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AM5728: Oscillator stray capacitance

yf liao
Expert 1080 points

Part Number: AM5728

Hi,

On AM5728 GP EVM, several crystal capacitor design is different. According to applicaton note SLLA122, need to account for the Cstray capacitance(=Cboard+Cdevice) of the circuit, when choosing the capacitor for the crystal. 

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I calculated the Cstray(=Cboard+Cdevice) value based on the crystal Cload, and CL1,CL2 value for each crystal used in GP EVM, I found every Cboard+Cdevice is different. For example,

Y2, Cload is 18pf, CL1, CL2 used is 18pF, thus Cstray calculated is 9pF.

Y3, Cload is 8pf, CL1, CL2 used is 12pF, Cstray calculated is 2pF. 

Y1, Cload is 6pf, CL1, CL2 used is 10pF, Cstray calculated is 1pF.

It appears that Y3, Y1, Cstray is obvious low, which violating the typcial value mentioned in SLLA122(2pF~5pF + 3pF~6pF).

I understand the Cstray capacitance is related with the crystal itself and the trace layout, but can you confirm what Cdevice+Cboard value is typically used when calculating the capacitor for the crystal?

  • 0
    TI__Guru**** 393215 points
    Hi,

    All necessary requirements for AM572x input clocks are given in section 6.1.1 of the AM572x Datasheet SR2.0 Rev. C. Please follow them.
  • 0 in reply to Biser Gatchev-XID
    Expert 1080 points
    Hi Biser,
    I think this question is more on PCB desgin aspect, also it's more about experience value where we could start from, and those value won't be specified by datasheet.
    I am asking this question because I haven't see consistent desgin algorithm in the GP EVM among several crystal circuit, so could you help to reach out the board designer on this?
  • 0 in reply to yf liao
    TI__Mastermind 42545 points
    Hi,
    EVM designs are generally completed while the part documentation is still in a preliminary state. We try to follow the documentation that we have but often we do not have the opportunity to revise the board once the documentation is completed. This is one of the reasons why EVMs are considered software development platforms and not reference designs. As Biser stated, you should follow the recommendations in the data manual. I will try and locate the designer of that board and ask him the question.
    Regards, Bill
  • 0 in reply to Bill Taboada
    Expert 1080 points

    Hi Bill,

    Thanks for your follow up, please do let us know the updates, thanks.