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ICS650GI-36LFT Datasheet(PDF) 3 Page - Integrated Circuit Systems |
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ICS650GI-36LFT Datasheet(HTML) 3 Page - Integrated Circuit Systems |
3 / 7 page Networking & PCI Clock Source MDS 650-36 D 3 Revision 030206 In te gr ated C i rcuit Syste m s ● 525 R ace Street, San Jose, CA 9 5126 ● tel (40 8) 29 7-120 1● www.icst.com ICS650-36 External Components Decoupling Capacitor As with any high performance mixed-signal IC, the ICS650-36 must be isolated from system power supply noise to perform optimally. A decoupling capacitor of 0.01µF must be connected between each VDD and the PCB ground plane. Series Termination Resistor Clock output traces over one inch should use series termination. To series terminate a 50 Ω trace (a commonly used trace impedance), place a 33 Ω resistor in series with the clock line, as close to the clock output pin as possible. The nominal impedance of the clock output is 20 Ω. Crystal Load Capacitors The device crystal connections should include pads for small capacitors from X1 to ground and from X2 to ground. These capacitors are used to adjust the stray capacitance of the board to match the nominally required crystal load capacitance. Because load capacitance can only be increased in this trimming process, it is important to keep stray capacitance to a minimum by using very short PCB traces (and no vias) between the crystal and device. Crystal capacitors must be connected from each of the pins X1 and X2 to ground. The value (in pF) of these crystal caps should equal (CL -6 pF)*2. In this equation, CL= crystal load capacitance in pF. Example: For a crystal with a 16 pF load capacitance, each crystal capacitor would be 20 pF [(16-6) x 2 = 20]. PCB Layout Recommendations Observed the following guidelines for optimum device performance and lowest output phase noise: 1) The 0.01µF decoupling capacitors should be mounted on the component side of the board as close to the VDD pin as possible. No vias should be used between the decoupling capacitors and VDD pins. The PCB trace to VDD pins should be kept as short as possible, as should the PCB trace to the ground via. 2) The external crystal should be mounted just next to the device with short traces. The X1 and X2 traces should not be routed next to each other with minimum spaces, instead they should be separated and away from other traces. 3) Place the 33 Ω series termination resistor (if needed) close to the clock output to minimize EMI. 4) An optimum layout is one with all components on the same side of the board, minimizing vias through other signal layers. Other signal traces should be routed away from the ICS650-36. This includes signal traces just underneath the device, or on layers adjacent to the ground plane layer used by the device. 14 REF Output Reference 25 MHz clock output. Weak internal pull-down when tri-state. 15 GND Power Connect to ground. 16 VDD Power Connect to +3.3 V. Pin Number Pin Name Pin Type Pin Description |
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