Specifications

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3.2 DUT Connection Configuration:
In order to make short repeatability small, there are test fixtures that use the 4T connection configuration (for example,
Agilent 16044A). By employing this technique, the effect of contact resistance is reduced and short repeatability is sig-
nificantly improved. As a result, the range of accurate low impedance measurements is expanded down to a low mil-
liohm region.
Figure below shows the difference between the 2T connection and the 4T connection. In a 2T connection, the contact
resistance that exists between the fixture’s contact electrodes and the DUT, is measured 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.
2-Terminal and 4-Terminal connector 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 detected without being affected by the contact resistance. As a result, the 4T con-
nection method can eliminate 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 (jωM), 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).
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Appendix Additional Error