PHYSICAL DESIGN
AND
DEBUGGING
EMI (also called crosstalk)
is
caused
by
the magnetic field
that
exists around any current-
carrying conductor. The magnetic flux from one conductor can induce current
in
another
conductor, resulting in transient voltage.
Several precautions can minimize EMI:
• Running a ground line between two adjacent lines wherever they traverse a long section
of the circuit board. The ground line should be grounded at both ends.
• Running ground lines between the lines of an address bus or a data bus if either
of
the
following conditions exists:
- The bus
is
on an external layer of the board.
- The bus
is
on an internal layer but not sandwiched between power and ground planes
that
are at most
10
mils away.
• Avoiding closed loops
in
signal paths (see Figure 11-7). Closed loops cause excessive
current and create inductive noise, especially
in
the circuitry enclosed by a loop.
ESI
is
caused by the capacitive coupling of two adjacent conductors. The conductors act as
the plates of a capacitor; a charge built up on one induces the opposite charge on the other.
The following steps reduce
ESI:
•
Separating signal lines
so
that
capacitive coupling becomes negligible.
• Running a ground line between two lines to cancel the electrostatic fields.
G30107
Figure 11-7. Avoid Closed-Loop Signal Paths
11-6
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