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H
Effective Impedance Measurement
Using OPEN/SHORT/LOAD
Correction
Application Note 346-3
Introduction
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
OPEN/SHORT/LOAD correction.
How OPEN/SHORT/LOAD correction differs from
OPEN/SHORT correction
Here we compare the principle of the OPEN/SHORT/LOAD correction with
the OPEN/SHORT correction.
1. 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.
Figure 1. OPEN/SHORT
Correction Model
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.
1
Zm-Zs
Zdut= ----------------------------------
l-(Zm-Zs)Yo
where,
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:
s Using an HP test fixture
s Measurements at the front panel terminals
s Measurements using an HP test cable compensated for
electrical length
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:
s Scanner/multiplexer/matrix switches
s Component handlers
s Custom-made test fixture
s Non-standard length cable test leads
s External DC bias circuitry
s Balun transformer
s Additional filters and amplifiers
In addition, the OPEN/SHORT correction has the following
severe limitations:
s Not able to correlate measurement values from different
test instruments
s Not able to improve measurement repeatability
To solve these test limitations and issues, the OPEN/SHORT/LOAD correc-
tion is necessary.
2. OPEN/SHORT/LOAD correction
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
Figure 2. OPEN/SHORT/LOAD
Correction Model
With the assumption that a DUT with an impedance of Z2 is connected to
the front panel terminals, the instrument would measure an impedance val-
ue of Z1. The following relationship defines Z1.
Given:Z1=V1/I1 and Z2=V2/I2
AV2+BI2 AZ2+B
Z1= -------------------------- = --------------------------
CV2+DI2 CZ2+D
The parameters of A,B,C and D can be removed when using the
following definitions:
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
test setup.
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:
Zstd(Zo-Zsm) (Zxm-Zs)
Zdut= ----------------------------------------------------------------------
(Zsm-Zs) (Zo-Zxm)
(Note that each parameter has real and imaginary components.)
The OPEN/SHORT/LOAD correction function is built into the following HP
LCR meters/analyzers:
s HP 4263B s HP 4286A
s HP 4278A s HP 4291B
s HP 4279A s HP 4395A with opt. 010 and HP 43961A
s HP 4284A s HP 4396B with opt. 010 and HP 43961A
s HP 4285A
3
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.
1. OPEN correction
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.
2. SHORT correction
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.
3. LOAD correction
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 100 to 1k 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
4
impedance value. However, when the DUT's impedance value is very low
or very high, it is recommended to use a standard DUT of 100 or 1k ,
whose value isn't close to the impedance of the DUT.
When a low or high impedance standard DUT is used, the reference value
(described later) of the standard DUT cannot be obtained accurately, and it
may cause the abnormal measurement results, not the true value
of the DUT.
(3) Referencing the standard DUT
To perform the OPEN/SHORT/LOAD correction, it is necessary to measure
the standard DUT for a reference value (known value) beforehand. When
measuring it, it is important to use a high accuracy instrument and to set
its measurement conditions (such as integration time or averaging time) so
that it can measure as accurately as possible. To minimize the measure-
ment error, the standard DUT should be measured using a direct-
connected test fixture after performing the OPEN/SHORT correction.
4. Measurement condition of impedance measurement instruments
when performing the OPEN/SHORT/LOAD correction
Impedance measurement equipment with the OPEN/SHORT/LOAD correc-
tion function will automatically set the measurement condition (such as
integration time or averaging time) so that it can perform error correction
with highest accuracy.
If performing the OPEN/SHORT/LOAD correction for instruments not
equipped with the OPEN/SHORT/LOAD correction function using an exter-
nal controller, the measurement conditions should be set to measure cor-
rection data as accurately as possible.
Example of actual OPEN/SHORT/LOAD correction
Figure 3 shows the measurement example to extend the HP 4285A's
measurement terminal using 16048E(4m cable). In this case, it is necessary
to perform the OPEN/SHORT/LOAD correction since the HP 4285A cannot
compensate the 4m extension cable with its cable length correction func-
tion. Figure 4 shows the comparison of measurement error between 100pF
capacitor measurement with the OPEN/SHORT correction and that with
Figure 3. Cable Extension
Using an HP 16048E (4m)
5
the OPEN/SHORT/LOAD correction. A 47pF capacitor is used as a stan-
dard DUT. This measurement result shows the error cannot be minimized
sufficiently with the OPEN/SHORT correction, but can be compensated
with an OPEN/SHORT/LOAD correction as shown by the plot.
