When identifying and controlling
electromagnetic interference (EMI),
discussion of common-mode and differentialmode
interference will likely dominate.
Differential-mode interference is a signal
that appears on two lines of a closed loop,
but current flow is in opposite directions.
This kind of interference essentially appears
in series with the desired signal. The solution
is an inductor in series with the high
side (and/or low side) of the line and a shunt
capacitor across the lines.
A common-mode EMI signal appears on
the two signal lines simultaneously in the
same direction and phase. The signal on each
line returns through a common ground. The
solution, a common-mode choke, produces
equal and opposite magnetic fields that cancel
one another. A capacitor from each line to
ground is also used.
Reprints
