations besides the instrument affect accuracy. Effects not noticeable when
working with higher voltages are very important with microvolt signals.
The Model
153
only reads the signal received at its input; therefore, it is important that this Signal
be
properly transmitted from the source. The following paragraphs indicate
factors which
affect accuracy: t hermal emf's, shielding and circuit connections.
2-10. THERMAL EMF'S.
a. Thermal emf's (thermo-electric potentials) are generated by t hermal gradients be­
tween any two junctions of dissimilar metals. These can
be
significatn compared
to
the
signals which the Model
153
can measure.
b. Thermal emf's can cause t he following problems:
1 . Metal instability or zero offset much higher than normal. Note, though, the
Model
153
may have some offset (paragraph 2-3).
2.
Meter
is
very sensitive to ambient t emperature variations.
This
is seen by touch­
ing the cirCUit, by putting a heat source near the circuit, or by a regular pattern
of
instability, corresponding to heating and air conditioning systems or changes in
sun­
light.
9