ICS844246DI Data Sheet

FEMTOCLOCK® CRYSTAL-TO-LVDS FREQUENCY SYNTHESIZER w/FANOUT BUFFER

ICS844246DGI REVISION A NOVEMBER 28, 2012

11

©2012 Integrated Device Technology, Inc.

Recommendations for Unused Input and Output Pins

Inputs:

LVCMOS Control Pins

All control pins have internal pullups or pulldowns; additional

resistance is not required but can be added for additional protection.

A 1k

 resistor can be used.

Outputs:

LVDS Outputs

All unused LVDS output pairs can be either left floating or terminated

with 100

 across. If they are left floating, there should be no trace

attached.

LVDS Driver Termination

For a general LVDS interface, the recommended value for the

termination impedance (ZT) is between 90 and 132. The actual

value should be selected to match the differential impedance (Z0) of

your transmission line. A typical point-to-point LVDS design uses a

100

 parallel resistor at the receiver and a 100 differential

transmission-line environment. In order to avoid any transmission-

line reflection issues, the components should be surface mounted

and must be placed as close to the receiver as possible. IDT offers a

full line of LVDS compliant devices with two types of output structures:

current source and voltage source. The standard termination

schematic as shown in Figure 2A can be used with either type of

output structure. Figure 2B, which can also be used with both output

types, is an optional termination with center tap capacitance to help

filter common mode noise. The capacitor value should be

approximately 50pF. If using a non-standard termination, it is

recommended to contact IDT and confirm if the output structure is

current source or voltage source type. In addition, since these outputs

are LVDS compatible, the input receiver’s amplitude and

common-mode input range should be verified for compatibility with

the output.

LVDS Termination

LVDS

Driver

LVDS

Driver

LVDS

Receiver

LVDS

Receiver

ZT

C

ZO  ZT

ZO  ZT

ZT

2

ZT

2

Figure 2A. Standard Termination

Figure 2B. Optional Termination