Figure 4. Comparison
Between OPEN/SHORT Correction
and OPEN/SHORT/LOAD Correction
OPEN/SHORT/LOAD correction with an external controller
The OPEN/SHORT/LOAD correction can be accomplished with simple key
operation when using an impedance measurement instruments equipped
with the OPEN/SHORT/LOAD correction function. When using instruments
not equipped with the OPEN/SHORT/LOAD correction function, it is possi-
ble to perform the OPEN/SHORT/LOAD correction by executing the correc-
tion calculation with an external controller. But this method has the
following tradeoff problems as the comparison with the correction of a
measurement instrument alone.
s Complicated operation
s Slow measurement speed due to data transfer time
Figure 5 shows an example program to execute the OPEN/SHORT/LOAD
correction for capacitor measurement using the HP 4194A Impedance Ana-
lyzer. In this program, the measurement is performed at one frequency
point with a manual trigger mode.
Line 130-190 setting measurement condition.
Line 210-270 Measuring impedance of OPEN condition in G-B mode.
Line 300-330 Measuring impedance of SHORT condition in R-X mode.
Line 350-610 Measuring impedance of the standard DUT after inputting
Cs-D or Cp-D value of valued standard DUT.
Line 660-690 selecting DUT mode(Cs-D or Cp-D),
Line 710-10l0 Executing correction calculation after measuring
impedance of DUT, then displaying the result.
6
Conclusion
This application note shows the principle of the OPEN/SHORT/LOAD
correction and some points to be considered in the execution. With the
proper OPEN/SHORT/LOAD correction, measurement comes
higher in accuracy.
"Reference" Impedance measurement handbook (5091-3000)
7
Figure 5. Sample Program for HP 4194A OPEN/SHORT/LOAD Correction
8
9
Appendix. Correction Capability of HP Instruments
Model No. Correction Capability Cable Length Correction
HP 4192A OPEN/SHORT Correction 0m/1m
HP 4194A OPEN/SHORT Correction 0m/1m
HP 4195A(*) OPEN/SHORT Correction none (Electrical Length Correction)
with HP 41951A
HP 4263A(*) OPEN/SHORT Correction 0m/lm/2m
OPEN/SHORT/LOAD Correction
(via HP-IB)
HP 4263B OPEN/SHORT/LOAD Correction 0m/1m/2m/4m
HP 4274A(*) OPEN/SHORT Correction 0m /lm
/HP 4275A(*)
HP 4276A(*) OPEN/SHORT Correction 0m/1m
/HP 4277A(*)
HP 4278A OPEN/SHORT/LOAD Correction 0m/1m/2m
Multi Channel Correction (Opt.301)
HP 4279A OPEN/SHORT/LOAD Correction 0m/1m/2m
Multi Channel Correction
HP 4284A OPEN/SHORT/LOAD Correction 0m/lm
Multi Channel Correction (Opt.301) 0m/1m/2m/4m (Opt.006)
HP 4285A OPEN/SHORT/LOAD Correction 0m/1m/2m
Multi Channel Correction (Opt.301)
HP 4286A OPEN/SHORT/LOAD Correction none (Electrical Length Correction)
HP 4291A(*) OPEN/SHORT/LOAD Correction none (Electrical Length Correction)
HP 4291B OPEN/SHORT/LOAD Correction none (Electrical Length Correction)
HP 4395A with OPEN/SHORT/LOAD Correction none (Electrical Length Correction)
Opt.010,43961A
HP 4396B with OPEN/SHORT/LOAD Correction none (Electrical Length Correction)
Opt.010,43961A
(*) Obsolete
10
H
For more information about Hewlett-
Packard test & measurement products,
applications, services, and for a
current sales office listing, visit
our web site, http://www.hp.com/go/tmdir.
You can also contact one of the following
centers and ask for a test and
measurement sales representative.
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Data subject to change
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