User manual

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6-28 Source-Measure Concepts 2400 Series SourceMeter

User’s Manual

WARNING To prevent injury or death, a safety shield must be used to pre-

vent physical contact with a guard plate or guard shield that is

at a hazardous potential (>30Vrms or 42.4V peak). This safety

shield must completely enclose the guard plate or shield and

must be connected to safety earth ground.

Figure 6-17B shows

the metal case of a test fixture being used as a safety shield.

Inside the test fixture, a triaxial cable can be used to extend guard to the DUT. The

center conductor of the cable is used for In/Out HI, the inner shield is used for

guard, and the outer shield is used for In/Out LO and is connected to the safety

shield (which is connected to safety earth ground).

A coaxial cable can be used if the guard potential does not exceed 30Vrms (42.4V

peak). The center conductor is used for In/Out HI, and the outer shield is used for

guard. For higher guard potentials, use a triaxial cable as previously explained.

Figure 6-17 shows how cable guard can eliminate leakage current through the

insulators in a test fixture. In Figure 6-17A, leakage current (I

) flows through the

insulators (R

and R

) to In/Out LO, adversely affecting the low-current (or high-

resistance) measurement of the DUT.

In Figure 6-17B, the driven guard is connected to the metal guard plate for the insu-

lators. Since the voltage on either end of R

is the same (0V drop), no current can

flow through the leakage resistance path. Thus, the SourceMeter only measures the

current through the DUT.

Cable guard should be used when sourcing or measuring low current (<1µA).

NOTE When using shielded, triaxial, or coaxial cabling with guard, the CABLE

guard setting must be used to prevent oscillations. CABLE guard is the

factory default setting.

Ohms guard

The OHMS guard selection provides a low-impedance (<1Ω), high current (up to

50mA) driven guard. This lets you perform in-circuit ohms measurements of the

DUT where other parallel resistive paths are present. These measurements are

typically performed in Delta or Wye configurations.

NOTE Ohms guard is not available for the 1A, 3A, and 5A (2420, 2425, 2430,

and 2440) ranges (source and measure). Ohms guard cannot be

selected if already on range. Conversely, if ohms guard is already

selected, the 1A, 3A, and 5A (2420, 2425, 2430, and 2440) ranges can

not be selected. See also Section 4, “6-wire ohms measurements,” and

Section 2, “Ohms guard.”