Front Panel Operation
2-42
WARNING
A hazardous voltage (400V) may auto-
matically be set for the ohms function
when AUTO V-Source is selected. Table
2-11 identifies the ohms ranges that use
the high voltage.
2.7.4 Multiple display
There is one multiple display that is unique to the ohms func-
tion.
Measure/Source: When this NEXT display is selected, the
amps measurement and V-Source value are shown on the
secondary display. The resistance measurement is shown on
the primary display.
2.7.5 Ohms measurement considerations
Some considerations for making accurate resistance and re-
sistivity measurements are summarized in the following
paragraphs. High resistance measurements (above 1M
Ω
)
may exhibit problematic background currents (see paragraph
2.21) and can be improved by using the Alternating Polarity
Test Sequence (see paragraph 2.14). Additional measure-
ment considerations are summarized in paragraph 2.21. For
comprehensive information on precision measurements, re-
fer to the Low Level Measurements handbook, which is
available from Keithley.
LEAKAGE RESISTANCE
Even though the FVMI method for resistance measurements
minimizes the effects of leakage resistance, there some cases
where leakage can affect the measurement. For example, test
fixture leakage paths may appear in parallel with the device
being measured, introducing errors in the measurement.
These errors can be minimized by using proper insulating
materials (such as Teflon) in test fixture terminals and keep-
ing them clean and moisture free.
Leakage currents in the test fixture can be cancelled by per-
forming a REL on the current component of the measure-
ment (see Cancelling Test Fixture Leakage Current in
paragraph 2.7.1).
VOLTAGE COEFFICIENT
The measured value of a high-megohm resistor will often
vary with the applied voltage. Such variation in resistance is
known as the voltage coefficient, and is usually expressed in
percent/volt or ppm/volt values. To obtain consistent test re-
sults, these resistors should always be biased at the same
voltage.
The Model 6517A can be used to characterize such resis-
tance changes by measuring the resistance with a number of
different applied voltages. Once the variations are known,
the voltage coefficient of the resistor being tested can be de-
termined.
TEST VOLTAGE and ELECTRIFICATION TIME
Test Voltage —
Typically specified test voltages to be ap-
plied to the insulator sample are 100V, 250V and 1000V.
Higher test voltages are sometimes used, however the maxi-
mum voltage that can be applied to the Model 8009 is 1000V,
which is the maximum output of the Model 6517A V-Source.
Unless otherwise specified, the applied direct voltage to the
insulator sample should be 500V.
Electrification Time —
Electrification time (also known as
bias time) is the total time that the specified voltage is ap-
plied to the insulator sample when the measurement is taken.
For example, for an electrification time of 60 seconds, the
measurement is to be taken after the insulator sample is sub-
jected to the applied test voltage for 60 seconds. The conven-
tional arbitrary electrification time is 60 seconds. Keep in
mind that special studies or experimentation may dictate a
different electrification time.
CURRENT MEASUREMENT CONSIDERATIONS
Ohms measurements are performed by forcing voltage and
measuring current (FVMI). Thus, accurate measurements re-
quire accurate current measurements. Current measurement
considerations are covered in paragraph 2.6.3.
NOTE
Capacitive inputs will increase preamplifier
noise, resulting in increased noise across the
voltage source terminals. See page 2-29 for
details.
2.8
Charge measurements (Q)
The Model 6517A is equipped with four coulombs ranges to
resolve charges as low as 10fC (10
-14
C) and measure as high
as 2.1µC. In the coulombs function, an accurately known ca-
pacitor is placed in the feedback loop of the amplifier so that
the voltage developed is proportional to the integral of the in-
put current in accordance with the formula:
The voltage is scaled and displayed as charge.
V
1
C
---- idt
∫
Q
S
C
-------
=
=
