Keysight B1505A Configuration and Connection Guide 5-51Connection Guide for Wafer Prober and Your Own Test FixtureAbout Cable ConnectionsTo Make Connection to Reduce Leakage CurrentTo reduce the leakage current caused by connection cables, the guard technique iseffective. Connect the probing needles to the coaxial cables as shown below:1. Cut and trim end of the coaxial cable such as N1254A-503 and N1254A-506, so thatthe center conductor does not touch the outer conductor (connected to the guardterminal).2. Connect the center conductor to tail of the probing needle. Never connect the outerconductor to the probing needle. However the outer conductor should be extended asclose as possible to the probing needle.3. Connect the outer conductor to the outer conductor of the probing needle if it is acoaxial probing needle.Example The following example connection can be used to reduce the leakage current. Extend theouter conductor as close as possible to the probing needle. This also reduces the inducednoise.WARNING Do not touch the guard terminal with bare hands because you may be shocked byhigh voltage. The potential of the guard terminal is equal to the output voltage.CAUTION Never connect the guard terminal to any other output, including circuit common, frameground, or the terminals of any other unit.Guarding Guarding reduces the leakage current between the measurement points and instrument.This is important when you measure low current.The following figure shows the theory of guarding. The buffer amplifier (1) keeps thepotential of the guard conductor at the same potential as the force conductor, so currentdoes not flow between the force and guard conductors. Therefore, the current measured bySMU is same as current at measurement point because no current is leaked.Probing needleContact padOuter conductorCenter conductorCoaxial cableABufferGuard Force DUT
