Echelon FT 3150 Smart Transceiver - Manual (Page 105)

FT 3120 / FT 3150 Smart Transceiver Data Book 99
EMI Design Issues
Careful PCB layout is important to ensure that a FT Smart Transceiver based-device will achieve the desired level
of EMC. A typical FT Smart Transceiver based-device will have several digital signals switching in the 1-40MHz
range. These signals will generate voltage noise near the signal traces, and will also generate current noise in the
signal traces and power supply traces. The goal of good device design is to keep this voltage and current noise from
coupling out of the product's package.
It is very important to minimize the leakage capacitance from circuit traces in the device to any external pieces of
metal near the device, because this capacitance provides a path for the digital noise to couple out of the product's
package. Figure 4.5 shows the leakage capacitances to earth ground from a device's logic ground (C
leak,GND
) and
from a digital signal line in the device (C
leak,SIGNAL
). If the FT Smart Transceiver based-device is housed inside of
a metal chassis, then that metal chassis will probably have the largest leakage capacitance to other nearby pieces of
metal. If the device is housed inside of a plastic package, then PCB ground guarding must be used to minimize
C
leak,SIGNAL
. Effective guarding of digital traces with logic ground reduces C
leak,SIGNAL
significantly, which reduces
the level of common-mode RF currents driven onto the network cable.
Figure 4.5 Parasitic Leakage Capacitances to Earth Ground
When a device is mounted near a piece of metal, especially metal that is earth grounded, any leakage capacitance
from fast signal lines to that external metal will provide a path for RF currents to flow. When V
gate
is pulled down
to logic ground, the voltage of logic ground with respect to earth ground will increase slightly. When V
gate
pulls up
to V
CC
, logic ground will be pushed down slightly with respect to earth ground. As C
leak,SIGNAL
increases, a larger
Twisted
Pair
Network
Leakage
Capacitances to
Earth Ground
C
leak,GND
C
load
Smart Transciever
V cc
Node
Logic
Ground
Floating Device on TP/FT-10 Network
C
decouple
C
leak,SIGNAL
V
gate
NET_A
NET_B
Vcc
GND
CHASSIS
GND
C
leak,CHASSIS