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using a XTAL with the CDCS502

mapefer3
Expert 5725 points
Other Parts Discussed in Thread: CDCS502

Dear Sir/Madam,

I am currently designing a very frequency sensitive circuit using your

CDCS502 clock gen chips. There are several particular quartz crystals I need to use with load capacitances ranging from 12pF to 18pF. In the CDCS502's data-sheet it says it is possible to connect small external capacitors in order to match the crystal load capacitance to the on-chip capacitance, but it doesn't say how to calculate the value of these external capacitors. I have assumed that for the 18pF crystal, two 8pF capacitors between Xin/Xout and GND will be sufficient, but I would like to double check. Am I right, or are the required external capacitor sizes double that?

  • 0
    TI__Expert 5725 points

    If the load capacitance of the XTAL is 10pF, the CDCS502 provides a typical XTAL load of 10pF; that means that the CDCS502 provides 20pF to GND in XIN and another 20pF to GND in XOUT.

    If the chosen XTAL requires 18pF load, when using the CDCS502, a capacitor of 16pF should be placed in XIN and another one of 16pF at XOUT. Therefore in each of those pins there will be effectively 36pF, so the XTAL in total has 18pF load.

    The above applies considering that the XIN and XOUT input capacitance, tracks and pads from the XTAL to the CDCS502 pin are not adding any additional capacitance. Corrections might be needed in order to take this into account.

    Back to the second example, if the estimated XIN input capacitance is 6pF, then instead of adding 16pF to GND, 10pF should be added in this pin. When the estimated XOUT is 2pF typical, then 14pF to GND should be added to provide a total of 16pF in this pin.