www.fairchildsemi.com

2

FIGURE 1. Driver Output Levels

FIGURE 2. Application of RS-422 Data to an LVDS Receiver

The R1, R2, and R3 resistor divider network collectively is

a total differential load of 100

Ω to match the characteristic

impedance of the transmission line and reduces the typical

RS-422 differential signal amplitude of 3.3V down to a

330mV level. Essentially, the resistor network is a 10:1

attenuation circuit. Refer to the equation below to calculate

the voltage drops across the resistors.

The majority (90%) of the RS-422 voltage is dropped

across resistors R1 and R2, leaving the remaining voltage

to be dropped across R3 which is compatible to the LVDS

receiver.

(Current through the resistor network)

= 3.3V / (R1 + R2 = R3)

= 3.3V / 100Ω

= 33 mA

(Voltage across R1)

(Voltage across R2)

(Voltage across R3)

V1

= I

V2

= I

V3

= I

= (33mA)(45Ω)

= (33mA)(45Ω)

= (33mA)(10Ω)

= 1.485V

= 1.485V

= 330mV