Table of Contents
Advertisement
Section 4: Basic measurements
Model 6517B Electrometer Reference Manual
Multiple display
There is one multiple display that is unique to the ohms function.
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.
Ohms measurement considerations
Some considerations for making accurate resistance and resistivity measurements are summarized in
the following paragraphs. High resistance measurements (above 1 M) may exhibit problematic
background currents and can be improved by using the alternating polarity test sequence (see
Other
measurement considerations
(on page 4-35) for additional information). For comprehensive
information on precision measurements, refer to the Low Level Measurements Handbook, which is
available from Keithley Instruments.
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 by keeping them clean and moisture free.
Leakage currents in the test fixture can be cancelled by performing a REL on the current component
of the measurement.
Voltage coefficient
The measured value of a high-megohm resistor often varies 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 results, these resistors should always be biased at the same
voltage.
The Model 6517B can be used to characterize such resistance 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 determined.
Test voltage and electrification time
Test voltage
Typically specified test voltages to be applied to the insulator sample are 100 V, 250 V and 1000 V.
Higher test voltages are sometimes used, however the maximum voltage that can be applied to the
Model 8009 is 1000 V, which is the maximum output of the Model 6517B V-source. Unless otherwise
specified, the applied direct voltage to the insulator sample should be 500 V.
Electrification time
Electrification time (also known as bias time) is the total time that the specified voltage is applied 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 subjected to the applied test
voltage for 60 seconds. The conventional arbitrary electrification time is 60 seconds. Keep in mind
that special studies or experimentation may dictate a different electrification time.
4-30
6517B-901-01 Rev. C / August 2015
Advertisement
Table of Contents
