Page 122 - The Art and Science of Analog Circuit Design

James M, Bryant

purposes of resistance calculation in cases where the skin depth is less
than one-fifth the conductor thickness, we can assume that all the HF
current flows in a layer the thickness of the skin depth, and is uniformly
distributed.

SKIN EFFECT

At high frequencies inductive effects cause currents to flow
only in the surface of conductors.
HF Currents flow only
in thin surface layers.

CONDUCTOR

Skin depth at frequency f in a conductor of resistivity p ohm-metre
and permittivity \i henry/metre is

In copper the skin depth is //r

the skin resistance is 2.6X10" Vf fl/sq
(Remember that current may flow in both sides of a PCB
[this is discussed later] and that the skin resistance formula
is only valid if the skin depth is less than the conductor thickness.

Figure &-3.

Skin effect has the effect of increasing the resistance of conductors at
quite modest frequencies and must be considered when deciding if the
resistance of wires or PC tracks will affect a circuit's performance. (It
also affects the behavior of resistors at HF.)

Good HF analog design must incorporate stray capacitance. Wherever
two conductors are separated by a dielectric there is capacitance. The
formulae for parallel wires, concentric spheres and cylinders, and other
more exotic structures may be found in any textbook but the commonest
structure, found on all PCBs, is the parallel plate capacitor.

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