2-8 Measurement Concepts Model 6485 Picoammeter Instruction Manual
Noise and safety shields — Figure 2-4 shows typical measurement shielding. A noise
shield is used to prevent unwanted signals from being induced on the picoammeter input.
Amps measurements below 1µA may benefit from effective shielding. Typically, the noise
shield is connected to picoammeter input LO. Additionally, Figure 2-4 shows LO con-
nected to earth ground via the ground link.
WARNING The maximum safe voltage between picoammeter LO and chassis
ground (common mode voltage) is 42V. The Model 6485 does not inter-
nally limit the LO-to-chassis voltage. Exceeding 42V can create a
shock hazard.
If it is possible for the DUT or external supply to present more than
42V 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 42V.
CAUTION The LO to chassis breakdown voltage is 500V. Exceeding this voltage
may cause damage to the instrument.
Figure 2-4
Shielding for measurements (unguarded)
HI
LO
DUT
INPUT
(220V Peak)
Metal Noise
Shield
A. Noise Shield
HI
LO
DUT
Metal Noise
Shield
B. Safety Shield
Metal Safety
Shield
Safety
Earth
Ground
INPUT
(220V Peak)
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