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ICS843001A21 Datasheet(PDF) 9 Page - Integrated Circuit Systems |
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ICS843001A21 Datasheet(HTML) 9 Page - Integrated Circuit Systems |
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9 / 15 page  843001AG-21 www.icst.com/products/hiperclocks.html REV. A OCTOBER 26, 2005 9 Integrated Circuit Systems, Inc. ICS843001-21 FEMTOCLOCKS™CRYSTAL-TO-3.3V LVPECL FREQUENCY SYNTHESIZER TERMINATION FOR 3.3V LVPECL OUTPUT 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 ter- mination 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. There- fore, terminating resistors (DC current path to ground) or current sources must be used for functionality. These FIGURE 3B. LVPECL OUTPUT TERMINATION FIGURE 3A. LVPECL OUTPUT TERMINATION outputs are designed to drive 50 Ω transmission lines. Matched impedance techniques should be used to maxi- mize operating frequency and minimize signal distor- tion. 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 compat- ibility across all printed circuit and clock component pro- cess variations. INPUTS: CRYSTAL INPUT: 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 1k Ω resistor can be tied from XTAL_IN to ground. TEST_CLK INPUT: For applications not requiring the use of the test clock, it can be left floating. Though not required, but for additional protection, a 1k Ω resistor can be tied from the TEST_CLK to ground. LVCMOS CONTROL PINS: All control pins have internal pull-ups or pull-downs; additional resistance is not required but can be added for additional protection. A 1k Ω resistor can be used. RECOMMENDATIONS FOR UNUSED INPUT AND OUTPUT PINS OUTPUTS: LVCMOS OUTPUT: All unused LVCMOS output can be left floating. We recommend that there is no trace attached. LVPECL OUTPUT 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. |
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