Specifications

5-7
Contact resistance
As described in Section 4.4, contact resistance between the DUT’s electrodes and the contact elec-
trodes of the test fixture causes D measurement error. While the contact resistance of the 2T test fix-
ture directly adds to the measured impedance as a positive D error, the contact resistance at the Hp
and Lc electrodes of a 4T test fixture cause a negative D error (see Figure 4-10.) When a capacitor
that has a very low D is measured using a 4T test fixture, a negative D value is displayed depending
on the magnitude of the D measurement error due to a contact resistance.
Improper short compensation
When short compensation is performed based on an improper short measurement value, a negative
D value may be displayed. Major causes of an improper short measurement are a contact resistance
at the test fixture’s electrodes and a residual resistance of the shorting bar. As described in
Section 4.3, the resistance (Rs) and reactance (Xs) values obtained by short measurement are stored
in the instrument and removed from the measured impedance of the DUT by performing the short
compensation. If the Rs value is greater than the resistance component (Rxm) of the DUT’s imped-
ance, the corrected resistance (Rxm – Rs) becomes a negative value and, as a result, a negative D
value is displayed. To avoid this problem, clean the test fixture’s electrodes to minimize the contact
resistance and use a shorting bar with the lowest possible residual resistance.
Improper cable length correction
When cable length correction is not properly performed for the test cables used, a negative D value
may be displayed at high frequencies because a phase angle measurement error is caused by the
cables. The error increases in proportion to the square of the measurement frequency. After the
cable length correction is performed, a small phase error may remain and cause a negative D value
because the characteristics of test cables are slightly different for the respective cables. The
open/short/load compensation can minimize the measurement error due to the differences between
the cables.
Complicated residuals
Using a long cable, a component scanner, or a component handler has the propensity to cause a neg-
ative D display due to complicated residuals. When complex residual impedance and stray admit-
tance exist in the connection circuit between the DUT and the calibration plane of the impedance
measuring instrument, the characteristics of the connection circuit do not match the open/short
compensation circuit model (see Figure 4-4.) Since the open/short compensation cannot effectively
remove the measurement error due to the complex residuals and strays, a D measurement error
causes a negative D display. The open/short/load compensation is an effective method for eliminat-
ing measurement errors caused by complicated residuals.