Specifications

A-3
Open offset error:
The term, Yo•Zx × 100 is called open offset error. If the same analysis is carried out with admittance,
then it can be concluded that this term also affects the absolute admittance error, by adding an offset.
Open repeatability (Yo) is determined from the variations in multiple measurements of the test fixture
in open condition. 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 resistance and a
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
New devices:
Recently, the debut of extremely low ESR capacitors and the trend to use capacitors at much higher
frequencies, have made low impedance measurements more strongly demanded than in the past. 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 mea-
suring low impedance. Notice that when the measured impedance is less than 100 mΩ, short offset
error influences the entirety of the test fixture’s additional 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%. Since the proportional error is minimal in low frequencies, the additional
error will be 10% as well.
For the additional error of test fixtures, up until now, it was common to just specify the proportional
error (A). As shown in the 10 Ω measurement case (same figure down below), if the measured
impedance is large in comparison to the test fixture’s short repeatability, then short offset error can
be ignored completely. This is the reason why open and short offset error was not specified previ-
ously. Test fixtures that are only specified with proportional error are due to this reason. On the
contrary, for measured impedance from 1 Ω to 10 kΩ proportional error (A) alone is sufficient to
express the test fixture’s additional error.
Relationship between proportional error, short offset error and frequency when measuring low impedance