ICS853031 Datasheet, PDF (9/19 Pages) Integrated Circuit Systems – LOW SKEW, 1-TO-9 DIFFERENTIAL-TO-2.5V/3.3V LVPECL/ECL FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
ICS853031
LOW SKEW, 1-TO-9
DIFFERENTIAL-TO-2.5V/3.3V LVPECL/ECL FANOUT BUFFER
APPLICATION INFORMATION
WIRING THE DIFFERENTIAL INPUT TO ACCEPT SINGLE ENDED LEVELS
Figure 2 shows how the differential input can be wired to accept
single ended levels. The reference voltage V_REF ~ V_CC/2 is
generated by the bias resistors R1, R2 and C1. This bias circuit
should be located as close as possible to the input pin. The ratio
of R1 and R2 might need to be adjusted to position the V_REF in
the center of the input voltage swing. For example, if the input
clock swing is only 2.5V and V = 3.3V, V_REF should be 1.25V
CC
and R2/R1 = 0.609.
V_CC
CLK_IN
C1
0.1uF
R1
1K
+
V_REF
-
R2
1K
FIGURE 2. SINGLE ENDED SIGNAL DRIVING DIFFERENTIAL INPUT
TERMINATION FOR 3.3V LVPECL OUTPUTS
The clock layout topology shown below is a typical termina-
tion for LVPECL outputs. The two different layouts mentioned
are recommended only as guidelines.
FOUT and nFOUT are low impedance follower outputs that
generate ECL/LVPECL compatible outputs.Therefore, terminat-
ing 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.
Z_o = 50Ω
FOUT
FIN
Z_o = 50Ω
50Ω
RTT =
1
((V_OH + V_OL) / (V_CC – 2)) – 2
Z_o
50Ω
V_CC - 2V
RTT
FOUT
3.3V
125Ω
125Ω
Z_o = 50Ω
FIN
Z_o = 50Ω
84Ω
84Ω
FIGURE 3A. LVPECL OUTPUT TERMINATION
FIGURE 3B. LVPECL OUTPUT TERMINATION
853031AY
www.icst.com/products/hiperclocks.html
REV. B SEPTEMBER 16, 2004
9