A ground plane is recommended when using high fre-
quency devices like the LTC1688/LTC1689. A 0.1µF
ceramic bypass capacitor less than 0.25 inch away from
pin is also recommended. Special care should be
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
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
CATEGORY 5 CABLE
Figure 14. Test Circuit for Cable Speed Evaluation
Figure 15. 4000 Feet, 0.5Mbps, LTC1518 Receiver
Figure 17. 1000 Feet, 2Mbps, LTC1518 ReceiverFigure 16. 4000 Feet, 1Mbps, LTC1518 Receiver