7

LTC490

S

APPLICATI

I FOR ATIO

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). 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 re-

flected 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/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.

Rt

DRIVER

DX

RECEIVER

RX

Rt = 120

Ω

Rt = 47

Ω

Rt = 470

Ω

LTC490 • TA10

PROBE HERE

Figure 9. Termination Effects

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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 LTC490. One way to eliminate the

unwanted current is by AC coupling the termination resis-

tors as shown in Figure 10.

LTC490 • TA11

120

Ω

RECEIVER

RX

C

C = LINE LENGTH (FT)

× 16.3pF

Figure 10. 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).

Fault Protection

All of LTC’s RS485 products are protected against ESD

transients up to 2kV using the human body model (100pF,

1.5k

Ω). However, some applications need more

protection. The best protection method is to connect a

bidirectional TransZorb

(Figure 11). A TransZorb

TransZorb