Page 126 - The Art and Science of Analog Circuit Design

James M. Bryant

Figure 9-10,

At LF magnetic shielding requires Mu-Metal which is
heavy, expensive and vulnerable to shock.

At HF a conductor provides effective magnetic shielding
provided the skin depth is less than the conductor thickness.
PC foil is an effective magnetic shield above 10-20 MHz.

than the thickness of the shield. In breadboards a piece of copper-clad
board, soldered at right angles to the ground plane, can make an excellent
HF magnetic shield, as well as being a Faraday shield.
Magnetic fields are dipole fields, and therefore the field strength di-
minishes with the cube of the distance. This means that quite modest
separation increases attenuation a lot. In many cases physical distance is
all that is necessary to reduce magnetic coupling to acceptable levels.

Grounds

KirchofPs Law tells us that return currents in ground are as important
as signal currents in signal leads. We find here another example of the
"superconductor assumption"—too many engineers believe that all
points marked with a ground symbol on the circuit diagram are at the
same potential. In practice ground conductors have resistance and induc-
tance—and potential differences. It is for this reason that such bread-
boarding techniques as matrix board, prototype boards (the ones where
you poke component leads into holes where they are gripped by phos-
phor-bronze contacts) and wire-wrap have such poor performance as
analog prototyping systems.
The best analog breadboard arrangement uses a "ground plane"—a
layer of continuous conductor (usually copper-clad board). A ground

The net current at any point in a circuit is zero.
OR
What flows in flows out again.
OR
Current flows in circles.
THEREFORE
All signals are differential.
AND
Ground impedance matters.
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