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Technical Information
Basic Principles of Capacitance Measurement
Principle of four-terminal pair measurement
Generally, in connection methods using common terminal structures, mutual inductance,
interference between measurement signals, and unnecessary residual elements (especially
at higher frequencies) significantly affect measurements.
The E4981A adopts a four-terminal pair structure to reduce the limitations on
measurements due to these factors and to facilitate stable and accurate measurements.
Figure E-7 illustrates the principle of four-terminal pair measurement. The UNKNOWN
terminals are four coaxial connector terminals.
Figure E-7
•
HCUR: Current high terminal
•
HPOT: Voltage high terminal
•
LPOT: Voltage low terminal
•
LCUR: Current low terminal
Principle of Four-terminal Pair Measurement
The four-terminal pair measurement method has advantages in both low- and
high-impedance measurements. The outer shield conductors serve as the return path of the
measurement signal current (not grounded). Current of the same amplitude flows through
the core conductor and the surrounding shield conductor in opposite directions, and
therefore no external magnetic field occurs around either conductor. In other words, the
magnetic field caused by the inner conductor and that caused by the outer conductor cancel
each other completely. The measurement signal current does not cause any induction field
and, therefore, the test leads do not increase the error due to their self-inductance or the
mutual inductance between different leads.
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Appendix E