10

LTC1688/LTC1689

Layout Considerations

A ground plane is recommended when using high fre-

quency devices like the LTC1688/LTC1689. A 0.1

µF

ceramic bypass capacitor less than 0.25 inch away from

taken to route the differential outputs very symmetrically

in order to obtain the same parasitic capacitances and thus

maintain good propagation delay skew.

Parasitic capacitance from each input to its corresponding

outputs should also be minimized. Any excess capaci-

tance could result in slower operation or even instability.

Channel output pairs should be kept away from other

output pairs to avoid parasitic coupling.

Data Rate vs Cable Length

Cable length and quality limit the maximum data rate in a

twisted pair system. Category 5 unshielded twisted pair is

a good choice for high speed data transmission, as it

exhibits superior bandwidth over other cables of similar

cost.

Driver and receiver bandwidth affects the maximum data

rate only over distances of less than 100', even for the best

cables. The LTC1688/LTC1689 RS485 drivers and

LTC1518/LTC1519 52Mbps RS485 receivers are the fast-

est in the industry. The LTC1688/LTC1689 drivers can

reach speeds over 100Mbps, with a rise and fall time of

just 2ns. At speeds in excess of 52Mbps, the non-RS485

Lucent Technologies’ BRF2A receiver is recommended.

Detailed information on data rate vs cable length is pro-

vided by the cable manufacturer. They characterize their

cables for bit rate and 0% to 50% rise time vs cable length,

allowing a rapid comparison of various cable types.

The following oscilloscope waveforms illustrate how a

cable attenuates the signal and slows its rise time at

different lengths. Also shown are the driver input and

receiver output.

1688/89 F14

RECEIVER

1/4 LTC1688

1/4 LTC1689

100

Ω

100

Ω

CATEGORY 5 CABLE

UNDER TEST

DRIVER

Figure 14. Test Circuit for Cable Speed Evaluation

2

µs/DIV

1688/89 F15

2V/DIV

DRIVER INPUT

RECEIVER INPUT

RECEIVER OUTPUT

CABLE DELAY

Figure 15. 4000 Feet, 0.5Mbps, LTC1518 Receiver

2

µs/DIV

1688/89 F16

2V/DIV

DRIVER INPUT

RECEIVER INPUT

RECEIVER OUTPUT

CABLE DELAY

Figure 17. 1000 Feet, 2Mbps, LTC1518 Receiver

Figure 16. 4000 Feet, 1Mbps, LTC1518 Receiver

500ns/DIV

1688/89 F17

2V/DIV

DRIVER INPUT

RECEIVER INPUT

RECEIVER OUTPUT

CABLE DELAY