ICS849S625I Datasheet, PDF (14/24 Pages) Integrated Device Technology – Ten selectable differential LVPECL or LVDS outputs

ICS849S625I Data Sheet
CRYSTAL-TO-LVPECL/LVDS CLOCK SYNTHESIZER
LVDS Driver Termination
A general LVDS interface is shown in Figure 2. Standard termination
for LVDS type output structure requires both a 100 parallel resistor
at the receiver and a 100 differential transmission line environment.
In order to avoid any transmission line reflection issues, the 100
resistor 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 2 can be used with either
type of output structure. If using a non-standard termination, it is
recommended to contact IDT and confirm if the output is a current
source or a voltage source type structure. In addition, since these
outputs are LVDS compatible, the input receivers amplitude and
common mode input range should be verified for compatibility with
the output.
LVDS Driver
100Ω
+
LVDS
Receiver
–
100Ω Differential Transmission Line
Figure 2. Typical LVDS Driver Termination
Termination for 3.3V LVPECL Outputs
The clock layout topology shown below is a typical termination for
LVPECL outputs. The two different layouts mentioned are
recommended only as guidelines.
The differential outputs are low impedance follower outputs that
generate ECL/LVPECL compatible outputs. Therefore, terminating
resistors (DC current path to ground) or current sources must be
used for functionality. These outputs are designed to drive 50
transmission lines. Matched impedance techniques should be used
to maximize operating frequency and minimize signal distortion.
Figures 3A and 3B show two different layouts which are
recommended only as guidelines. Other suitable clock layouts may
exist and it would be recommended that the board designers
simulate to guarantee compatibility across all printed circuit and clock
component process variations.
3.3V
Z_o = 50Ω
3.3V
+
LVPECL
Z_o = 50Ω
R1
50Ω
RTT =
1
((V_OH + V_OL) / (V_CC – 2)) – 2
* Z_o
_
Input
R2
50Ω
V_CC - 2V
RTT
Figure 3A. 3.3V LVPECL Output Termination
3.3V
LVPECL
3.3V
R3
R4
125Ω
125Ω
3.3V
Z_o = 50Ω
+
Z_o = 50Ω
R1
84Ω
_
R2
84Ω
Input
Figure 3B. 3.3V LVPECL Output Termination
ICS849S625BYI REVISION A OCTOBER 1, 2012
14
©2012 Integrated Device Technology, Inc.