Figure 2-4 Shielding For Unguarded Measurements - Keithley 6514 Instruction Manual

Noise and safety shields — Figure 2-4 shows typical shielding for unguarded measure-
ments. A noise shield is used to prevent unwanted signals from being induced on the electrom-
eter input. Measurements that may benefit from effective shielding include unguarded volts and
ohms, amps below 1uA, and low level coulombs.
Typically, the noise shield is connected to electrometer input LO. However, better noise per-
formance may be achieved by connecting the noise shield to both input LO and chassis ground.
Electrometer LO can be connected to chassis ground by installing the ground link between the
COMMON and CHASSIS binding posts.
A safety shield is required whenever a hazardous voltage (>30V) is present on the noise
shield or when the test circuit (DUT) is floated above earth ground at a hazardous voltage level
(see "Floating Measurements"). Connections for the safety shield is shown in Figure 2-4B. The
metal safety shield must completely surround the noise shield or floating test circuit, and it must
be connected to safety earth ground using #18 AWG or larger wire.
Figure 2-4
Shielding for
unguarded
measurements
A. Noise Shield
B. Safety Shield
HI
INPUT
250V PK
LO
HI
Chassis
Ground
INPUT
250V PK
LO
Measurement Concepts
Metal Noise Shield
DUT
Metal Noise Shield
Metal Safety Shield
DUT
Safety
Earth
Ground
2-7