7

LTC486

S

APPLICATI

I FOR ATIO

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 representation that the

interconnection of its circuits as described herein will not infringe on existing patent rights.

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

Ωcables. 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/(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.

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

Figure 10. AC Coupled Termination

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC486 • TA10

PROBE HERE

Figure 9. Termination Effects

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 120

Ω 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.

LTC486 • TA11

C = LINE LENGTH (FT)

× 16.3pF

120

Ω

RECEIVER

RX

C

LTC486 • 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 Dirvers Are Off

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