Datasheet

CLKoutX
CLKoutX*
50:
50:Trace
50:
Load
Vcc - 2V
Vcc - 2V
LVPECL
Driver
CLKoutX
CLKoutX*
120:120:
0.1 PF
0.1 PF
LVPECL
Reciever
100:Trace
(Differential)
LVPECL
Driver
82:
120:
Vcc
82:
120:
Vcc
CLKoutX
CLKoutX*
0.1 PF
0.1 PF
LVDS
Receiver
50:
100:Trace
(Differential)
LVDS
Driver
50:
Vbias
LMK01000
SNAS437G –FEBRUARY 2008–REVISED OCTOBER 2009 www.ti.com
Figure 7. Differential LVDS Operation, AC Coupling
LVPECL drivers require a DC path to ground. When AC coupling an LVPECL signal use 120 Ω emitter resistors
close to the LVPECL driver to provide a DC path to ground as shown in Figure 11. For proper receiver operation,
the signal should be biased to the DC bias level (common mode voltage) specified by the receiver. The typical
DC bias voltage (common mode voltage) for LVPECL receivers is 2 V. A Thevenin equivalent circuit (82 Ω
resistor connected to Vcc and a 120 Ω resistor connected to ground with the driver connected to the junction of
the 82 Ω and 120 Ω resistors) is a valid termination as shown in Figure 8 for Vcc = 3.3 V. Note: this Thevenin
circuit is different from the DC coupled example in Figure 6.
Figure 8. Differential LVPECL Operation, AC Coupling, Thevenin Equivalent
Termination for Single-Ended Operation
A balun can be used with either LVDS or LVPECL drivers to convert the balanced, differential signal into an
unbalanced, single-ended signal.
It is possible to use an LVPECL driver as one or two separate 800 mV p-p signals. When DC coupling one of the
LMK01000 family LVPECL drivers, the termination should still be 50 Ω to Vcc - 2 V as shown in Figure 9. Again
the Thevenin equivalent circuit (120 Ω resistor connected to Vcc and an 82 Ω resistor connected to ground with
the driver connected to the junction of the 120 Ω and 82 Ω resistors) is a valid termination as shown in Figure 10
for Vcc = 3.3 V.
Figure 9. Single-Ended LVPECL Operation, DC Coupling
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