Datasheet

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
LMK03000, LMK03000C, LMK03000D, LMK03001
LMK03001C, LMK03001D, LMK03033, LMK03033C
SNAS381O –NOVEMBER 2006–REVISED MARCH 2013 www.ti.com
8.7.2 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 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. If the receiver does
not automatically bias its input, one way to do this is with the termination circuitry in Figure 8-7.
When using AC coupling with LVDS outputs, there may be a startup delay observed in the clock output
due to capacitor charging. Figure 8-7 employs 0.1 µF capacitors. This value may need to be adjusted to
meet the startup requirements for a particular application.
Figure 8-7. 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 8-8. 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 8-8 for Vcc = 3.3 V. Note this Thevenin circuit is different from the DC coupled example in Figure 8-
6.
Figure 8-8. Differential LVPECL Operation, AC Coupling, Thevenin Equivalent
34 Application Information Copyright © 2006–2013, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: LMK03000 LMK03000C LMK03000D LMK03001 LMK03001C LMK03001D LMK03033
LMK03033C