Agilent Technologies B1500A Training Manual page 54

Why Use Guarded (Triax) Cables?
Shield (Ground)
Leakage
MOSFET Subthreshold
mA
uA
nA
Id
pA
fA
Vg
The triax cable is a special low dielectric loss, high impedance cable. This cable may be used down
to fA levels when properly used with a guarded probe. The guard voltage tracks the force voltage
exactly, so that no voltage drop can exist between guard and force. This eliminates the capacitive
loading that would otherwise limit low current measurements.
If low impedance coax cables are used with outer layer at ground potential, two limitations will be
immediately apparent. The cable leakage will limit the low current measurement floor. In addition,
when the voltage is swept, the sudden change will cause additional cable charging. This distorts the
low current portion of a MOS subthreshold curve as shown.
RULES:
Unguarded coax cable is OK for measurements above 1 nA.
Triax cable or coax with outer layer at guard potential should be used for measurements below 1 nA.
Guard
Force
Id
Triax Cables
Eliminate cable low current errors.
5-34
Shield (Ground)
MOSFET Subthreshold
mA
uA
nA
pA
fA
Vg Coax Cables
Module 5
Basic Measurement
Force