Keithley 6485 - Capacitor leakage current; Measuring high resistance with external bias source

Model 6485 Picoammeter Instruction Manual Applications Guide I-19
NOTE The details on page I-12 on range change transients may be particularly rele-
vant to this application.
Capacitor leakage current
Figure I-15 shows how to measure the leakage current for a capacitor. The magnitude of
the leakage is dependent on the type of dielectric and the applied voltage. A resistor and a
diode are used to limit noise for the measurement.
For this test, a fixed bias voltage is to be applied to the capacitor for a specified time to
allow the capacitor to fully charge (current decays exponentially with time). The leakage
current is then measured. After the measurement, the voltage source is set to output 0V for
a specified time to allow the capacitor to discharge.
Figure I-15
Connections; capacitor leakage current test
Measuring high resistance with external bias source
The 6485 Picoammeter can be used to make high resistance (>1GΩ) measurements when
used with an external voltage source. High resistance measurement applications include
insulation resistance testing and resistivity measurements of insulators.
To measure high resistance, a constant voltage source is placed in series with the unknown
resistance and the picoammeter. Since the voltage drop across the picoammeter is negligi-
ble, essentially all the voltage appears across the unknown resistance. The resulting cur-
rent is measured by the picoammeter. The resistance is then calculated using Ohm's Law:
where: V is the sourced test voltage
I is the measured current
Programmable
V-Source
A
+
-
6485
Picoammeter
Metal
Shield
HI
LO
HI
LO
Equivalent Circuit
R
V
I
----=