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Esd design issues, Designing systems for esd immunity – Echelon LonWorks Router User Manual

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Most of the RF noise originates in the CPU portion of the RTR-10

router—which effectively means the entire board. Most of the RF noise

originates with the Series 5000 router rchip.

Most of the EMI will be radiated by the network cable and the power

cable.

Filtering is generally necessary to keep RF noise from getting out on the

power cable.

EMI radiators should be kept away from the RTR-10 router or Series

5000 router chip to prevent internal RF noise from coupling onto the

radiators.

The RTR-10 router must be well grounded to ensure that its built-in EMI

filtering works properly. Likewise, a Series 5000 router must be well

grounded.

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 10 kV.

Keyboards, connectors, and enclosures provide paths for static discharges to

reach ESD-sensitive components, such as the Neuron Chip in the RTR-10 or a

Series 5000 router.

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 20 kV discharge will arc about 10 mm

(0.4 inches) through dry air, but the same discharge can travel over 20 mm (0.8

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LONWORKS Router Design Issues