10

LTC1685

Very inexpensive unshielded telephone grade twisted pair

is shown in Figure 13. In spite of the noticeable loss at the

receiver input, the LTC1685 can still transfer 30Mbps at

100ft of telephone grade UTP. Note that under all these

conditions, the LTC1685 can pass through a single data

pulse equal to the inverse of the data rate (e.g., 20ns for

50Mbps data rate).

Even at distances of 4000ft, 1Mbps data rates are possible

using the LTC1685 and category 5 UTP. Figure 14a shows

a 1

µs pulse propagated down 4000ft of category 5 UTP.

Notice both the DC and the AC losses at the receiver input.

The DC attenuation is due to the parasitic resistance of the

cable. Figure 14b shows a 1Mbps square wave over

4000ft. To transmit at this speed but using longer cable

lengths, see the LTC1686/LTC1687 high speed RS485

full-duplex transceivers.

Figure 13. 100ft of Telephone Grade UTP: 30Mbps

Figure 12. 1000ft of Category 5 UTP: 4Mbps

1685 F13

DRIVER INPUT

DIFFERENTIAL

RECEIVER

INPUT

RECEIVER

OUTPUT

20ns/DIV

2V/DIV

2V/DIV

2V/DIV

1685 F12

DRIVER

INPUT

RECEIVER

OUTPUT

200ns/DIV

2V/DIV

2V/DIV

Figure 14b. 4000ft of Category 5 UTP: 1Mbps Square Wave

Figure 14a. 4000ft of Category 5 UTP: 1

µs Pulse

High Speed Backplane Transmission

The LTC1685 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 LTC1685

is 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 impedance of the trace. For best

results, the signal should be routed differentially. The true

and complement outputs of the LTC1685 should be routed

on adjacent layers 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

1685 F14a

RECEIVER

INPUT

DRIVER

INPUT

RECEIVER

OUTPUT

1

µs/DIV

1V/DIV

5V/DIV

2V/DIV

CABLE DELAY

1685 F14b

DRIVER

INPUT

RECEIVER

OUTPUT

1

µs/DIV

5V/DIV

2V/DIV