LTC487ISW(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LTC487ISW
(Rev.:RevA)

Quad Low Power RS485 Driver

Linear Technology 

LTC487

APPLICATI S I FOR ATIO

Losses in a transmission line are a complex combination

of DC conductor loss, AC losses (skin effect), leakage, and

AC losses in the dielectric. In good polyethylene cables

such as the Belden 9841, the conductor losses and dielec-

tric losses are of the same order of magnitude, leading to

relatively low overall loss (Figure 7).

10

1.0

Cable Termination

The proper termination of the cable is very important. If the

cable is not terminated with its characteristic impedance,

distorted waveforms will result. In severe cases, distorted

(false) data and nulls will occur. A quick look at the output

of the driver will tell how well the cable is terminated. It is

best to look at a driver connected to the end of the cable,

since this eliminates the possibility of getting reflections

from two directions. Simply look at the driver output while

transmitting square wave data. If the cable is terminated

properly, the waveform will look like a square wave

(Figure 9).

PROBE HERE

0.1

0.1

1.0

10

100

FREQUENCY (MHz)

LTC487 • TA08

Figure 7. Attenuation vs Frequency for Belden 9841

When using low loss cables, Figure 8 can be used as a

guideline for choosing the maximum line length for a given

data rate. With lower quality PVC cables, the dielectric loss

factor can be 1000 times worse. PVC twisted pairs have

terrible losses at high data rates (> 100kbs) and greatly

reduce the maximum cable length. At low data rates

however, they are acceptable and much more economical.

10k

1k

100

10

10k

100k

1M 2.5M 10M

DATA RATE (bps)

LTC487 • TA09

Figure 8. Cable Length vs Data Rate

DX

DRIVER

Rt

RECEIVER

RX

Rt = 120Ω

Rt = 47Ω

Rt = 470Ω

Figure 9. Termination Effects

LTC487 • TA10

If the cable is loaded excessively (47Ω), the signal initially

sees the surge impedance of the cable and jumps to an

initial amplitude. The signal travels down the cable and is

reflected back out of phase because of the mistermination.

When the reflected signal returns to the driver, the ampli-

tude will be lowered. The width of the pedestal is equal to

twice the electrical length of the cable (about 1.5ns/foot).

If the cable is lightly loaded (470Ω), the signal reflects in

phase and increases the amplitude at the driver output. An

input frequency of 30kHz is adequate for tests out to

4000 feet of cable.

6

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