LTC2856-2 datasheet(13/20 Pages) LINER

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LTC2856-1/LTC2856-2

LTC2857-1/LTC2857-2

LTC2858-1/LTC2858-2

285678fe

APPLICATIONS INFORMATION

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

sensitive 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 distance

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 differ-

ent performance limiting factors in data transmission. In the

ﬂat region of the curve, maximum distance is determined

by resistive losses in the cable. The downward 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 maximum 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.

Figure 8. Supply Current vs Data Rate

Cable Termination

Proper cable termination is very important for good

signal ﬁdelity. If the cable is not terminated with its char-

acteristic impedance, reﬂections will result in distorted

waveforms.

RS485 transceivers typically communicate over twisted-

pair cables with characteristic impedance ranging from

100

Ω 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.

Figure 9. Cable Length vs Data Rate

(RS485 Standard Shown in Solid Vertical Lines)

DATA RATE (kbps)

102

103

104

105

285678 F08

RDIFF = 54Ω

CL = 1000pF

CL = 100pF

285678 F09

DATA RATE (bps)

10k

10M

100k

100M

100

10k

LOW-EMI MODE

MAX DATA RATE

RS485 MAX

DATA RATE

NORMAL

MODE MAX

DATA RATE

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