ICS843N3960I Datasheet, PDF (11/20 Pages) Integrated Device Technology – Fourth Generation FemtoClock

ICS843N3960I Data Sheet
FEMTOCLOCK® NG CRYSTAL-TO-3.3V LVPECL CLOCK GENERATOR
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 50Ω
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
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 1kΩ resistor can be used.
CLK/nCLK Inputs
For applications not requiring the use of the differential input, both
CLK and nCLK can be left floating. Though not required, but for
additional protection, a 1kΩ resistor can be tied from CLK to ground.
Crystal Inputs
For applications not requiring the use of the crystal oscillator input,
both XTAL_IN and XTAL_OUT can be left floating. Though not
required, but for additional protection, a 1kΩ resistor can be tied from
XTAL_IN to ground.
Outputs:
LVPECL Outputs:
All unused LVPECL outputs can be left floating. We recommend that
there is no trace attached. Both sides of the differential output pair
should either be left floating or terminated.
LVCMOS Outputs
The unused LVCMOS output can be left floating. We recommend that
there is no trace attached.
ICS843N3960DGI REVISION B SEPTEMBER 24, 2012
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©2012 Integrated Device Technology, Inc.