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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 electric