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Crystal for DM8148

Chunte Li
Prodigy 100 points

Hi,

One of the crystal requirements is the 5pF for maximum shunt capacitance. It is difficult to find 5pF maximum shunt capacitance and meet all other requirements. Most of the crystals have the maximum shunt capacitance of 7pF. Does the 5pF maximum shunt capacitance have to be followed, or 7pF maximum shunt capacitance is okay to use? If 5pF maximum shunt capacitance has to be followed, can you recommend some crystals that meet all the requirements? Thank you.

Best Regards,

Chun-Te Li

   

  • 0
    TI__Mastermind 43875 points

    Where did you see the requirement for 5pF?

    The EVM uses 10pF.

    BR

    Steve

  • 0 in reply to Steve Clynes
    TI__Mastermind 43875 points

    Never mind. I found the reference in the DS.

    I am currently chasing this for you, but as I mentioned the EVM uses 10pF.

    BR,

    Steve

  • 0
    TI__Mastermind 43875 points

    Is the 7pF for the exact frequency you are using, or is this a generic number from the crystal manufacturer's data sheet? Many manufacturers will quote the max capacitance over all ranges of operating frequencies when in fact the capacitance might be lower at different actual frequencies.

    Can you please check the actual capacitance for the specific operating frequency please?

    Can you point me to a data sheet you are considering please?

    BR,

    Steve

  • 0 in reply to Steve Clynes
    Prodigy 100 points

    Hi Steve,

    Thanks for the information. The 7pF is from various data sheets of crystals I found from the internet for the maximum shunt capacitance with the frequencies I am interested in. That is, 20MHz for DM8148. Though I found some crystals with 5pF maximum value, but the ESR of the crystals are much higher than the required value in DM8148 data sheet. Eventually, I found TXC 7A series crystal,which seems to meet all the requirements stated in the DM8148 data sheet. The web address is http://txccrystal.com/images/pdf/7a.pdf

    It will be great if you can help me to double check the spec. from the data sheet against the requirements from DM8148 to ensure I did not miss anything. Thank you.

    Best Regards,

    Chun-Te Li 

  • 0 in reply to Chunte Li
    TI__Intellectual 1040 points

    Chunte Li said:

    Hi Steve,

    Thanks for the information. The 7pF is from various data sheets of crystals I found from the internet for the maximum shunt capacitance with the frequencies I am interested in. That is, 20MHz for DM8148. Though I found some crystals with 5pF maximum value, but the ESR of the crystals are much higher than the required value in DM8148 data sheet. Eventually, I found TXC 7A series crystal,which seems to meet all the requirements stated in the DM8148 data sheet. The web address is http://txccrystal.com/images/pdf/7a.pdf

    It will be great if you can help me to double check the spec. from the data sheet against the requirements from DM8148 to ensure I did not miss anything. Thank you.

    Best Regards,

    Chun-Te Li 

    Hi Chun-Te,

    Most of the time on crystal vendor's websites, the crystal parameters (ESR, Cshunt, etc.) are listed for a range of freq. (8 ~ 80MHz for example in the above TXC link).  And for a given freq. these crystal parameters turns out to be much better.  But for this, we have to request the crystal vendor to provide this data.

    I've been working with the design team to come up with a solution that will make designing the oscillator circuit a little easier.  We can relax the Cshunt 5pF requirement to 7pF with a max ESR of 40 Ω and parallel load capacitance range between 12pF~20pF.

    Also note:  The CL calculation in the datasheet is not correct. 
    It should be:
                        CL = (C1 * C2) / (C1+C2) + Cshunt

    Best regards,
    Scott