LTC486

6

S

APPLICATI

I FOR ATIO

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,

EN

12

EN

4

LTC486 • TA07

120

Ω

DX

1

2

3

SHIELD

120

Ω

RX

RX

SHIELD

3

DX

EN

12

EN

4

2

EN

EN

4

1

2

3

RX

RX

3

DX

EN

12

EN

4

1

DX

1/4 LTC488

1/4 LTC486

1/4 LTC488

1/4 LTC486

2

1

12

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, with

relatively low overall loss (Figure 7).

FREQUENCY (MHz)

0.1

0.1

1

10

1

10

100

LTC486 • TA08

Figure 7. Attenuation vs Frequency for Belden 9841

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

LTC486 • TA09

2.5M

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., 47

Ω), 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., 470

Ω),

Figure 8. Cable Length vs Data Rate

Figure 6. Typical Connection