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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 source current so that the meter measures the
sum of the two currents:
I
= I
+ I
M
S
I0
where; I
is the measured current.
M
I
is the source current.
S
I
is the internal input offset current.
I0
Internal current noise — Peak-to-peak noise over any one minute interval will be within
400aA during 90% of the interval for following conditions:
•
The triax input connectors of the Remote PreAmp capped, and on the 1pA range for at
least three minutes.
•
Auto filter on.
•
Auto zero off.
•
Source delay set to 0 sec.
External offset current — Offset currents can also be generated from external effects, such
as electrochemical effect (discussed below). The external offset current also adds to the source
current, and the ammeter again measures the sum of the currents:
I
= I
+ I
+ I
M
S
I0
EO
where; I
is the external offset current.
EO
As long as the internal and external offsets remain stable for a reasonable period of time, the
Relative feature of the SourceMeter can be used to zero out offset current. With the source cur-
rent (I
) set to zero, the ammeter will only measure and display the offset current reading.
S
When REL is enabled, the display will zero. What happens is that the offset current reading is
acquired as the rel value and is subtracted from present and future readings. When the source
current is applied, the displayed reading will not include the offset:
I
= I
+ I
+ I
- rel
M
S
I0
EO
I
= I
M
S
where; rel is the rel value (I
As long as the rel value equals the sum of the offsets, only the source current will be dis-
played. See Section 7 for details on Relative.
Offset current drift is a function of time and/or temperature. Therefore, when offset current
drifts significantly, you will have to re-zero the ammeter.
+ I
).
I0
EO
Measurement Considerations
F-7
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