User`s guide

6-2
EMI Design Issues
The high-speed digital signals associated with microcontroller designs can generate
unintentional Electromagnetic Interference (EMI). High-speed voltage transitions
generate RF currents that can cause radiation from a product if a length of wire or piece
of metal can serve as an antenna.
Products that use the LPT-10 transceivers together with a Neuron Chip will generally
need to demonstrate compliance with EMI limits established by various regulatory
agencies. In the USA, the FCC
6
requires that unintentional radiators comply with Part
15 level “A” for industrial products, and level “B” for products that can be used in
residential environments. Similar regulations are imposed in most countries throughout
the world
7,8
.
Designing Systems for EMC (Electromagnetic Compatibility)
Careful design of application electronics is important to ensure that an LPT-10
transceiver will achieve the desired level of EMC. In a link power network, the nodes
"float" relative to local safety/earth ground because the single point of earth ground in
the network is at the LPI-10 module. Since no explicit connection to earth ground is
allowed at a link power node, the usual EMC techniques involving grounding do not
apply. The techniques for designing RF-quiet link power nodes are very similar to
those used with battery-powered palmtop computers, since palmtops have no explicit
earth ground connection and have cables that connect them to other devices.
Since link power nodes are not allowed to have an explicit earth ground connection, it
becomes very important to minimize the "leakage" capacitance from circuit traces in
the node to any external pieces of metal near the node. Figure 6.1 shows the leakage
capacitances to earth ground from a node's logic ground (C
leak,GND
) and from a digital
signal line in the node (C
leak,SIGNAL
).