Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes 2562 Meas are Low in High R
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A G R E AT E R M E A S U R E O F C O N F I D E N C E
Problem: Measure-
ments are Low in
High Resistance
Measurements
There can be several reasons for measure- Remedy cancel the offset by selecting the correct
ments that read low when making high resis- Use the force voltage and measure current reading range and using the zero function
tance measurements. technique to measure the high resistance. In of the meter.
this technique, the unknown resistance is put B. Use the alternate polarity technique. Off-
Probable Cause: in series with a picoammeter or electrometer set currents may be due to charge stored
Low Insulation Resistance ammeter and a constant voltage is applied. in the material (dielectric absorption),
The insulation resistance of the test fix- This voltage can come from the picoamme- static or triboelectric charge, or piezo-
ture could be in parallel with the device un- ter's internal source function or an external electric effects, and can be equal to or
der test. source. Since the ammeter has very low re- greater than the current stimulated by
sistance, essentially all the voltage appears the applied voltage. To use the alternat-
Remedy across the unknown resistance. The resulting ing polarity method, make the measure-
Use a test fixture and connecting cables current is measured by the ammeter and the ment once and record the result. Then
with higher insulation resistance. The use of resistance calculated using Ohm's law. reverse the polarity of the applied voltage
a driven guard also effectively increases the and measure again. Taking the average
shunt resistance. Probable Cause: Offset Current of the readings should give an accurate
result. The polarity reversal process can
Probable Cause: Remedies be repeated any number of times, and the
Low Input Resistance in the Meter A. Adjust the zero baseline of the meter. Me- resistance calculated based on a weighted
The input resistance of the voltmeter ters often have some offset in the front average of the most recent current meas-
could be too low for the measurement. end circuitry. After power-up, allow the urements.
meter to reach thermal equilibrium, then
Specifications are subject to change without notice.
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