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-11 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 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.
Designing Systems for EMC (Electromagnetic Compatibility)
Careful design of application electronics is important to ensure that an LPT-11
transceiver will achieve the desired level of EMC. In a link power network, the
devices “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 device, the usual EMC techniques involving
grounding do not apply. The techniques for designing RF-quiet link power devices
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 devices are not allowed to have an explicit earth ground connection,
it becomes very important to minimize the “leakage” capacitance from circuit traces
in the device to any external pieces of metal near the device. Figure 6.1 shows the
leakage capacitances to earth ground from a device's logic ground (Cleak,
GND
) and
from a digital signal line in the device (Cleak,
SIGNAL
).
6-2
Design
Issues