lmk04000 National Semiconductor Corporation, lmk04000 Datasheet - Page 48

• Manufacturer Part Number: lmk04000
• Description: Low-noise Clock Jitter Cleaner With Cascaded Plls
• Manufacturer: National Semiconductor Corporation

www.national.com
Some LVDS receivers may have internal biasing on the in-
puts. In this case, the circuit shown in Figure 11 is modified
by replacing the 50 Ω terminations to Vbias with a single 100
Ω resistor across the input pins of the receiver, as shown in
Figure 12. When using AC coupling with LVDS outputs, there
may be a startup delay observed in the clock output due to
capacitor charging. The previous figures employ a 0.1 µF ca-
pacitor. This value may need to be adjusted to meet the
startup requirements for a particular application.
FIGURE 12. LVDS Termination for a Self-Biased Receiver
LVPECL drivers require a DC path to ground. When AC cou-
pling an LVPECL signal use 120 Ω emitter resistors close to
the LVPECL driver to provide a DC path to ground as shown
in Figure 13. For proper receiver operation, the signal should
be biased to the DC bias level (common mode voltage) spec-
ified by the receiver. The typical DC bias voltage for LVPECL
receivers is 2 V. A Thevenin equivalent circuit (82 Ω resistor
connected to V
with the driver connected to the junction of the 82 Ω and 120
Ω resistors) is a valid termination as shown in Figure 13 for
V
DC coupled example in Figure 10.
FIGURE 13. Differential LVPECL Operation, AC Coupling,
17.8.3 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.
FIGURE 11. Differential LVDS Operation, AC Coupling,
Thevenin Equivalent, External Biasing at the Receiver
CC
= 3.3 V. Note this Thevenin circuit is different from the
External Biasing at the Receiver
CC
and a 120 Ω resistor connected to ground
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It is possible to use an LVPECL driver as one or two separate
800 mVpp signals. When using only one LVPECL driver of a
CLKoutX/CLKoutX* pair, be sure to properly terminated the
unused driver. When DC coupling one of the LMK04000 fam-
ily clock LVPECL drivers, the termination should be 50 Ω to
V
circuit is also a valid termination as shown in Figure 15 for Vcc
= 3.3 V.
When AC coupling an LVPECL driver use a 120 Ω emitter
resistor to provide a DC path to ground and ensure a 50 Ω
termination with the proper DC bias level for the receiver. The
typical DC bias voltage for LVPECL receivers is 2 V (See
Section 17.9.2 Driving CLKin Pins with a Single-Ended
Source). If the companion driver is not used it should be ter-
minated with either a proper AC or DC termination. This latter
example of AC coupling a single-ended LVPECL signal can
be used to measure single-ended LVPECL performance us-
ing a spectrum analyzer or phase noise analyzer. When using
most RF test equipment no DC bias point (0 VDC) is required
for safe and proper operation. The internal 50 Ω termination
of the test equipment correctly terminates the LVPECL driver
being measured as shown in Figure 16.
CC
FIGURE 14. Single-Ended LVPECL Operation, DC
FIGURE 15. Single-Ended LVPECL Operation, DC
FIGURE 16. Single-Ended LVPECL Operation, AC
- 2 V as shown in Figure 14. The Thevenin equivalent
Coupling, Thevenin Equivalent
Coupling
Coupling
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