LVPECL

R2

150:

R1

150:

50:

50:

15MB200

0.1 PF

0.1 PF

LVPECL

R2

150:

R1

150:

50:

50:

15MB200

DS08MB200

SNLS197D – MAY 2006 – REVISED MARCH 2013

www.ti.com

APPLICATIONS

Interfacing LVPECL to LVDS

An LVPECL driver consists of a differential pair with coupled emitters connected to GND via a current source.

This drives a pair of emitter-followers that require a 50 ohm to VCC-2.0 load. A modern LVPECL driver will

typically include the termination scheme within the device for the emitter follower. If the driver does not include

the load, then an external scheme must be used. The 1.3 V supply is usually not readily available on a PCB,

therefore, a load scheme without a unique power supply requirement may be used.

Figure 7. DC Coupled LVPECL to LVDS Interface

Figure 7 is a separated

π termination scheme for a 3.3 V LVPECL driver. R1 and R2 provides proper DC load for

the driver emitter followers, and may be included as part of the driver device. The DS08MB200 includes a 100

ohm input termination for the transmission line. The common mode voltage will be at the normal LVPECL

levels – around 2 V. This scheme works well with LVDS receivers that have rail-to-rail common mode voltage,

VCM, range. Most Texas Instruments LVDS receivers have wide VCM range. The exceptions are noted in devices’

respective datasheets. Those LVDS devices that do have a wide VCM range do not vary in performance

significantly when receiving a signal with a common mode other than standard LVDS VCM of 1.2 V.

Figure 8. AC Coupled LVPECL to LVDS Interface

An AC coupled interface is preferred when transmitter and receiver ground references differ more than 1 V. This

is a likely scenario when transmitter and receiver devices are on separate PCBs. Figure 8 illustrates an AC

coupled interface between a LVPECL driver and LVDS receiver. R1 and R2, if not present in the driver device,

provide DC load for the emitter followers and may range between 140-220 ohms for most LVPECL devices for

this particular configuration. The DS08MB200 includes an internal 100 ohm resistor to terminate the transmission

line for minimal reflections. The signal after ac coupling capacitors will swing around a level set by internal

biasing resistors (i.e. fail-safe) which is either VDD/2 or 0 V depending on the actual failsafe implementation. If

internal biasing is not implemented, the signal common mode voltage will slowly wander to GND level.

8

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