Keithley 2520, Руководство пользователя, Страница: 303 / 329

Model 2520 User’s Manual Measurement Considerations F-19
Offset currents
Internal offset current — The ideal ammeter should read zero when its input terminals
are left open. Practical ammeters, such as those in the Model 2520, do have some small
current that flows when the input is open. This current is known as the input offset current,
and it is caused by bias currents of active devices as well as by leakage currents through
insulators within the instrument.
The internal input offset current adds to the photodiode detector current so that the meter
measures the sum of the two currents:
I
M
= I
PD
+ I
I0
where: I
M
is the measured current.
I
PD
is the photodiode current.
I
I0
is the internal input offset current.
The offset current of the Model 2520 is sufficiently low so as to be negligible in most mea-
surement situations. Use the lowest measurement range possible to minimize offset cur-
rent effects.
External offset current — Offset currents can also be generated from external effects.
The external offset current also adds to the desired current, and the ammeter again mea-
sures the sum of the currents:
I
M
= I
PD
+ I
I0
+ I
EO
where: I
EO
is the external offset current.
The two main sources for external offset currents are leaky photodiodes and increased
photodiode leakage current caused by bias voltage. Generally, lower voltage bias values
result in lower photodiode leakage currents.
Dielectric absorption
Dielectric absorption in an insulator can occur when a voltage across that insulator causes
positive and negative charges within the insulator to polarize because various polar mole-
cules relax at different rates. When the voltage is removed, the separated charges generate
a decaying current through circuits connected to the insulator as they recombine.
To minimize the effects of dielectric absorption on current measurements, avoid applying
voltages greater than a few volts to insulators being used for sensitive current measure-
ments. In cases where this practice is unavoidable, it may take minutes or even hours in
some cases for the current caused by dielectric absorption to dissipate.