SN75LVDS32, SN75LVDS9637

HIGH-SPEED DIFFERENTIAL LINE RECEIVERS

SLLS360B – JUNE 1999 – REVISED JUNE 2001

10

POST OFFICE BOX 655303

APPLICATION INFORMATION

using an LVDS receiver with RS-422 data

Receipt of data from a TIA/EIA-422 line driver may be accomplished using a TIA/EIA-644 line receiver with the

addition of an attenuator circuit. This technique gives the user a very high-speed and low-power 422 receiver.

If the ground noise between the transmitter and receiver is not a concern (less than

±1 V), the answer can be

as simple as shown below in Figure 5. The use of a resistor divider circuit in front of the LVDS receiver attenuates

the 422 differential signal to LVDS levels.

The resistors present a total differential load of 100

Ω to match the characteristic impedance of the transmission

line and to reduce the signal 10:1. The maximum 422 differential output signal or 6 V is reduced to 600 mV. The

high input impedance of the LVDS receiver prevents input bias offsets and maintains a better than 200-mV

differential input voltage threshold at the inputs to the divider. This circuit is used in front of each LVDS channel

that also receives 422 signals.

R1

45.3

Ω

R2

45.3

Ω

R3

5.11

Ω

R4

5.11

Ω

A

B

Y

’LVDS32

NOTE A: The components used were standard values.

R1, R2 = NRC12F45R3TR, NIC Components, 45.3 Ohm, 1/8W, 1%, 1206 Package

R3, R4 = NRC12F5R11TR, NIC Components, 5.11 Ohm, 1/8W, 1%, 1206 Package

The resistor values do not need to be 1% tolerance. However, it can be difficult locating a supplier of resistors having values less than

100

Ω in stock and readily available. The user may find other suppliers with comparable parts having tolerances of 5% or even 10%.

These parts are adequate for use in this circuit.

Figure 5. RS-422 Data Input to an LVDS Receiver Under Low Ground Noise Conditions

If ground noise between the RS-422 driver and LVDS receiver is a concern, then the common-mode voltage

must be attenuated. The circuit must then be modified to connect the node between R3 and R4 to the LVDS

receiver ground. This modification to the circuit increases the common-mode voltage from

±1 V to greater than

±4.5 V.