F-4 Measurement Considerations Models 2500 and 2502 User’s Manual
Generated currents
Any extraneous generated currents in the test system will add to the desired current, caus-
ing errors. Currents can be internally generated, as in the case of instrument input offset
current, or they can come from external sources such as insulators and cables.
Offset currents
Internal offset current — The ideal ammeter should read zero when its input terminals
are left open. Practical ammeters, however, 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 source current so that the meter measures the
sum of the two currents:
I
M
= I
S
+ I
I0
where: I
M
is the measured current.
I
S
is the source current.
I
I0
is the internal input offset current.
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
M
= I
S
+ I
I0
+ I
EO
where: I
EO
is the external offset current.
As long as the internal and external offsets remain stable for a reasonable period of time,
the relative feature of the Model 2500 can be used to zero out offset current. With the
source current (I
S
) set to zero, the ammeter will only measure and display the offset cur-
rent reading. 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 read-
ings. When the source current is applied, the displayed reading will not include the offset:
I
M
= I
S
+ I
I0
+ I
EO
- rel
I
M
= I
S
where: rel is the rel value (I
I0
+ I
EO
).
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 cur-
rent drifts significantly, you will have to re-zero the ammeter.
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