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DP83848K: About Load Capacitance of 25MHz Crystal

Part Number: DP83848K

Hello,

According to datasheet(page 66), load capacitance of 25MHz Crystal is 25~40pF.

My customer confirmed the crystal which they will use is obtained by 10pF matching by the crystal maker.

(Question)

(1) Is there any problem with 10pF as Load Capacitance?

(2) According to application report(SNLA290) (page 8), recommendation load capacitance is 8 pF ≤ CLOAD ≤ 20 pF.

Or according to application report(SNLA079D), requirement of load capacitance is 15~40pF.

Which is correct?  Datasheet? Or Application report?

(3) According to datasheet(page 65), 20-pF load crystal resonator should be used.

But load capacitance of 25MHz Crystal is 25~40pF in datasheet(page66). Which is correct?

Regards,

Tao_2199

  • Hi Tao,

    The 25-40pF and 15-40pF ranges are for the internal capacitance of the crystal or oscillator itself. Cload refers to the external loading capacitors, whose range is 8pF - 20pF. We are investigating the disconnect between the datasheet and app note for the 15 or 25 pF minimum value for the crystal/oscillator.

     

    Thank you,

    Nikhil

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and it is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

     

  • Hello Nikhil,

    Thank you for comments.

    I have a question.

     

    >The 25-40pF and 15-40pF ranges are for the internal capacitance of the crystal or oscillator itself.

     What is meaning the internal capacitance of the crystal?

    According to datasheet(page 65), load capacitance is described following.

     

    CL is calculated from the following formula.

    CL(Load Capacitance)=(CL x CL2/(CL1 + CL2) + Cstray

    Cstray: the stray capacitance in the oscillator circuit, which will normally be in the 2pF to 5pF range

    C1C233pFCtsray5pF

    CL=(33 x 33)/(2 x 33) +5 = 21.5pF

    The above is almost as calculated.  

     

    Is load capacitance(CL) meaning above formula or internal capacitance(below)?

    C0: Shunt Capacitance

    C1: Motional Capacitance-

     

     

    Regards,

    Tao_2199

  • Hi Tao,

    You may calculate load capacitance as in section 3.7 of SNLA290. "the internal capacitance of the crystal" refers to the load capacitance spec of the crystal itself (not including any external hardware). Does this answer your questions?

    Thank you,

    Nikhil

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and it is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

  • Hello,

    Thank you for comments.

    But I can't make it clear yet.

    (Question)

    (4) Like this time, if there is a difference in CL(load capacitance) range between the app note and the data sheet, which CL range should be adopted for DP83848?

    (5)The range “8pF - 20pF” in 7 Summary of SNLA290,

    Is this meaning CL(load capacitance) or CL1/CL2?

    However, in both cases it is very different from the other CL ranges.

    Or  is there no problem to use it as CL range for DP83848?

    (6) My customer confirmed the CL(load capacitance) of crystal which they will use  is obtained by 10pF  with matching by the crystal maker.

    In this case, is there no problem  to use it outside the CL range of the data sheet?

    (7)Your last mail,

     "We are investigating the disconnect between the datasheet and app note for the 15 or 25 pF minimum value for the crystal/oscillator."

    Which is minimum of CL(load capacitance) ?

     

    Regards,

    Tao_2199

  • Hi Tao,

    15 pF is the recommended minimum internal load capacitance for the crystal. The range "8-20pF" refers to the external load capacitors, CL1 and CL2. Please adopt these values for the crystal and the external components for the DP83848 design. 

    Does this answer your questions?

    Thank you,

    Nikhil

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and it is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

  • Hi Tao,

    I am covering for nikhil, Do you have some additional questions? If not should i close the thread.

    Regards,

    Sreenivasa

  • Hello Sreenivasa,

     

    Thank you for support.

     

    I have informed to customer about your comment and will close this thread.

    I appreciate your support for this problem solving.

     

     

    Regards,

    Tao_2199

  • Hi Tao,

    hank you for updating. I will close the thread from my side.

    Regards,

    Sreenivasa