Guard - Keithley SourceMeter 2400 User Manual

6-34 Source-Measure Concepts

Guard

WARNING GUARD is at the same potential as output HI. Thus, if hazardous volt-
ages are present at output HI, they are also present at the GUARD
terminal.
NOTE See Section
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
The driven guard (available at the rear panel GUARD terminal) is always enabled and pro-
vides 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 cur-
rent (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 leak-
age 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.
WARNING To prevent injury or death, a safety shield must be used to prevent
physical contact with a guard plate or guard shield that is at a hazard-
ous potential (>30Vrms or 42.4V peak). This safety shield must com-
pletely enclose the guard plate or shield and must be connected to
safety earth ground.
being used as a safety shield.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
2400 Series SourceMeter
2, "Guarding methods," for details on guarded test connections as
Section
Figure 6-18B shows the metal case of a test fixture
®
User's Manual
13, "Output-off states."