Keithley 6485 Instruction Manual page 30

2-8 Measurement Concepts
Noise and safety shields —
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,
nected to earth ground via the ground link.
WARNING
CAUTION
Figure 2-4
Shielding for measurements (unguarded)
HI
INPUT
(220V Peak)
LO
A. Noise Shield
Figure 2-4
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.
The LO to chassis breakdown voltage is 500V. Exceeding this voltage
may cause damage to the instrument.
Metal Noise
Shield
DUT
(220V Peak)
B. Safety Shield
Model 6485 Picoammeter Instruction Manual
shows typical measurement shielding. A noise
HI
INPUT
LO
Figure 2-4 shows LO con-
Metal Noise
Shield
Metal Safety
Shield
DUT
Safety
Earth
Ground