LTC2857HDD-1 データシートの表示（PDF） - Analog Devices

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LTC2857HDD-1

20Mbps and Slew Rate Limited 15kV RS485/RS422 Transceivers

Analog Devices 

LTC2856-1/LTC2856-2

LTC2857-1/LTC2857-2

LTC2858-1/LTC2858-2

APPLICATIONS INFORMATION

75

RDIFF = 54Ω

70

65

CL = 1000pF

60

55

CL = 100pF

50

45102

103

104

105

DATA RATE (kbps)

285678 F08

Figure 8. Supply Current vs Data Rate

High Speed Considerations

A ground plane layout is recommended. A 0.1µF bypass

capacitor less than one-quarter inch away from the VCC

pin is also recommended. The PC board traces connected

to signals A/B and Z/Y should be symmetrical and as short

as possible to maintain good differential signal integrity.

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 sen-

sitive inputs to reduce feedback effects that might cause

noise, jitter or even oscillations. For example, in the full-

duplex LTC2857-1, DI and A/B should not be routed near

the driver or receiver outputs.

The logic inputs have 100mV of hysteresis to provide

noise 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 line termination also reduce

glitches caused by driver transitions.

Cable Length vs Data Rate

For a given data rate, the maximum transmission dis-

tance is bounded by the cable properties. A typical curve

of cable length vs data rate compliant with the RS485

standard is shown in Figure 9. Three regions of this curve

reflect different performance limiting factors in data trans-

mission. 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

standard. The dashed line at 250kbps shows the maxi-

mum data rate of the low-EMI LTC2856-2, LTC2857-2,

and LTC2858-2. The dashed line at 20Mbps shows the

maximum data rates of the LTC2856-1, LTC2857-1 and

LTC2858-1.

10k

LOW-EMI MODE

1k

MAX DATA RATE

NORMAL

MODE MAX

100

DATA RATE

RS485 MAX

DATA RATE

10

10k

100k

1M

10M

100M

DATA RATE (bps)

285678 F09

Figure 9. Cable Length vs Data Rate

(RS485 Standard Shown in Solid Vertical Lines)

Cable Termination

Proper cable termination is very important for good signal

fidelity. If the cable is not terminated with its characteristic

impedance, reflections will result in distorted waveforms.

RS485 transceivers typically communicate over twisted-

pair cables with characteristic impedance ranging from

100Ω to 120Ω. Each end of the network should be termi-

nated with a discrete resistor matching the characteristic

impedance or with an LTC2859/LTC2861 transceiver with

integrated termination capability.

For more information www.linear.com/LTC2856-1

285678fg

13

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