AM450e User Manual
AEMULUS
18
3.2.1 Guard Terminal
The purpose of Guard is to reduce current leakage into sensitive node, from SMU output
connector all the way to the DUT terminal, via cables, PCB, and/or test fixture. This current
leakage can affect the measurement results if not being taken carefully. Measurement test
time can become very long in order to fully charge up the leakage current caused by the
parasitic paths, before actual test results of the DUT can be taken.
There are known current leakage issues from Force-High to Force-Low conductors in a coaxial
cable. Imperfect insulators between center and outer (shield) conductors result in current
leakage. In this case, guarding can be used to eliminate parasitic capacitances.
Figure 5: Coaxial Cable Connection
Guard terminal can effectively solve the current leakage issues in a triaxial cable. For triaxial
able, there is an additional layer (driven guard) that exactly tracks the voltage sweep (force
line) which eliminates cable charging since the inner and outer parts of the cable are always
kept at the same potential. It eliminates any noise since the driven guard is connected to a
separate low noise amplifier inside the SMU.
Figure 6: Triaxial Cable Connection
