873990AY
www.idt.com
REV. B JULY 25, 2010
9
ICS873990
LOW VOLTAGE, LVCMOS/
CRYSTAL-TO-LVPECL/ECL CLOCK GENERATOR
APPLICATION INFORMATION
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 3.3V LVPECL OUTPUTS
FIGURE 3B. LVPECL OUTPUT TERMINATION
FIGURE 3A. LVPECL OUTPUT TERMINATION
As in any high speed analog circuitry, the power supply pins
are vulnerable to random noise.The ICS873990 provides sepa-
rate power supplies to isolate any high switching
noise from the outputs to the internal PLL. V
CC, VCCA, and VCCO
should be individually connected to the power supply
plane through vias, and bypass capacitors should be
used for each pin. To achieve optimum jitter performance,
power supply isolation is required.
Figure 2 illustrates how
a 10
Ω resistor along with a 10μF and a .01μF bypass
capacitor should be connected to each V
CCA pin.
POWER SUPPLY FILTERING TECHNIQUES
FIGURE 2. POWER SUPPLY FILTERING
10
Ω
V
CCA
10
μF
.01
μF
3.3V
.01
μF
V
CC