LTC486CS 데이터 시트보기 (PDF) - Linear Technology
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LTC486CS Datasheet PDF : 8 Pages
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LTC486
APPLICATI S I FOR ATIO
PROBE HERE
Rt
DX
DRIVER
RECEIVER
RX
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
Figure 9. Termination Effects
LTC486 • TA10
the signal reflects in phase and increases the amplitude at
the driver output. An input frequency of 30kHz is adequate
for tests out to 4000 ft. of cable.
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. When no data is
being sent 33mA of DC current flows in the cable . This DC
current is about 220 times greater than the supply current
of the LTC486. One way to eliminate the unwanted current
is by AC coupling the termination resistors as shown in
Figure 10.
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, 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/(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. 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 120Ω, the differential inputs to the
receiver are shorted together, not left floating.
If the receiver output must be forced to a known state,
the circuits of Figure 11 can be used.
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
5V
110Ω 130Ω 130Ω 110Ω
RECEIVER
RX
120Ω
C
RECEIVER
RX
5V
1.5k
140Ω
RECEIVER
RX
C = LINE LENGTH (FT) × 16.3pF
LTC486 • TA11
Figure 10. AC Coupled Termination
The coupling capacitor allows high frequency energy to
flow to the termination, but blocks 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
100k
5V
1.5k
C
120Ω
RECEIVER
RX
LTC486 • TA12
Figure 11. Forcing “0” When All Dirvers Are Off
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.
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