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SLLA120
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Interfacing Between LVPECL, VML, CML, and LVDS Levels
3.1 LVPECL Interface Structures
LVPECL is derived from ECL and PECL and typically uses 3.3 V and ground supply voltage.
The current Texas Instruments serial gigabit solution device that has an integrated LVPECL
driver is the TNETE2201 device.
3.1.1 LVPECL Output Stage
The typical output of an LVPECL driver consists of a differential pair with the emitters connected
to ground via a current source. This differential pair drives a pair of emitter-followers which
provide the current to Output+ and Output–. The LVPECL output stage requires a termination
resistance of 50 Ω to the bias voltage of V
CC
– 2 V. This implies that the emitters of the pair of
followers are around V
CC
– 1.3 V, because there is a 0.7-V drop across the base-emitter
junction. The 0.7-V drop across the 50-Ω termination resistors results in a 14-mA flow through
the PECL driver. Because the output impedance of the driver is lower than 50 Ω, care must be
taken that correct termination is used to reduce voltage reflections that occur due to this
mismatch.
Output +
Output –
GND
Current Source
V
CC
50 Ω 50 Ω
V
CC
*2 V
Figure 2. Typical Structure for an LVPECL Output Stage