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Appendix D: Measurement Considerations
I
= I
+ I
M
S
Where: I
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
M
S
where: I
As long as the internal and external offsets remain stable for a reasonable period of time, the
relative feature of the Model 6482 can be used to zero out offset current. With the source current
(I
) set to zero, the ammeter will only measure and display the offset current reading. When REL is
S
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
M
S
I
= I
M
S
where: rel is the rel value (I
When the rel value equals the sum of the offsets, only the source current will be displayed. See
Section 5
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.
Electrochemical effects
Error currents also arise from electrochemical effects when ionic chemicals create weak batteries
on a circuit board. These batteries could generate a few nanoamps of current between
conductors. Ionic contamination may be the result of body oils, salts, or solder flux. The problem is
further enhanced by high humidity (moisture) that decreases insulation resistance.
When building test fixtures, select insulators that resist water absorption, and use the fixture in a
moderate humidity environment. Also, be sure that all insulators are kept clean and free of
contamination.
Humidity
Excess humidity can reduce insulation resistance on PC boards and in test connection insulators.
Reduction in insulation resistance can, of course, seriously affect high-
impedance measurements. Also, humidity (moisture) can combine with contaminants to produce
offset currents caused by electrochemical effects (see
effects of moisture, keep humidity to a minimum (ideally <50 percent), and keep components and
connectors in the test system clean.
Triboelectric effects
Triboelectric currents are generated by charges created between a conductor and an insulator due
to friction. Here, free electrons rub off the conductor and create a charge imbalance that causes
the current flow. For example, bending a triaxial cable causes friction between the center
conductor (HI) and its surrounding insulator resulting in triboelectric currents.
Triboelectric currents can be minimized as follows:
D-4
I0
is the measured current.
M
I
is the source current.
S
I
is the internal input offset current.
I0
+ I
I0
EO
is the external offset current.
EO
+ I
- rel
I0
EO
+ I
I0
for details on relative.
Return to
Model 6482 Dual-Channel Picoammeter / Voltage Source Reference Manual
).
EO
"Electrochemical
Section Topics
effects"). To minimize the
6482-901-01 Rev. A / August 2012
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