LTC487



487fc

applicaTions inForMaTion

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 220 times greater than

the supply current of the LTC487. One way to eliminate

the unwanted current is by AC coupling the termination

resistors as shown in Figure 10.

LTC487 TA11

C = LINE LENGTH (FT) 16.3pF

120

RECEIVER

RX

C

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 ca-

pacitor 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).

Receiver Open-Circuit Fail-Safe

Some data encoding schemes require that the output of

the receiver maintains a known state (usually a logic 1)

when the data is finished transmitting and all drivers on

the line are forced into three-state. All LTC RS485 receivers

have a fail-safe feature which guarantees the output to be

in a logic 1 state when the receiver inputs are left floating

(open-circuit). However, when the cable is terminated with

120Ω, the differential inputs to the receiver are shorted

together, not left floating. Because the receiver has about

70mV of hysteresis, the receiver output will maintain the

last data bit received.

If the receiver output must be forced to a known state, the

circuits of Figure 11 can be used.

LTC487 TA12

140

RECEIVER

RX

5V

1.5k

RECEIVER

RX

5V

110

130

110

130

120

RECEIVER

RX

C

5V

100k

1.5k

Figure 11. Forcing ‘0’ When All Drivers Are Off

The termination resistors are used to generate a DC

bias which forces the receiver output to a known state,

in this case a logic 0. The first method consumes about

208mW and the second about 8mW. The lowest power

solution is to use an AC termination with a pull-up resis-

tor. Simply swap the receiver inputs for data protocols

ending in logic 1.

Fault Protection

All of LTC’s RS485 products are protected against ESD

transients up to 2kV using the human body model

(100pF, 1.5kΩ). However, some applications need more

protection. The best protection method is to connect a

bidirectional TransZorb from each line side pin to ground

(Figure 12).