AN1318 - APPLICATION NOTE

6/9

As described for LVPECL to interface from PECL to LVDS a thevenin equation should be applied. The

Thevenin equation resistor terminates the transmission line Z near the receiver.

- The line characteristics impedance is:

- The DC condition in point A is VCC - 2V

- The DC levels at the LVDS input B are located within the LVDS input common mode range.

The LVDS input swing decreases depending on R2 and R3

8. INTERFACING LVDS TO PECL.

The direct translation between LVDS and PECL/LVPECL signals is not possible. This is because the

LVDS output common mode and differential voltage are not compatible with PECL input levels. Devices

like MC100(LV)EL17 should be used to translate these signals.

Figure 8: Interfacing LVDS to PECL/LVPECL Using the MC100(LV)EL17 device

9. INTERFACING ECL TO LVDS.

The ECC output requires a DC path to VEE. The pull down resistors are connected to VEE. The

Thevenin resistor pair represents the termination of the transmission line Z (R1 || R2). For example,

R1=270Ohm and R2=75Ohm making a parallel of 50Ohm and generates a DC level of 1.2V in the static

entry point of the LVDS circuitry.

Figure 9: Interfacing ECL to LVDS

ZR1

R2

R3

+

()

||

=

pointA

R1

() R1 R2 R3

++

()

⁄

2V

() VCC

()

⁄

=

pointB

R3

() R1 R2 R3

++

()

⁄

VIL

() VCC

()

⁄

=

pointA

in

pointB

R3

() R2 R3

+

()

⁄

∗Vswing

=

in

Vswing

3.3V

5V

Z = 50

Ω

LVDS

MC100(LV)EL17

Z = 50

Ω

R 100

Ω

To PECL

Z = 50

Ω

ECL

3.3V

LVDS

3.3V

VEE

VEE

R1

270

Ω

10 pF

10 pF

Z = 50

Ω

R2

75

Ω

R1

270

Ω

R2

75

Ω