# What is Impedance?

Latest Updated:01/01/2006

## Question:

What is Impedance?

## Answer:

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 (j

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.

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 (j

^{2}= -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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