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Router User’s Guide
6-3
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Early EMI testing of prototypes at a certified outdoor range is an extremely
important step in the design of level “B” products. This testing ensures that
grounding and enclosure design questions are addressed early enough to avoid most
last-minute changes.
ESD Design Issues
Electrostatic Discharge (ESD) is encountered frequently in industrial and commercial
use of electronic systems. Reliable system designs must consider the effects of ESD and
take steps to protect sensitive components. Static discharges occur frequently in low-
humidity environments when operators touch electronic equipment. The static voltages
generated by humans can easily exceed 10kV. Keyboards, connectors, and enclosures
provide paths for static discharges to reach ESD sensitive components such as the
Neuron Chip. This section describes techniques to design ESD immunity into RTR-10
router-based products.
Designing Systems for ESD Immunity
ESD hardening includes the following techniques:
•
Provide adequate creepage and clearance distances to prevent ESD hits from
reaching sensitive circuitry;
• Provide low impedance paths for ESD hits to ground;
• Use diode clamps or transient voltage suppression devices for accessible, sensitive
circuits
The best protection from ESD damage is circuit inaccessibility. If all circuit components
are positioned away from package seams, the static discharges can be prevented from
reaching ESD sensitive components. There are two measures of "distance" to consider
for inaccessibility: creepage and clearance.
Creepage
is the shortest distance between
two points along the contours of a surface.
Clearance
is the shortest distance between
two points through the air. An ESD hit generally arcs farther along a surface than it
will when passing straight through the air. For example, a 20kV discharge will arc
about 0.4 inches (10 mm) through dry air, but the same discharge can travel over 0.8
inches (20mm) along a clean surface. Dirty surfaces can allow arcing over even longer
creepage distances.