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Renesas Electronics America - Knowledgebase

What is Impedance?

Latest Updated:01/01/2006


What is Impedance?


The electrical resistance against direct current is expressed as R (resistance: Ω), and the electrical resistance against alternating current is expressed as Z, which is named as impedance.
In addition to resistance R, inductance L (H) exists parasitically on a signal line in the direction in which the signal flows and capacitance C (F) exists parasitically between the line and ground or other signal lines.

When expressing the impedances of L and C in a complex number with imaginary number j (j2 = -1) on the vertical axis, the impedance of L is jωL and that of C is -j (1/ωC) (ω = 2πf).
Therefore, the impedance of L against direct current (f = 0 Hz) is 0 and that of C infinite.
Against alternating current, the higher frequency f becomes, the larger the impedance of L becomes and the smaller that of C.
Note that combined impedance of L + C, j (ωL - 1/ωC), is known as reactance. The impedance, which is generally termed, is a combination of R, a real part, and reactance, an imaginary part.

If you simplify the causes of parasitism in the transmission path between A and B and between that path and GND as shown in the diagram below, impedance characteristics (R + j ωL) occur in the direction in which the signal flows and impedance characteristics (R + j(ωL - 1/ ωC)) occur with respect to GND.

Therefore, with direct current, the impedance between A and B is only the real part R, and an isolation state exists with respect to GND.
With alternating current, the impedance between A and B increases in proportion as the frequency rises, and the impedance with respect to the ground, C, becomes smaller, so it may attenuate the signal which reaches B.
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