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USING AN LVDS RECEIVER WITH RS-422 DATA

45.3 Ω

5.11 Ω

’LVDS32

SN55LVDS32 , SN65LVDS32

SN65LVDS3486 , SN65LVDS9637

SLLS262Q – JULY 1997 – REVISED JULY 2007

APPLICATION INFORMATION (continued)

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

addition of an attenuator circuit. This technique gives the user a 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 in Figure 14 . 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 greater 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.

NOTE: The components used were standard values. (1) R1, R2 = NRC12F45R3TR, NIC components, 45.3 Ω , 1/8 W, 1%,

1206 package (2) R3, R4 = NRC12F5R11TR, NIC components, 5.11 Ω , 1/8 W, 1%, 1206 package (3) 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 14. 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, 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.

The devices are generally used as building blocks for high-speed point-to-point data transmission where ground

differences are less than 1 V. Devices can interoperate with RS-422, PECL, and IEEE-P1596. Drivers/receivers

approach ECL speeds without the power and dual-supply requirements.

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