LTC2850MP datasheet(12/18 Pages) AD

12 / 18 page

LTC2850/LTC2851/LTC2852

285012fe

For more information www.linear.com/LTC2850

APPLICATIONS INFORMATION

To minimize capacitive effects, the differential signals

should be separated by more than the width of a trace

and should not be routed on top of each other if they are

on different signal planes.

Care should be taken to route outputs away from any

sensitive inputs to reduce feedback effects that might

cause noise, jitter, or even oscillations. For example, in

the full-duplex devices, DI and A/B should not be routed

near the driver or receiver outputs.

Thelogicinputshave150mVofhysteresistoprovidenoise

immunity. Fast edges on the outputs can cause glitches in

the ground and power supplies which are exacerbated by

capacitive loading. If a logic input is held near its threshold

(typically 1.5V), a noise glitch from a driver transition may

exceed the hysteresis levels on the logic and data input

pins causing an unintended state change. This can be

avoided by maintaining normal logic levels on the pins

and by slewing inputs through their thresholds by faster

than 1V/µs when transitioning. Good supply decoupling

and proper driver termination also reduce glitches caused

by driver transitions.

285012 F14

DATA RATE (bps)

10k

10M

100k

100M

100

10k

RS485/RS422

MAX DATA RATE

LTC2850/

LTC2851/LTC2852

MAX DATA RATE

Figure 14. Cable Length vs Data Rate

(RS485/RS422 Standard Shown in Solid Line)

Cable Length vs Data Rate

For a given data rate, the maximum transmission distance

is bounded by the cable properties. A curve of cable length

vs data rate compliant with the RS485/RS422 standards

is shown in Figure 14. Three regions of this curve reflect

different performance limiting factors in data transmis-

sion. In the flat region of the curve, maximum distance

is determined by resistive losses in the cable. The down-

ward sloping region represents limits in distance and

data rate due to AC losses in the cable. The solid vertical

line represents the specified maximum data rate in the

RS485/RS422 standards. The dashed lines at 20Mbps

show the maximum data rates of the LTC2850, LTC2851

and LTC2852.

Figure 13. Supply Current vs Data Rate

DATA RATE (Mbps)

0.1

100

285012 F13

RDIFF = 54

CL = 1000pF

CL = 100pF

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