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Model 6517B Electrometer Reference Manual
Section 6: Test sequences
Alternating polarity resistance/resistivity test
The alternating polarity resistance/resistivity test is designed to improve high resistance/resistivity
measurements. These measurements are prone to large errors due to background currents. By using
an alternating stimulus voltage, it is possible to eliminate the effects of these background currents.
This test measures surface or volume resistivity or resistance, as selected in the CONFIGURE
RESISTANCE menu. See Resistance and resistivity measurements for the test circuit diagrams and
connections to the Model 8009. Refer to the Model 8009 Instruction Manual for information on
installing the insulator sample in the test fixture.
When this test is run, the V-source alternates between two voltages (V-OFS + V-ALT) and (V-OFS -
V-ALT) at timed intervals (MEAS-TIME). Current measurements are taken at the end of each of these
alternations and after calculation of Icalc resistance values are computed. Icalc is a weighted average
of the latest four current measurements, each at the end of a separate alternation. The resistance
value is then converted to a resistivity value if the meter has been configured for resistivity
measurements. The first few readings can be rejected (DISCARD XXX RDGS) as the sample or
resistance achieves a steady-state response to the alternating voltage. After this, the alternation
continues until a specified number of readings (STORE XXX RDGS) have been stored in the buffer.
The time required to complete a sequence is (STORE + DISCARD + 4) * MEAS-TIME. For example,
a sequence alternating at 15 second intervals, discarding 3 readings, and storing 3 readings takes2.5
minutes.
The following figure shows an example of the alternating polarity test using the test parameters
shown and the resulting sample current from a typical high resistance sample. Note that the sample
currents shown exhibit some capacitive decay, as many high resistance samples also tend to have
significant capacitance.
When the alternating polarity sequence is first armed by pressing SEQuence and then ENTER, the
settings for the current measurements made internally to the sequence are preset to the settings for
the amps function. If the amps function is set to a specific range, the sequence defaults to that range.
If the amps function is autoranging, the sequence defaults to autoranging. The range can be changed
after the sequence is armed by pressing the manual or AUTO range keys. The alternating polarity
sequence does not autorange past the 2 nA range. If the resistance/resistivity to be measured is high
and a more sensitive range is required, the user must set this range manually using the manual
RANGE keys. (For the 20 pA and 200 pA ranges, use a measure time of at least 15 seconds).
While in the armed condition, the sequence parameters may be changed (CONFIG-SEQ), the range
may be changed, the output result type may be changed (resistance, surface of volume resistivity),
and the resistivity parameters edited. To run the sequence, press TRIG after arming. Pressing EXIT
after arming disarms the sequence, and returns the Model 6517B to the function in use when it was
armed.
During execution, the sequence displays "--------" until the first reading becomes available and is sent
to the buffer. After this, the latest calculated value is displayed. If, at the end of any alteration the
current exceeds the amps range in use, the error +618 Resistivity Out Of Limit occurs and the
sequence aborts, returning it to the function in use before it was last armed. If the alternating polarity
sequence calculates a current of zero, "<Infinity>" is displayed, but the sequence continues. A lower
current range should be selected.
The alternating polarity test is selected and configured from the CONFIGURE SEQUENCE menu
(APPLICATIONS; R/RESISTIVITY; ALT-POLARITY).
Figure 73: Alternating polarity test example
6517B-901-01 Rev. C / August 2015
6-7
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