LTC2854 datasheet(16/22 Pages) LINER

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LTC2862A

2862af

For more information www.linear.com/LTC2862A

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 faster than 1V/μs.

Good supply decoupling and proper driver termination

also reduce glitches caused by driver transitions.

RS485 Cable Length vs Data Rate

Many factors contribute to the maximum cable length

that can be used for RS485 or RS422 communication,

including driver transition times, receiver threshold, duty

cycle distortion, cable properties and data rate. A typical

curve of cable length versus maximum data rate is shown

in Figure 10. Various regions of this curve reflect different

performance limiting factors in data transmission.

Atfrequenciesbelow100kbps,themaximumcablelengthis

determined by DC resistance in the cable. In this example,

a cable longer than 4000ft will attenuate the signal at the

far end to less than what can be reliably detected by the

receiver.

For data rates above 100kbps the capacitive and inductive

properties of the cable begin to dominate this relationship.

The attenuation of the cable is frequency and length

dependent, resulting in increased rise and fall times at

the far end of the cable. At high data rates or long cable

lengths, these transition times become a significant part

of the signal bit time. Jitter and intersymbol interference

aggravate this so that the time window for capturing valid

data at the receiver becomes impossibly small.

The boundary at 20Mbps in Figure 10 represents the

guaranteed maximum operating rate of the LTC2862A-1.

Thedashedverticallineat10Mbpsrepresentsthespecified

maximum data rate in the RS485 standard. This boundary

is not a limit, but reflects the maximum data rate that the

specification was written for.

It should be emphasized that the plot in Figure 10 shows

a typical relation between maximum data rate and cable

length. Results with the LTC2862A will vary, depending on

cable properties such as conductor gauge, characteristic

impedance, insulation material, and solid versus stranded

conductors.

Low EMI 250kbps Data Rate

The LTC2862A-2 features slew rate limited transmitters

for low electromagnetic interference (EMI) in sensitive

applications. The slew rate limit circuit maintains

consistent control of transmitter slew rates across voltage

and temperature to ensure low EMI under all operating

conditions. Figure 11 demonstrates the reduction in

high frequency content achieved by the 250kbps mode

compared to the 20Mbps mode.

The 250kbps mode has the added advantage of reducing

signal reflections in an unterminated network, and there-

by increasing the length of a network that can be used

without termination. Using the rule of thumb that the rise

time of the transmitter should be greater than four times

the one-way delay of the signal, networks of up to 140

feet can be driven without termination.

applicaTions inForMaTion

Figure 10. Cable Length vs Data Rate (RS485/RS422 Standard

Shown in Vertical Dashed Line)

Figure 11. High Frequency EMI Reduction of Slew Limited

250kbps Mode Compared to Non Slew Limited 20Mbps Mode

DATA RATE (bps)

10k

100

10k

100k

10M

2862A F10

100M

LOW EMI

MODE

LTC2862A-2

RS485

STANDARD

SPEC

FREQUENCY (MHz)

–120

–40

–60

–80

–100

–20

–60

–20

–40

2862A F11

NON SLEW LIMITED

SLEW LIMITED

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