7
LTC487
U
S
A
O
PPLICATI
W
U
U
I FOR ATIO
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
Ω
resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 220 times greater than
the supply current of the LTC487. One way to eliminate the
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 10.
LTC487 • TA11
C = LINE LENGTH (FT) x 16.3pF
120
Ω
RECEIVER
RX
C
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
Ω
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
Ω
×
C).
Receiver Open-Circuit Fail-Safe
Some data encoding schemes require that the output of
the receiver maintains a known state (usually a logic 1)
when the data is finished transmitting and all drivers on the
line are forced into three-state. All LTC RS485 receivers
have a fail-safe feature which guarantees the output to be
in a logic 1 state when the receiver inputs are left floating
(open-circuit). However, when the cable is terminated
with 120
Ω
, the differential inputs to the receiver are
shorted together, not left floating. Because the receiver
has about 70mV of hysteresis, the receiver output will
maintain the last data bit received.
If the receiver output must be forced to a known state, the
circuits of Figure 11 can be used.
LTC487 • TA12
140
Ω
RECEIVER
RX
5V
1.5k
RECEIVER
RX
5V
110
Ω
130
Ω
110
Ω
130
Ω
120
Ω
RECEIVER
RX
C
5V
100k
1.5k
Figure 11. Forcing ‘0’ When All Drivers Are Off
The termination resistors are used to generate a DC bias
which forces the receiver output to a known state, in this
case a logic 0. The first method consumes about
208mW and the second about 8mW. The lowest power
solution is to use an AC termination with a pull-up resistor.
Simply swap the receiver inputs for data protocols ending
in logic 1.
Fault Protection
All of LTC’s RS485 products are protected against ESD
transients up to 2kV using the human body model
(100pF, 1.5k
Ω
). However, some applications need more
protection. The best protection method is to connect a
bidirectional TransZorb
®
from each line side pin to ground
(Figure 12).
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.
TransZorb is a registered trademark of General Instruments, GSI