9

LTC491

S

APPLICATI

I FOR ATIO

Cable Termination

The proper termination of the cable is very important.

If the cable is not terminated with it’s 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 13).

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC491 • TA14

PROBE HERE

Figure 13. 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.

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 120

Ω 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 LTC491. One way to eliminate the

unwanted current is by AC coupling the termination resis-

tors as shown in Figure 14.

LTC491 • TA15

120

Ω

RECEIVER

RX

C

C = LINE LENGTH (ft) x 16.3pF

Figure 14. AC Coupled Termination

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 capacitor

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 120

Ω 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/(120

Ω × C).