
8737AG-11
www.idt.com
REV. C AUGUST 9, 2010
8
ICS8737-11
LOW SKEW,
÷÷÷÷÷1/÷÷÷÷÷2
DIFFERENTIAL-TO- 3.3V LVPECL CLOCK GENERATOR
APPLICATION INFORMATION
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
CC = 3.3V, V_REF should be 1.25V and R2/R1 = 0.609.
FIGURE 2. SINGLE ENDED SIGNAL DRIVING DIFFERENTIAL INPUT
R2
1K
V
CC
CLK_IN
+
-
R1
1K
C1
0.1uF
V_REF
V
CC - 2V
50
Ω
50
Ω
RTT
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
RTT =
Z
o
1
((V
OH + VOL) / (VCC – 2)) – 2
3.3V
125
Ω
125
Ω
84
Ω
84
Ω
Z
o = 50Ω
Z
o = 50Ω
FOUT
FIN
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 gen-
erate 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
Ω 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 lay-
outs may exist and it would be recommended that the board
designers simulate to guarantee compatibility across all printed
circuit and clock component process variations.
TERMINATION FOR LVPECL OUTPUTS
FIGURE 3B. LVPECL OUTPUT TERMINATION
FIGURE 3A. LVPECL OUTPUT TERMINATION
WIRING THE DIFFERENTIAL INPUT TO ACCEPT SINGLE ENDED LEVELS