Specifications

Until recently, to allow for additional error in test fixtures it was common to just specify the propor-
tional 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 com-
pletely. 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 mea-
sured impedance from 1 Ω to 10 kΩ, proportional error (A) alone is sufficient to express the test fix-
ture’s additional error.
A.3.2 DUT connection configuration
In order to make short repeatability small, there are test fixtures that use the 4T connection configu-
ration (for example, Agilent 16044A). By employing this technique, the effect of contact resistance is
reduced and short repeatability is significantly improved. As a result, the range of accurate low
impedance measurements is expanded down to a low milliohm region.
Figure A-5 shows the difference between the 2T connection and the 4T connection. In a 2T connec-
tion, the contact resistance that exists between the fixture’s contact electrodes and the DUT, is mea-
sured together with the DUT’s impedance. Contact resistance cannot be eliminated by compensation
because the contact resistance value changes each time the DUT is contacted.
Figure A-5. Two-terminal and four-terminal connection techniques
In a 4T connection, the voltage and current terminals are separate. Since the voltmeter has high
input impedance, no current flows into the voltage terminals. Hence, the voltage that is applied
across the DUT can be accurately detected without being affected by the contact resistance. Also, the
current that flows through the DUT flows directly into the current terminal and is accurately detect-
ed without being affected by the contact resistance. As a result, the 4T connection method can elimi-
nate the effect of contact resistance and realize a small short repeatability. By using a 4T test fixture,
it is possible to measure low impedance with better accuracy than that which can be measured with
a 2T test fixture.
The 2T test fixture can be used up to a higher frequency than the 4T test fixture. Since the 2T test
fixture has a simple DUT connection configuration, the effects of residuals and mutual coupling
(jwM), which cause measurement error to increase with frequency, are smaller than those of the 4T
test fixture and can be effectively reduced by compensation. Thus, the 2T connection is incorporated
in test fixtures designed for use in the higher frequency region (typically up to 40 or 110 MHz.)
H
p
H
c
L
c
L
p
V
A
Measurement
instrument
Fixture
Contact
point
Shielding plate
DUT
DUT
(a) Two terminal test fixture
H
p
H
c
L
c
L
p
V
A
Measurement
instrument
Contact
point
( b) Four terminal test fixture
Shielding plate
Fixture
A-4