15MB200

R2

50:

R1

50:

50:

50:

LVPECL

LVPECL

R2

150:

R1

150:

50:

50:

15MB200

0.1 PF

0.1 PF

DS15MB200

SNLS196E – NOVEMBER 2005 – REVISED MARCH 2013

www.ti.com

Figure 9. 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 9 illustrates an AC

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

this particular configuration. The DS15MB200 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

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

Interfacing LVDS to LVPECL

An LVDS driver consists of a current source (nominal 3.5mA) which drives a CMOS differential pair. It needs a

differential resistive load in the range of 70 to 130 ohms to generate LVDS levels. In a system, the load should

be selected to match transmission line characteristic differential impedance so that the line is properly

terminated. The termination resistor should be placed as close to the receiver inputs as possible. When

interfacing an LVDS driver with a non-LVDS receiver, one only needs to bias the LVDS signal so that it is within

the common mode range of the receiver. This may be done by using separate biasing voltage which demands

Figure 10. DC Coupled LVDS to LVPECL Interface

(2)

The bias networks shown above for LVPECL drivers and receivers may or may not be present within the driver device. The LVPECL

driver and receiver specification must be reviewed closely to ensure compatibility between the driver and receiver terminations and

common mode operating ranges.

10

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