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Measurement Concepts 2-7

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.

DUT

INPUT

250V PK

INPUT

250V PK

Metal Noise Shield

A. Noise Shield

B. Safety Shield

Metal Safety Shield

Safety

Earth

Ground

Chassis

Ground

Figure 2-4

Shielding for

unguarded

measurements

Keithley 6514 User Manual

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