lmk03000disqx National Semiconductor Corporation, lmk03000disqx Datasheet - Page 25

• Manufacturer Part Number: lmk03000disqx
• Description: Precision Clock Conditioner With Integrated Vco
• Manufacturer: National Semiconductor Corporation

FIGURE 8. Differential LVPECL Operation, DC Coupling,
FIGURE 7. Differential LVPECL Operation, DC Coupling
Thevenin Equivalent
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3.7.2 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 re-
ceiver 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, how-
ever the proper DC bias point needs to be established at the
receiver. One way to do this is with the termination circuitry in
Figure 9.
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 10. 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 (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 termi-
nation as shown in Figure 10 for Vcc = 3.3 V. Note this
Thevenin circuit is different from the DC coupled example in
Figure 8.
FIGURE 10. Differential LVPECL Operation, AC Coupling,
FIGURE 9. Differential LVDS Operation, AC Coupling
Thevenin Equivalent
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