Ron Wellman suggests that it is a truth universally
acknowledged that ESD applied to a connector pin
will almost always lead to failure. Without precautions
this may be so but for low frequency connections
it is possible to protect not only against permanent
damage but also against loss of data as required by
the EMC tests under the EMC directive. A combination
of filtering and clamping very close to the connector
can provide the required protection. The closeness
to the connector is however vital. The threat from the
discharge comes not only from its amplitude but from
its very fast rise time (0.7ns). At this speed a path to
ground within the equipment of only a couple of inches
presents an appreciable impedance and will constitute
a radiating antenna broadcasting to the rest of the
equipment. Filters built in to the connector with feed
through capacitors are best but they should not spread the
energy out over such a long period that it is sensed
as a valid signal. If you are fighting to keep the
bandwith up while providing protection the best scheme
is to use a modest filter in the connector to slow the
edge of the waveform, closely followed by clamping diodes
and then a further filter to reduce the clamp level to
below the signal threshold. This can be expensive
and bulky if you have a lot of signals but it can be done.
Nick Rouse
