I-8
Applications Guide
Model 6485 Picoammeter Instruction Manual
shows an example of AC electrostatic coupling. An electrostatic voltage 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 cur-
rent can be calculated with the following equation:
Figure I-4
Electrostatic coupling
For example, two conductors, each with lcm
2
area and spaced lcm apart by air, will have
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 gen-
erated.
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.
illustrates an example of shielding. Made from a conductive material, the shield
is always connected to the low impedance input of the electrometer or picoammeter. If cir-
cuit low is floating above ground, observe special safety precautions to prevent anyone
from touching the shield. (See
“Floating measurements,” page 2-12
i
C
dV
d
-------
V
dC
dt
-------
+
=
V
Coupling
capacitance
Electrostatic
voltage source
Ground-referenced
signal conductor
C
i
i = C + V
dV
dt
dC
dt
Содержание 6485
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Страница 44: ...1 20 Getting Started Model 6485 Picoammeter Instruction Manual ...
Страница 66: ...3 6 Measurements Model 6485 Picoammeter Instruction Manual ...
Страница 138: ...9 20 Remote Operation Model 6485 Picoammeter Instruction Manual ...
Страница 159: ...11 CommonCommands ...
Страница 164: ...11 6 Common Commands Model 6485 Picoammeter Instruction Manual ...
Страница 165: ...12 SCPISignalOriented MeasurementCommands ...
Страница 190: ...14 12 SCPI Reference Tables Model 6485 Picoammeter Instruction Manual ...
Страница 214: ...16 14 Calibration Model 6485 Instruction Manual ...
Страница 219: ...A Specifications ...
Страница 221: ...B StatusandErrorMessages ...
Страница 227: ...C GeneralMeasurement Considerations ...
Страница 233: ...D DDCEmulationCommands ...
Страница 247: ...E ExamplePrograms ...
Страница 250: ...E 4 Example Programs Model 6485 Picoammeter Instruction Manual ...
Страница 251: ...F IEEE 488BusOverview ...
Страница 266: ...F 16 IEEE 488 Bus Overview Model 6485 Picoammeter Instruction Manual ...
Страница 267: ...G IEEE 488andSCPI ConformanceInformation ...
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