LTC486IN(RevA) 데이터 시트보기 (PDF) - Linear Technology
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LTC486IN Datasheet PDF : 8 Pages
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LTC486
APPLICATI S I FOR ATIO
EN
43
1
DX
DX
120Ω
SHIELD
SHIELD
EN
4
2
120Ω
RX
3
RX
2
1
12
12
EN
EN
1/4 LTC486
43
1
DX
DX
EN
EN
4
1
1/4 LTC488
3
RX
RX
2
2
12
12
EN
1/4 LTC486
EN
1/4 LTC488
LTC486 • TA07
Figure 6. Typical Connection
Losses in a transmission line are a complex combination
10k
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-
1k
tric losses are of the same order of magnitude, with
relatively low overall loss (Figure 7).
10
100
1
0.1
0.1
1
10
100
FREQUENCY (MHz)
LTC486 • 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.
Cable Termination
The proper termination of the cable is very important. If the
cable is not terminated with its characteristic impedance,
10
10k
100k
1M 2.5M 10M
DATA RATE (bps)
LTC486 • TA09
Figure 8. Cable Length vs Data Rate
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).
If the cable is loaded excessively (e.g., 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 amplitude will be lowered. The width of the
pedestal is equal to twice the electrical length of the cable
(about1.5ns/ft). If the cable is lightly loaded (e.g., 470Ω),
6
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