Specifications

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2.3 Open Offset Error:
The term, Yo Zx  100 is called open offset error. If Zx is multiplied to this term, then ∆Y = Yo. This term affects the
absolute admittance error, by adding an offset. Open repeatability, Yo, is determined from the variations in multiple
admittance measurements of the test fixture in open condition. After performing open compensation, the measured val-
ues of the open condition will distribute around 0 S in the complex admittance plane. As shown in the figure below, the
maximum value of the admittance vector in the complex admittance plane is defined as open repeatability. The larger
open repeatability is, the more difficult it is to measure large impedance values. Open repeatability is made up of a stray
conductance and stray capacitance part, which become larger as the frequency becomes higher.
Definition of open repeatability
3. New Market Trends and the Additional Error for Test Fixtures
3.1 New Devices:
Recently, the debut of extremely low ESR capacitors, and the trend to use capacitors at much higher frequencies, have
increased demand for low impedance measurements. As a result, the test fixture’s short repeatability has become
increasingly important. In the figure below, the relationship between proportional error, short offset error and frequency
are shown when measuring low impedance of 100 mΩ and 10 Ω. Notice that when the measured impedance is less
than 100 mΩ, short offset error influences the entirety of the test fixture’s inherent error. As shown in the figure below,
when the DUT's impedance is 100 mΩ and the test fixture’s short repeatability is 10 mΩ, the short offset error will be
10 percent. Since the proportional error is minimal in low frequencies, the additional error will be 10 percent.
Until recently, to allow for additional error in test fixtures it was common to just specify the proportional error (A). As
shown in the 10 Ω measurement case, if the measured impedance is large in comparison to the test fixture’s short
repeatability, then the short offset error can be ignored completely. This is the reason why open and short offset error
was not previously specified. This is the reason for test fixtures that are only specified with proportional error. On the
contrary, for measured impedance from 1 Ω to 10 kΩ, proportional error (A) alone is sufficient to express the test fix-
ture’s additional error.
Relationship between proportional error, short offset error and frequency when measuring low impedance
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Appendix Additional Error