29
4.2 Open/Short Compensation
For those precision impedance-measuring instruments, the open and short compensation need to be used to
reduce the parasitic effect of the test fixture. The parasitic effect of the test fixture can be treated like the simple
passive components in figure 4.7(a). When the DUT is open, the instrument gets the conductance Yp = Gp +
j
Cp (Figure 4.7(b)). When the DUT is short, the instrument gets the impedance Zs = Rs + j
Ls (Figure 4.7(c)).
After the open and short compensation, the 889B has Yp and Zs that can then be used for the real Zdut
calculation (Figure 4.7(d)).
H
CUR
H
POT
L
CUR
L
POT
Zdut
C
o
R
s
L
s
G
o
Z
m
Redundant
Impedance (Z
s
)
Parastic
Conductance
(Y
o
)
Parastic of the Test Fixture
(a) Parastic Effect of the Test Fixture
Y
o
= G
o
+ j
C
o
R
s
+ jω<<1/( G
o
+jωC
o
)
Z
s
=R
s
+ j
ωLs
Z
m
Y
o
Zdut
Z
m
- Z
s
Zdut =
1-(Z
m
-Z
s
)Y
o
(d) Compensation Equation
Z
s
Figure 4.7
4.3 Selecting the Series or Parallel Mode
According to different measuring requirement, there are series and parallel modes to describe the
measurement results. It is depending on the high or low impedance value to decide what mode to be used.
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