Datasheet

CLKoutX
CLKoutX*
LVPECL
Receiver
120:
100:Trace
(Differential)
120:
Vcc
Vcc
LVPECL
Driver
82:82:
CLKoutX
CLKoutX*
LVPECL
Receiver
50:
100:Trace
(Differential)
50:
Vcc - 2 V
Vcc - 2 V
LVPECL
Driver
CLKoutX
CLKoutX*
LVDS
Receiver
100:
100:Trace
(Differential)
LVDS
Driver
LMK01000
www.ti.com SNAS437G –FEBRUARY 2008–REVISED OCTOBER 2009
Figure 4. Differential LVDS Operation, DC Coupling
For DC coupled operation of an LVPECL driver, terminate with 50 Ω to Vcc - 2 V as shown in Figure 5.
Alternatively terminate with a 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 resitors) as shown in
Figure 6 for Vcc = 3.3 V.
Figure 5. Differential LVPECL Operation, DC Coupling
Figure 6. Differential LVPECL Operation, DC Coupling, Thevenin Equivalent
Termination for AC Coupled Differential Operation
AC coupling allows for shifting the DC bias level (common mode voltage) when driving different receiver
standards. Since AC coupling prevents the driver from providing a DC bias voltage at the receiver it is important
to ensure the receiver is biased to its ideal DC level.
When driving LVDS receivers with an LVDS driver, the signal may be AC coupled by adding DC blocking
capacitors, however the proper DC bias point needs to be established at the receiver. One way to do this is with
the termination circuitry in Figure 7.
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