LTC486

9

486fc

APPLICATIONS INFORMATION

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

guidelineforchoosingthemaximumlinelengthforagiven

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 how-

ever, 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 imped-

ance, 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 mister-

mination. 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 (about

1.5ns/ft). If the cable is lightly loaded (e.g., 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 ft. of cable.

AC Cable Termination

Cable termination resistors are necessary to prevent un-

wanted reflections, but they consume power. The typical

differential output voltage of the driver is 2V when the

cable is terminated with two 120Ω resistors. When no

data is being sent 33mA of DC current flows in the cable.

This DC current is about 220 times greater than the supply

current of the LTC486. One way to eliminate the unwanted

current is by AC coupling the termination resistors as

shown in Figure 10.

Figure 8. Cable Length vs Data Rate

DATA RATE (bps)

10k

10

100

1k

10k

100k

1M

10M

486 F08

2.5M

Figure 9. Termination Effects

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Rt = 47

Rt = 470

486 F09

PROBE HERE

486 F10

C = LINE LENGTH (FT)

× 16.3pF

120

RECEIVER

RX

C

Figure 10. AC Coupled Termination