2-8
Measurement Concepts and Connections
Model 6487 Reference Manual
Noise and safety shields
shows typical measurement shielding. In
(A), a noise shield is used
to prevent unwanted signals from being induced on the picoammeter input. Amps
measurements below 1
μ
A may benefit from effective shielding. Typically, the noise shield
is connected to picoammeter input LO. Additionally,
(B) shows an added safety
shield connected to earth ground and Model 6487 chassis. This type of shielding should be
used whenever hazardous voltages will be present in the test circuit.
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The maximum safe voltage between picoammeter LO and chassis
ground (common mode voltage) is 505V peak. The Model 6487 does
not internally limit the LO-to-chassis voltage. Exceeding 505V can cre-
ate a shock hazard.
If it is possible for the DUT or external supply to present more than
505V to the input HI, it is imperative that the connection between
input LO and the external voltage source be sufficiently low impedance
and capable of carrying the short-circuit current of the source, in order
that the LO not exceed 505V.
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The LO to chassis breakdown voltage is 505V. Exceeding this voltage
may cause damage to the instrument.
Figure 2-6
Shielding for measurements (unguarded)
Using a test fixture
Whenever possible, use a shielded low-leakage test fixture to make precision measure-
ments and for safety when high voltages (>30V) are used.
HI
LO
DUT
INPUT
B. Safety Shield
Safety
Earth
Ground
Chassis
Ground
Metal Noise Shield
Metal Safety
Shield
HI
LO
DUT
INPUT
Metal Noise Shield
A. Noise Shield
