LTC490

8

490fb

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 LTC490. One way to eliminate the

unwanted current is by AC coupling the termination resis-

tors as shown in Figure 10.

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.

APPLICATIO S I FOR ATIO

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 is a silicon transient voltage

suppressor that has exceptional surge handling capabili-

ties, fast response time, and low series resistance. They

are available from General Instruments, GSI and come in

a variety of breakdown voltages and prices. Be sure to pick

a breakdown voltage higher than the common mode

voltage required for your application (typically 12V). Also,

don’t forget to check how much the added parasitic

capacitance will load down the bus.

TransZorb is a registered trademark of General Instruments, GSI

Figure 10. AC Coupled Termination

Figure 11. ESD Protection with TransZorbs

LTC490 • F10

120

Ω

RECEIVER

RX

C

C = LINE LENGTH (FT)

× 16.3pF

LTC490 • F11

120

Ω

DRIVER

Z

Y