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CDCE913: internal crystal load capacitors

Wolfgang R.
Intellectual 266 points
Part Number: CDCE913
Other Parts Discussed in Thread: CDCE949

Tool/software:

Hi all,

the CDCE913 internal load capacitors are programmable from 1pF to 20pF via register XCSEL.

Question: is this the capacitance per capacitor, or is it the resulting load capacitance (CL1 in series with CL2)?

I read "Practical Consideration on Choosing a Crystal for CDCE(L)9xx Family" (SLEA071) and "VCXO Application Guideline for CDCE(L)9xx Family" (SCAA085A).

I think it's per capacitor, but am not absoutely sure.

So to get effective 10pF load capacitance for the crystal I would program 16 pF (XCSEL = 10h), correct?

With CL1external = 2pF, CL2external = 6 pF and CL1internal = CL2internal = 16 pF we get:

CL1 = (16 + 2) pF, CL2 = (16 + 6) pF and thus CL = (18 x 22) / (18 + 22) pF = 9.9 pF

Correct?

Regards,
Wolfgang

  • 0
    TI__Mastermind 19000 points

    Wonfgang,

    The load capacitor setting is the load capacitance at the XIN and XOUT pins. The architecture is as shown here:

    For a 10pF load capacitance from the device, you want the register setting that corresponds to 10pF. As the application note states though, be sure to include the parasitic capacitance when determining the register setting for the device:

    My recommendation is to also include footprints for external capacitors (also recommended in the datasheet) for best matching of the required capacitance.

    Thanks,
    Kadeem

  • 0 in reply to Kadeem Samuel
    Intellectual 266 points

    So will programming XCSEL = 10h set the load capacitors CL1 and CL2 to 16 pF EACH?
    Or does the XCSEL-programmable capacity of 1-20 pF refer to the CLOn-Chip also mentioned in your formula above?

    This is my only question. All the rest is clear.

  • +1 in reply to Wolfgang R.
    TI__Mastermind 19000 points

    Wolfgang,

    The register setting sets BOTH the CL1 and CL2 load capacitors, with values such that if 16pF is selected, the CLon-chip is 16pF total.

    Thanks,
    Kadeem

  • 0 in reply to Kadeem Samuel
    Intellectual 266 points

    Thanks for clarification.
    So I had guessed it wrong (capacity referring to the individual capacitors) - good to know.

  • 0 in reply to Wolfgang R.
    Intellectual 266 points

    Just to add this for reference and as suggestion for a datasheet update:

    The CDCE949 datasheet (presumably the same oscillator) makes it clear in the description of XCSEL that it's the total capacitance
    - by adding a circuit diagram:

    - and the word "total" in footnote 8:

    Having this also in the CDCE913 datasheet would have made this thread obsolete.