LTC1485
10
1485fb
For more information www.linear.com/LTC1485
Figure 12. Termination Effects
APPLICATIONS INFORMATION
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.
1485 F12
DX
DRIVER RECEIVER
RX
PROBE HERE
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
R
t
per foot of cable length for 120Ω cables. With the cou-
pling 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 decrease 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.
1485 F13
C = LINE LENGTH (FT) • 16.3pF
120Ω
C
RX
RECEIVER
110Ω
RX
130Ω
130Ω
110Ω
5V
RX
1.5k
120Ω
5V
1.5k
RX
RECEIVER
120Ω
5V
C
100k
RECEIVER
RECEIVER
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 capaci-
tor 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
Figure 13. AC-Coupled Termination
Figure 14. Forcing “0” When All Drivers Are Off
