# HP Effective Impedance Measurement Using OPEN/SHORT/LOAD User Manual

Brand: HP, Pages: 12, PDF Size: 0.3 MB

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HEffective Impedance Measurement

Using OPEN/SHORT/LOAD

Correction

Application Note 346-3This literature was published years prior to the establishment of Agilent Technologies as a company independent from Hewlett-Packard

and describes products or services now available through Agilent. It may also refer to products/services no longer supported by Agilent. We

regret any inconvenience caused by obsolete information. For the latest information on AgilentÕs test and measurement products go to:

www.agilent.com/find/products

Or in the U.S., call Agilent Technologies at 1-800-452-4844 (8amÐ8pm EST)

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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.

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.

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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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:

n Using an HP test fixture

n Measurements at the front panel terminals

n 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:

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

severe limitations:

n Not able to correlate measurement values from different

test instruments

n Not able to improve measurement repeatability

To solve these test limitations and issues, the OPEN/SHORT/LOAD correc-

tion is necessary.

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.

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Figure 2. OPEN/SHORT/LOAD

Correction Model

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.

Given:Z

1=V

1/I

1 and Z

2=V

2/I

2

AV

2+BI

2 AZ

2+B

Z

1= -------------------------- = --------------------------

CV

2+DI

2 CZ

2+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:

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.

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.

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.

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 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.

2.

3.