LTC487

6

S

APPLICATI

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).

FREQUENCY (MHz)

0.1

0.1

1.0

10

1.0

10

100

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.

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

LTC487 • TA09

2.5M

Figure 8. Cable Length vs Data Rate

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).

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC487 • TA10

PROBE HERE

Figure 9. Termination Effects

If the cable is loaded excessively (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 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 (470

Ω), 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.