• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 32 subscribers
  • Views 1009 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DP83620 - Crystal capacitance requirements

Greg
Expert 2021 points
Other Parts Discussed in Thread: DP83620

I need some help understanding the crystal capacitance requirements in the DP83620 datasheet.  On page 64, it recommends a "25 MHz, parallel, 20 pF load crystal resonator".  On page 65 there is a table of crystal specifications, and the shunt capacitance requirement is listed as being between 25 and 40 pF.  From what I have seen in data sheets for crystals, a typical shunt capacitance is about 3 to 5 pF.  The greatest value I found was 7 pF, but it wasn't an exhaustive search by any means.  Is the minimum shunt capacitance really 25 pF?  If the table should have listed "Load Capacitance" rather than "Shunt Capacitance", the recommended 20 pF is outside the range listed in the table on page 65.

Thanks,

Greg

  • 0
    TI__Mastermind 19780 points
    Hi Greg,

    Shunt capacitance, when looking in a crystal's datasheet, isn't the load capacitors you see in the circuit diagram figure 9-3 labeled CL1 and CL2. This term may be confusing in our table.

    Here is a great app note on the topic: www.foxonline.com/.../xtaldesignnotes.pdf

    CL1 and CL2 present the specified parallel load capacitance to the crystal. CL1&2 are in series and need to be combined into their equivalent capacitor plus the effect of stray capacitance from the PCB and IC. This is in the "Calculation of Load Capacitance" section of the linked app note.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Expert 2021 points

    Hi Rob,

    Thanks for explaining that and thanks for the link.  It makes a lot of sense.  Since crystal circuits are simple and always the same, I always assumed the load capacitance spec of the crystal meant that the load capacitors should have that value.  The table is confusing because it is presented as specifications for a crystal.  Frequency, tolerance, and stability are all specifications of the crystal.  "Shunt Capacitance" should be "Load Capacitor Values", and is a specification for the circuit, not the crystal.  So if that is true, it is all perfectly clear to me now.

    Thanks,

    Greg

  • 0 in reply to Greg
    TI__Mastermind 19780 points

    Greg,

    Your understanding is correct.  A better name would be "Load Capacitor Values" in the table given.  It will be noted for future revisions.

    Regards,