9

LTC1688/LTC1689

High Speed Twisted-Pair Transmission

Data rates up to 100Mbps can be transmitted over short

distances using Category 5 UTP (unshielded twisted pair).

The cable distance will determine the maximum data rate.

Figures 11 and 12 show an 8ns pulse propagating over 25

feet of Category 5 UTP. Notice how the cable attenuates the

signal. Lucent Technologies’ BRF2A and BRS2A receivers

are recommended for these ultrahigh speed applications.

High Speed Backplane Transmission

The LTC1688/LTC1689 can be used in backplane point-to-

point and multipoint applications. At high data rates,

signals should be routed differentially and PC traces

should be terminated (see Figure 13). Note that the RS485

specification calls for characteristic impedances near 100

Ω;

therefore, PC trace transmission lines should be designed

with an impedance close to 100

Ω. If trace impedance is

much less than 100

Ω, and the trace is double terminated,

the part will experience excess heating. The propagation

delay could then fall outside the specified window. The

LT1720 dual UltraFast

high data rate backplane applications.

Figure 12. 100Mbps Differential Data Connection

Figure 11. 8ns Pulse Over 25 Feet Category 5 UTP

10ns/DIV

2V/DIV

2V/DIV

2V/DIV

5V/DIV

1688/89 F11

DRIVER INPUT

DRIVER OUTPUT

RECEIVER INPUT

RECEIVER OUTPUT

1688/89 F12

RECEIVER

1/4 LTC1688

100

Ω

100

Ω

+

–

25 FT CATEGORY 5 UTP

DRIVER

Figure 13. 100Mbps Backplane Transmission

BACKPLANE

RECEIVER

1/4 LTC1688

1/2 LT1720

100

Ω

1688/89 F13

DRIVER

TRANSMISSION LINE

UltraFast is a trademark of Linear Technology Corporation.