LTC2850/LTC2851/LTC2852

11

285012fc

APPLICATIONS INFORMATION

Receiver Input Resistance

The receiver input resistance from A or B to ground is

guaranteed to be greater than 96k (C, I-Grade). This is 8x

higher than the requirements for the RS485 standard and

thus this receiver represents a one-eighth unit load. This,

in turn, means that 8x the standard number of receivers,

or 256 total, can be connected to a line without loading

it beyond what is specified in the RS485 standard. The

receiver input resistance from A or B to ground on high

temperature H-Grade parts is greater than 48k providing

a one-quarter unit load. The high input resistance of the

receiver is maintained whether it is enabled or disabled,

powered or unpowered.

Supply Current

The unloaded static supply currents in these devices are

very low, typically under 500μA for all modes of opera-

tion. In applications with resistively terminated cables,

the supply current is dominated by the driver load. For

example, when using two 120

Ω terminators with a dif-

ferential driver output voltage of 2V, the DC load current

is 33mA, which is sourced by the positive voltage supply.

Power supply current increases with toggling data due to

capacitive loading and this term can increase significantly

at high data rates. Figure 13 shows supply current vs

data rate for two different capacitive loads for the circuit

configuration of Figure 4.

High Speed Considerations

A ground plane layout is recommended. A 0.1μF bypass

is also recommended. The PC board traces connected to

signals A/B and Z/Y should be symmetrical and as short

as possible to maintain good differential signal integrity.

To minimize capacitive effects, the differential signals

should be separated by more than the width of a trace

and should not be routed on top of each other if they are

on different signal planes.

Care should be taken to route outputs away from any

sensitive inputs to reduce feedback effects that might

cause noise, jitter, or even oscillations. For example, in

the full-duplex devices, DI and A/B should not be routed

near the driver or receiver outputs.

The logic inputs have 150mV of hysteresis to provide noise

immunity. Fast edges on the outputs can cause glitches in

the ground and power supplies which are exacerbated by

capacitive loading. If a logic input is held near its threshold

(typically 1.5V), a noise glitch from a driver transition may

exceed the hysteresis levels on the logic and data input

pins causing an unintended state change. This can be

avoided by maintaining normal logic levels on the pins

and by slewing inputs through their thresholds by faster

than 1V/μs when transitioning. Good supply decoupling

and proper driver termination also reduce glitches caused

by driver transitions.

Cable Length vs Data Rate

For a given data rate, the maximum transmission distance

is bounded by the cable properties. A curve of cable length

vs data rate compliant with the RS485/RS422 standards

is shown in Figure 14. Three regions of this curve reflect

different performance limiting factors in data transmis-

sion. In the flat region of the curve, maximum distance

Figure 13. Supply Current vs Data Rate

DATA RATE (Mbps)

0.1

20

60

70

80

1

10

100

285012 F13

50

40

30