Specifications

Typical equations for determining the test fixture's error are:
Ze = ±{ A + (Zs/Zx + Yo × Zx) × 100} (%)
De = Ze/100 (D ≤ 0.1)
Ze: Additional error for impedance (%)
De: Additional error for dissipation factor
A: Test fixture’s proportional error (%)
Zs/Zx × 100: Short offset error (%)
Yo × Zx × 100: Open offset error (%)
Zs: Test fixture’s short repeatability (Ω)
Yo: Test fixture’s open repeatability (S)
Zx: Measured impedance value of DUT (Ω)
Proportional error, open repeatability, and short repeatability are mentioned in the test fixture’s
operation manual and in the accessory guide. By inputting the measurement impedance and
frequency (proportional error, open repeatability, and short repeatability are usually a function of
frequency) into the above equation, the fixture’s additional error can be calculated.
A.2.1 Proportional error
The term, proportional error, A, is derived from the error factor, which causes the absolute imped-
ance error to be proportional to the impedance being measured. If only the first term is taken out of
the above equation and multiplied by Zx, then ΔZ = A × Zx (Ω). This means that the absolute value of
the impedance error will always be A times the measured impedance. The magnitude of proportional
error is dependent upon how precisely the test fixture is constructed to obtain electrically and
mechanically optimum matching with both the DUT and instrument. Conceptually, it is dependent
upon the simplicity of the fixture’s equivalent circuit model and the stability of residuals.
Empirically, proportional error is proportional to the frequency squared.
A.2.2 Short offset error
The term, Zs/Zx x 100, is called short offset error. If Zx is multiplied to this term, then ΔZ = Zs (Ω).
Therefore, this term affects the absolute impedance error, by adding an offset. Short repeatability,
Zs, is determined from the variations in multiple impedance measurements of the test fixture in
short condition. After performing short compensation, the measured values of the short condition
will distribute around 0 Ω in the complex impedance plane. The maximum value of the impedance
vector is defined as short repeatability. This is shown in Figure A-2. The larger short repeatability is,
the more difficult it is to measure small impedance values. For example, if the test fixture’s short
repeatability is ±100 mΩ, then the additional error of an impedance measurement under 100 mΩ
will be more than 100 percent. In essence, short repeatability is made up of a residual resistance and
a residual inductance part, which become larger as the frequency becomes higher.
Figure A-2. Definition of short repeatability
A-2