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LTC1686

52Mbps Precision Delay RS485 Fail-Safe Transceivers

Linear Technology 

LTC1686/LTC1687

APPLICATIONS INFORMATION

LTC1687

D1

R

2000 FT

LTC1687

R2 D

REPEATER

2000 FT

LTC1687

R3 D

REPEATER

2000 FT

LTC1687

R4 D

REPEATER

Figure 15a. 1.6Mbps, 8000 Feet (1.5 Miles) Using Three Repeaters

2000 FT

LTC1687

R5

1686/87 F15a

2V/DIV

DELAY OF 8000 FT

OF CABLE

DRIVER 1

INPUT

5V/DIV

2V/DIV

RECEIVER 5

OUTPUT

DRIVER 1

INPUT

5V/DIV

2µs/DIV

RECEIVER 5

OUTPUT

1686/87 F15b

Figure 15b. 1.6Mbps Pulse and Square Wave Signals

Over 8000 Feet Category 5 UTP Using Three Repeaters

2V/DIV

1V/DIV

1V/DIV

1V/DIV

5V/DIV

DRIVER 1

INPUT

RECEIVER 2

INPUT

RECEIVER 3

INPUT

RECEIVER 4

INPUT

RECEIVER 5

OUTPUT

2µs/DIV

1686/87 F16

Figure 16. Intermediate Signals of a 1µs Pulse

propagation of a 600ns pulse through the network of

Figure 15A. The bottom two traces show a 1.6Mbps

square wave. Notice that the duty cycle does not notice-

ably degrade. For the case of the single pulse, however,

there is a slight degradation of the pulse width.

By slowing down the data rate slightly to 1Mbps, one can

obtain minimal pulse width degradation as the signal

traverses through the repeater network. Figure 16 shows

that the output pulse (bottom trace) is nearly the same

width to the input pulse (top trace). The middle three

traces of Figure 16 show the signal at the end of each of the

first three 2000 feet sections of category 5 UTP. Notice

how the LTC1687 repeaters are able to regenerate the

signal with little loss. This implies that we can cascade

more repeater networks and potentially achieve 1Mbps

operation at total distances of over 10,000 feet! A higher

data rate can be achieved if the repeaters are spaced closer

together.

HIGH SPEED BACKPLANE TRANSMISSION

The LTC1686/LTC1687 can also be used in backplane

point-to-point transceiver applications, where the user

wants to assure operation even when the common mode

goes above or below the rails. It is advisable to terminate

the PC traces when approaching maximum speeds. Since

the LTC1686/LTC1687 are not intended to drive parallel

terminated cables with characteristic impedances much

less than that of twisted pair, both ends of the PC trace

must be series terminated with the characteristic imped-

ance of the trace. For best results, the signal should be

routed differentially. The true and complement outputs of

the LTC1686/LTC1687 should be routed on adjacent lay-

ers of the PC board. The two traces should be routed very

symmetrically, minimizing and equalizing parasitics to

nearby signal and power/ground layers. For single-ended

transmission, route the series terminated single-ended

trace over an adjacent ground plane. Then set the (by-

passed) negative input of the receiver to roughly 2.5V.

Note that single-ended operation might not reach maxi-

mum speeds.

LAYOUT CONSIDERATIONS

A ground plane is recommended when using high fre-

quency devices like the LTC1686/LTC1687. A 0.1µF ce-

ramic bypass capacitor less than 0.25 inch away from the

VDD pin is also recommended.

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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