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ADS1258: ADS1258 crystal circuit can not start up

Haroad
Intellectual 1600 points
Part Number: ADS1258

Hi,

    One of our customer use ADS1258 for ECG. they found about 10pcs ADS1258 out of 1000pcs, crystal circuit can not start up. They use the ref circuit of ADS1258 Figure39, but use 2pcs 18pf capacitors, not 4.7pf.  

    Pls advice how to optimize the design.

    And they have below questions:

   1) based on the  formula  CL=(C1*C2)/(C1+C2)+CS, Cs is parasitic capacitance in PBCA. Cs should be 10pf. Right?

   2) attached is the crystal spec, can you help check if it's ok for ADS1258?

Regards,

Haroad9H03200413 .pdf

  • 0
    TI__Mastermind 44680 points
    Hi Haroad,

    I'd recommend looking over the suggestions provided in this similar E2E post: e2e.ti.com/.../2321778


    To answer your specific questions though:

    1) Cs, the parasitic capacitance, will depend on the PCB layout, and even then it may still vary from board to board. On a good layout, I might expect Cs to be smaller than 10 pF, especially since the load capacitance is the parallel combination of Cs1 and Cs2 (the parasitic capacitance on each side of the crystal).

    2) In general, lower ESR crystal are more likely to start-up reliably. The 65kOhm ESR crystal they are using is a bit on the higher-end when compared to crystals known to work with the ADS1258. You might need to try tweaking the values of the load capacitors to get some crystals to start. Sometimes I've seen that increasing the capacitors may help, but in other cases decreasing the load capacitors has been more effective...it really depends on the specific circuit, layout, and crystal.

    You may find the Start Factor test helpful in determining the optimal value of the load capacitors, as described in... www.ti.com/.../slaa322c.pdf

    Best regards,
    Chris