28
Figure 4.4
4-Terminal Path (4TP)
4-Terminal Path connection solves the problem that caused by the test lead inductance. 4TP uses four coaxial
cables to isolate the current path and the voltage sense cable (Figure 4.5). The return current will flow through
the coaxial cable as well as the shield. Therefore, the magnetic flux that generated by internal conductor will
cancel out the magnetic flux generated by external conductor (shield). The 4TP connection increases the
measurement range from 1m
to 10M
.
(c) Typical Impedance
Measurement Range (
)
Figure 4.5
Eliminating the Effect of the Parasitic Capacitor
When measuring the high impedance component (i.e. low capacitor), the parasitic capacitor becomes an
important issue (Figure 4.6). In figure 4.6(a), the parasitic capacitor Cd is paralleled to DUT as well as the Ci
and Ch. To correct this problem, add a guard plane (Figure 4.6(b)) in between H and L terminals to break the
Cd. If the guard plane is connected to instrument guard, the effect of Ci and Ch will be removed.
(a) Parastic Effect
H
CUR
H
POT
L
POT
L
CUR
Cd
Connection
Point
DUT
C
h
C
l
Ground
(b) Guard Plant reduces
Parastic Effect
H
CUR
H
POT
L
POT
L
CUR
Guard
Plant
Figure 4.6
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