DS08MB200 Datasheet, PDF (7/8 Pages) National Semiconductor (TI) – Dual 800 Mbps 1:2/2:1 LVDS Mux/Buffer

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 V_CC-2.0
load. A modern LVPECL driver will typically include the ter-
mination 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.
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 resis-
tor 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
another power supply. Some receivers have required biasing
voltage available on-chip (V_T, V_TT or V_BB).
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FIGURE 2. DC Coupled LVPECL to LVDS Interface
Figure 2 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 (Note 16). 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, V_CM, range. Most National
Semiconductor LVDS receivers have wide V_CM range. The
exceptions are noted in devices’ respective datasheets.
Those LVDS devices that do have a wide V_CM range do not
vary in performance significantly when receiving a signal
with a common mode other than standard LVDS V_CM of 1.2
V.
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FIGURE 4. DC Coupled LVDS to LVPECL Interface
Figure 4 illustrates interface between an LVDS driver and a
LVPECL with a V_T pin available. R1 and R2, if not present in
the receiver (Note 16), provide proper resistive load for the
driver and termination for the transmission line, and V_T sets
desired bias for the receiver.
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FIGURE 3. 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 3 illustrates an AC coupled interface
between a LVPECL driver and LVDS receiver. R1 and R2, if
not present in the driver device (Note 16), provide DC load
for the emitter followers and may range between 140-220
ohms for most LVPECL devices for this particular configura-
tion. 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
V_DD/2 or 0 V depending on the actual failsafe implementa-
tion. If internal biasing is not implemented, the signal com-
mon mode voltage will slowly wander to GND level.
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FIGURE 5. AC Coupled LVDS to LVPECL Interface
Figure 5 illustrates AC coupled interface between an LVDS
driver and LVPECL receiver without a V_T pin available. The
resistors R1, R2, R3, and R4, if not present in the receiver
(Note 16), provide a load for the driver, terminate the trans-
mission line, and bias the signal for the receiver.
Note 16: 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 be-
tween the driver and receiver terminations and common mode operating
ranges.
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