
TWISTED
PAIR
NETWORK
LEAKAGE
CAPACITANCES
TO EARTH
GROUND
C
leak,GND
C
load
LPT-11
V
cc
NODE
LOGIC
GROUND
"FLOATING" DEVICE ON LINK POWER NETWORK
C
decouple
C
leak,SIGNAL
V
gate
NET_A
NET_B
Vcc
GND
Figure 6.1
Parasitic Leakage Capacitances to Earth Ground
The single most important technique for designing an EMI “quiet” floating device is
to use logic ground to guard any fast digital signal lines. Effective guarding of traces
with logic ground reduces Cleak,
SIGNAL
significantly, which in turn reduces the
level of common-mode RF currents driven onto the network cable.
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 Vcc, logic ground will be pushed down slightly with respect to earth
ground. As Cleak,
SIGNAL
increases, a larger current flows during V
gate
transitions,
and more common-mode RF current couples to the network twisted pair. This
common-mode RF current can generate EMI in the 30-300MHz frequency band in
excess of “B” levels even when Cleak,
SIGNAL
from a clock line to earth ground is less
than 1pF, so guarding of clock lines is essential for meeting Level “B” requirements.
From this discussion, it is apparent that minimizing Cleak,
SIGNAL
is very
important. By using 0.1µF or 0.01µF decoupling capacitors at each digital IC power
pin, Vcc and logic ground noise can be reduced. Logic ground can then be used as a
ground shield for other noisy digital signals and clock lines.
L
ON
W
ORKS
LPT-11 Transceiver User’s Guide
6-3
Содержание LonWorks LPT-11
Страница 20: ...2 10 Electrical Interface ...
Страница 40: ...5 14 Network Cabling and System Performance ...
Страница 70: ...B 4 Appendix B Differences Between LPT 10 and LPT 11 ...
Страница 76: ...C 6 Appendix C Checklist ...