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G-8
Applications Guide
Figure G-4
source in the vicinity of a conductor, such as a cable or trace on a PC board, generates a
current proportional to the rate of change of the voltage and of the coupling capacitance.
This current can be calculated with the following equation:
For example, two conductors, each with lcm
almost 0.1pF of capacitance. With a voltage difference of 100V and a vibration causing a
change of capacitance of 0.01pF/second (a 10% fluctuation), a current of 1pA will be
generated.
To reduce the effects of the fields, a shield can be built to enclose the circuit being mea-
sured. The easiest type of shield to make is a simple metal box or meshed screen that
encloses the test circuit. Shielded boxes are also available commercially.
Figure G-5
shield is always connected to the low impedance input of the electrometer or picoammeter.
If circuit low is floating above ground, observe special safety precautions to prevent any-
one from touching the shield, such as triaxial cable with the outer shield at earth potential.
shows an example of AC electrostatic coupling. An electrostatic voltage
Figure G-4
Electrostatic coupling
C
Coupling
capacitance
V
Electrostatic
voltage source
illustrates an example of shielding. Made from a conductive material, the
dV
dC
------ - V
------ -
=
+
i
C
d
dt
i
Ground-referenced
signal conductor
dV
i = C
dt
2
area and spaced lcm apart by air, will have
Model 6487 Reference Manual
dC
+ V
dt
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