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HEffective Impedance Measurement
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Figure 1. OPEN/SHORT
Since Zs <<1/Yo, stray admittance Yo can be measured when the test termi-
nals are open. Similarly residual impedance Zs can be measured when the
test terminals are shorted. using this correction data, Device-Under-
Test(DUT) measurement data Zm can be compensated with the following
equation. Then a true value, Zdut, can be derived from Zm by removing the
residuals of a test fixture.
Generally, impedance measurement instruments have a reference plane to
define the measurement accuracy at the UNKNOWN terminals of its front
panel. HP impedance measurement instruments have a cable length correc-
tion function which is applicable for defining the reference plane at the end
of the HP test leads. In the actual measurement, a test fixture is connected
to the reference plane. Test fixtures degrade the total measurement accura-
cy by their residual impedance. To improve this degradation, error correc-
tion should be applied. The OPEN/SHORT correction is the most popular
correction technique used in recent impedance measurement instruments,
But when complicated residuals exist (for example, when a scanner or a
handler is used), or when using an extension cable whose length cannot be
compensated with the cable length correction function, the OPEN/SHORT
correction cannot minimize error sufficiently. To minimize these errors, the
OPEN/SHORT/LOAD correction is very effective. This application note
describes effective impedance measurements using the
How OPEN/SHORT/LOAD correction differs from
Here we compare the principle of the OPEN/SHORT/LOAD correction with
the OPEN/SHORT correction.
In the OPEN/SHORT correction, the residuals of a test fixture can be mod-
eled as an equivalent circuit shown in Figure 1. 1.
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Zdut: True value of DUT
Zm: Measurement value of DUT
Yo: Admittance of OPEN condition
Zs: Impedance of SHORT condition
(Note that each parameter has real and imaginary components.)
As it has been shown, simple measurement errors can be mathematically
compensated by using the OPEN/SHORT correction. However, this specific
technique is usable only when performing measurements under the follow-
ing test conditions:
n Using an HP test fixture
n Measurements at the front panel terminals
n Measurements using an HP test cable compensated for
There are numerous test conditions where complicated impedance
parasitic cannot be modeled as the simple equivalent circuit in Figure 1.
The OPEN/SHORT correction will not truly compensate for errors
introduced in the following test situations:
n Scanner/multiplexer/matrix switches
n Component handlers
n Custom-made test fixture
n Non-standard length cable test leads
n External DC bias circuitry
n Balun transformer
n Additional filters and amplifiers
In addition, the OPEN/SHORT correction has the following
n Not able to correlate measurement values from different
n Not able to improve measurement repeatability
To solve these test limitations and issues, the OPEN/SHORT/LOAD correc-
tion is necessary.
The OPEN/SHORT/LOAD correction requires the measurement data of a
standard DUT with known values in addition to the OPEN/SHORT measure-
ment data. The residuals of a test fixture can be defined as a four-terminal
network expressed with A, B, C, D parameters as shown in Figure 2. 2.
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Figure 2. OPEN/SHORT/LOAD
With the assumption that a DUT with an impedance of Z
2 is connected to
the front panel terminals, the instrument would measure an impedance val-
ue of Z
1. The following relationship defines Z
1 and Z
1= -------------------------- = --------------------------
The parameters of A,B,C and D can be removed when using the
Zo: Measured value when the instrument terminals are open.
Zs: Measured value when the terminals are shorted.
Zsm: Measured value of the standard DUT when connected to the
Zstd: True (or expected) value of the reference DUT.
Zxm: Measured value of DUT.
Zdut: Corrected value of DUT.
All of the analysis yields an equation that corrects for impedance parasitcs:
Zdut= -------------------------- --------------------------------------------
(Note that each parameter has real and imaginary components.)
The OPEN/SHORT/LOAD correction function is built into the following HP
n HP 4263B
n HP 4278A
n HP 4279A
n HP 4284A
n HP 4285A
n HP 4286A
n HP 4291B
n HP 4395A with opt. 010 and HP 43961A
n HP 4396B with opt. 010 and HP 43961A
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Otherwise, when using other LCR meters or analyzers, a computer can be
programmed to make the OPEN/SHORT/LOAD corrections through HP-IB.
Consideration with OPEN/SHORT/LOAD correction
When performing OPEN/SHORT/LOAD correction, the following points
should be considered.
It is important to measure the stray admittance of a test fixture accurately
in the OPEN correction. When getting the OPEN correction data, the dis-
tance between measurement terminals should be the same as the distance
that is required for actually holding the DUT.
It is important to measure the residual impedance of a test fixture accu-
rately in the SHORT correction. When getting the SHORT correction data,
the measurement terminals should be shorted or connected to a shorting
device. When using the shorting device, the residual impedance must be
much less than impedance value of DUT.
In the LOAD correction, selection and measurement of the standard DUT
should be considered carefully.
(1) Selection of standard DUT
When selecting the standard DUT, there is no restriction that inductor must
be used for inductance measurement, or capacitor must be used for a
capacitance measurement. Any device can be used if its impedance value is
accurately known. It is important to use a stable DUT not susceptible to
influences of environment such as temperature or magnetic fields. From
this viewpoint, capacitors or resistors are better sited than inductors which
are more susceptible to the environment .
Especially in the case of measuring low loss (low D, high Q, low ESR)
DUTs, it is necessary to use as low loss standard DUT as possible. Since it
is difficult to get low loss inductor but easy to get capacitor, low loss
capacitors are recommended for the standard DUT.
(2) Impedance value of standard DUT
When measuring a DUT's various impedance values, it is recommended to
use a 100½ to 1k½ device as the standard DUT. It can be measured accu-
rately by impedance measurement instruments and is not susceptible to
contact resistance or residuals.
When measuring a DUT of one impedance value, it is recommended that
the standard DUT have a impedance value close to that of the DUT. By
using a standard DUT, we can reduce the non-linear errors near its 1.