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Altera_Forum's avatar
Altera_Forum
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8 years ago

MAX V GCLK Impedance

Hi, I'm using a MAX V device with a 100Mhz CLK on the GCLK input. We need to understand the impedance of the GCLK input such that we can match the crystal oscillator module to the MAX V input. In the MAX V datasheet the only information I can find WRT to the input impedance is a reference to a maximum capacitance of 8pF. My Crystal has a 25 Ohm output matched to 50 ohm transmission line. Obviously I don't want to get reflections or bad clock edges, so matching the impedance's between the parts seems very sensible.

Does anyone know where this information is?

For reference I've attached a copy of the Crystal Datasheets Circuit.

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  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    8 years ago

    The specified input capacitance is the pin impedance you are asking for.

    Source side series termination should be considered for longer traces, e.g. > 50 - 100 mm. If this applies to your design, supplement a series resistor that adds up to internal 25 ohms for a total impedance equal to the trace impedance.