Keysight Technologies B1500A Self-Paced Training Manual page 74

Module 2
Basic Measurement
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.
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