Datasheet

CLKoutX

CLKoutX*

LVDS

Receiver

100:

100:Trace

(Differential)

LVDS

Driver

LMK03000, LMK03000C, LMK03000D, LMK03001

LMK03001C, LMK03001D, LMK03033, LMK03033C

SNAS381O –NOVEMBER 2006–REVISED MARCH 2013

www.ti.com

8.7 TERMINATION AND USE OF CLOCK OUTPUTS (DRIVERS)

When terminating clock drivers keep in mind these guidelines for optimum phase noise and jitter

performance:

• Transmission line theory should be followed for good impedance matching to prevent reflections.

• Clock drivers should be presented with the proper loads. For example:

– LVDS drivers are current drivers and require a closed current loop.

– LVPECL drivers are open emitter and require a DC path to ground.

• Receivers should be presented with a signal biased to their specified DC bias level (common mode

voltage) for proper operation. Some receivers have self-biasing inputs that automatically bias to the

proper voltage level. In this case, the signal should normally be AC coupled.

It is possible to drive a non-LVPECL or non-LVDS receiver with a LVDS or LVPECL driver as long as the

above guidelines are followed. Check the datasheet of the receiver or input being driven to determine the

best termination and coupling method to be sure that the receiver is biased at its optimum DC voltage

(common mode voltage). For example, when driving the OSCin/OSCin* input of the LMK03000 family,

OSCin/OSCin* should be AC coupled because OSCin/OSCin* biases the signal to the proper DC level,

see Figure 8-1. This is only slightly different from the AC coupled cases described in 3.7.2 because the

DC blocking capacitors are placed between the termination and the OSCin/OSCin* pins, but the concept

remains the same, which is the receiver (OSCin/OSCin*) set the input to the optimum DC bias voltage

(common mode voltage), not the driver.

8.7.1 Termination for DC Coupled Differential Operation

For DC coupled operation of an LVDS driver, terminate with 100 Ω as close as possible to the LVDS

receiver as shown in Figure 8-4. To ensure proper LVDS operation when DC coupling it is recommend to

use LVDS receivers without fail-safe or internal input bias such as DS90LV110T. The LVDS driver will

provide the DC bias level for the LVDS receiver. For operation with LMK03000 family LVDS drivers it is

recommend to use AC coupling with LVDS receivers that have an internal DC bias voltage. Some fail-safe

circuitry will present a DC bias (common mode voltage) which will prevent the LVDS driver from working

correctly. This precaution does not apply to the LVPECL drivers.

Figure 8-4. Differential LVDS Operation, DC Coupling

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LMK03033C