Guard - Keithley 6430 Instruction Manual

5-22 Source-Measure Concepts

Guard

WARNING
The driven guard 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.
Guard modes — There are two programmable output impedance levels for the guard out-
put. 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 can-
cel the effects of parallel resistances when measuring a resistor element of a resistor network.
Cable guard or ohms guard is available at the GUARD banana jack on the mainframe. Guard
mode (Cable or Ohms) is selected from the V or I source configuration menu as explained in
Section 2.
Cable guard is always available at the Remote PreAmp, regardless of the guard setting.
Ohms guard is not available at the Remote PreAmp.
NOTE
Cable guard
WARNING
Cable guard 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. From the Remote PreAmp, cable guard is extended to the test
fixture using standard 3-slot triax cable (inner shield is guard). From the mainframe, cable
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.
Guard is at the same potential as input/output HI. Thus, if hazardous
voltages are present at input/output HI, they are also present at the guard
terminals.
Cable guard at the Remote PreAmp and guard accessed at the V,Ω GUARD terminal
on the mainframe are different signals and should never be shorted together.
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 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 5-12B shows the metal case of a test fixture being used as a safety
shield.