Datasheet
9
LTC1485
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Figure 12. Termination Effects
1485 F12
DX
DRIVER RECEIVER
RX
PROBE HERE
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
R
t
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 120Ω resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 10 times greater than
the supply current of the LTC1485. One way to eliminate
the unwanted current is by AC-coupling the termination
resistors as shown in Figure 13.
Figure 13. 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 120Ω 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 400 feet
in length. Be aware that the power savings start to de-
crease once the data rate surpasses 1/(120Ω • 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. The receiver of the LTC1485
has a fail-safe feature which guarantees the output to be in
a logic 1 state when the receiver inputs are left floating
(open-circuit).
If the receiver output must be forced to a known state, the
circuits of Figure 14 can be used.
Figure 14. Forcing “0” When All Drivers Are Off
1485 F14
110Ω
RX
130Ω
130Ω
110Ω
5V
RX
1.5k
120Ω
5V
1.5k
RX
RECEIVER
120Ω
5V
C
100k
RECEIVER
RECEIVER
1485 F13
C = LINE LENGTH (FT) • 16.3pF
120Ω
C
RX
RECEIVER