User manual

2400 Series SourceMeter

User’s Manual Source-Measure Concepts 6-27

Guard

WARNING GUARD is at the same potential as output HI. Thus, if hazard-

ous voltages are present at output HI, they are also present at

the GUARD

terminal.

NOTE See Section 2, “Guarding methods,” for details on guarded test connec-

tions as well as information on selecting guard modes.

For 6-wire ohms guard measurements, use the GUARD output-off state.

For details on the GUARD output-off state, see

Section 13, “Output-off

states.”

The driven guard (available at the rear panel GUARD terminal) is always enabled

and provides a buffered voltage that is at the same level as the Input/Output HI (or

Sense HI for remote sense) voltage. The purpose of guarding is to eliminate the

effects of leakage current (and capacitance) that can exist between input/output

high and low. In the absence of a driven guard, leakage in the external test circuit

could be high enough to adversely affect the performance of the SourceMeter.

Leakage current can occur through parasitic or non-parasitic leakage paths. An

example of parasitic resistance is the leakage path across the insulator in a coax

or triax cable. An example of non-parasitic resistance is the leakage path through

a resistor that is connected in parallel to the DUT.

There are two programmable output impedance levels for the guard output. The

high-impedance (~10kΩ) CABLE guard is used to reduce the effects of capacitance

and leakage current paths in the test circuit. The low-impedance (<1Ω) OHMS

guard is used to cancel the effects of parallel resistances when measuring a resistor

element of a resistor network.

Cable guard

The CABLE guard selection provides a high-impedance (~10kΩ) driven guard to

prevent positive feedback, which could cause oscillations when using shielded

cables. Cable guard is used to drive the shields of cables and test fixtures. Guard

is extended to a test fixture using a safety banana plug (such as the Model 8008-

BAN). Inside the test fixture, the guard can be connected to a guard plate or shield

that surrounds the DUT.