Rt = 470鈩?/div>
LTC490 鈥?TA10
Figure 9. Termination Effects
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
AC Cable Termination
Cable termination resistors are necessary to prevent un-
wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120鈩?resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 60 times greater than
the supply current of the LTC490. One way to eliminate the
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 10.
120鈩?/div>
C
RECEIVER
RX
C = LINE LENGTH (FT)
脳
16.3pF
LTC490 鈥?TA11
W
U
UO
Figure 10. AC Coupled Termination
The coupling capacitor must allow high frequency energy
to flow to the termination, but block DC and low frequen-
cies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
represents an electrical one-tenth wavelength. The value
of the coupling capacitor should therefore be set at 16.3pF
per foot of cable length for 120鈩?cables.
With the coupling capacitors in place, power is consumed
only on the signal edges, and not when the driver output
is idling at a 1 or 0 state. A 100nF capacitor is adequate for
lines up to 4000 feet in length. Be aware that the power
savings start to decrease once the data rate surpasses
1/(120鈩?/div>
脳
C).
Fault Protection
All of LTC鈥檚 RS485 products are protected against ESD
transients up to 2kV using the human body model (100pF,
1.5k鈩?. However, some applications need more
protection. The best protection method is to connect a
bidirectional TransZorb
廬
from each line side pin to ground
(Figure 11). A TransZorb
廬
is a silicon transient voltage
TransZorb
廬
is a registered trademark of General Instruments, GSI
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