Specifications
4.3.2 Open and short compensations
Open and short compensations are the most popular compensation technique used in recent LCR
measurement instruments. This method assumes that the residuals of the test fixture can be repre-
sented by the simple L/R/C/G circuit as shown in Figure 4-4 (a). When the DUT contact terminals of
the test fixture are open, as shown in Figure 4-4 (b), stray admittance Go + jωCo is measured as Yo
because residual impedance (Zs) is negligible, (1/Yo >> Zs). When the DUT contact terminals of the
test fixture are shorted, as shown in Figure 4-4 (c), the measured impedance represents residual
impedance Zs = Rs + jωLs because Yo is bypassed. As a result, each residual parameter is known
and, the DUT’s impedance (Zdut) can be calculated from the equation given in Figure 4-4 (d).
Note: Agilent’s impedance measurement instruments actually use a slightly different equation.
Refer to Appendix B for more detailed information.
This compensation method can minimize the errors when the actual residual circuit matches the
assumed model in the specific situations listed below:
• Measurement by connecting an Agilent test fixture to the UNKNOWN terminals
• Measurement with an Agilent test fixture connected by an Agilent test cable that is compensat-
ed for electrical length
In other situations, the open/short compensation will not thoroughly correct the measured values.
In addition, this method cannot correlate measurement results from different instruments. To
resolve these compensation limitations, the open/short/load compensation is required. Refer to
“Open/short/load compensation” described in Section 4.3.3.
.
Figure 4-4. Open/short compensation
(a) Test fixture residuals
H
c
L
p
L
c
H
p
Z
du t
C
o
G
o
Z
m
R
s
L
s
Stray
Residual
Test fixture residuals
impedance (Z
s
)
admittance (Y
o
)
(b) Open measurement
H
c
L
p
L
c
H
p
Open
C
o
G
o
Y
o
R
s
L
s
(R
s
+ jwL
s
<< )
1
G
o
+ jwC
o
Y
o
= G
o
+ jwC
o
H
c
L
p
L
c
H
p
Short
C
o
G
o
Z
s
R
s
L
s
(c) Short measurement
Z
s
= R
s
+ jwL
s
SHORT impedance << R
s
+ jwL
s
H
c
L
p
L
c
H
p
Z
dut
Y
o
Z
xm
Z
s
Z
dut
=
Z
xm
-Z
s
1- (Z
xm
-Z
s
)Y
o
(d) Open/short compensation formula
Z : Corrected DUT impedance
dut
Z : Measured DUT impedance
xm
Y : Stray admittance
o
Z : Residual impedance
s
4-4