LTC488/LTC489

8

4889fb

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 (> 100kbps), 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 8).

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 drive 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, causing

33mA of DC current to flow in the cable when no data

is being sent. This DC current is about 60 times greater

than the supply current of the LTC488/LTC489. One way

to eliminate the unwanted current is by AC coupling the

termination resistors as shown in Figure 9.

The coupling capacitor must allow high frequency energy

to flow to the termination, but block DC and low frequen-

cies. The dividing line between high and low frequency

depends on the length of the cable. The coupling capaci-

tor must pass frequencies above the point where the line

represents an electrical one-tenth wavelength. The value

of the coupling capacitor should therefore be set at 16.3pF

per foot of cable length for 120Ω cables. With the coupling

capacitors in place, power is consumed only on the signal

edges, and not when the driver output is idling at a 1 or 0

state. A 100nF capacitor is adequate for lines up to 4000

feet in length. Be aware that the power savings start to

decrease once the data rate surpasses 1/(120Ω)(C).

APPLICATIONS INFORMATION

Figure 8. Termination Effects

Figure 9. AC-Coupled Termination

4889 F08

DX

PROBE HERE

Rt = 120Ω

Rt = 47Ω

Rt = 470Ω

Rt

RX

RECEIVER

DRIVER

488/9 F09

C = LINE LENGTH (FT)(16.3pF)

120Ω

C

RX

RECEIVER