5.3 Eliminate the influence of stray impedance
Hc Hp Lp Lc
Shielding ground
Cd
Test terminal
Cx
Ch Cl
Metal conductor
Figure 5-1 Influence of stray capacitance
Hc Hp Lp Lc
Shielding ground
Shielding plate
Test terminal
Cx
Metal conductor
Figure 5-2 Eliminate the influence of stray capacitance
When the DUT has high impedance(such as small capacitance), the influence of stray capacitance
cannot be ignored. Figure 5-1 is an example of the use of 4 terminal pair measurement. In this figure,
Cd is connected with Cx in a parallel way and when a conductance plate is positioned under DUT,
capacitance Ch will connect with Cx in parallel after connecting with Cl in series and by this way
the measurement result will have errors. If a ground conductor is installed between high and low
terminals, Cd can be reduced to Min. Meanwhile if the ground terminal is connected to the
conductance plate, the influence of Ch and Cl will be eliminated.
When the DUT is low impedance (such as small inductance, large capacitance), a large current will
flow through test leads Hc and Lc. In this case, electromagnetic coupling between test leads
becomes the main source of test errors except the influence of the contact resistance on test
terminals. If this coupling cannot be eliminated, it will bring unexpected influence on test results.
Generally, contact resistance affects the resistance of impedance and electromagnetic affects the
reactance of impedance. Test terminals can adopt 4TP connection method. For 4 terminal-pair (4TP)
connection, the currents flow though Hc and Lc are equal in value and opposite in direction with
those flowing through each shielding terminals (the current reflow from Hc to shielding layer). By
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