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ADS1258: crystal have probability not start up

Jeff Chen71
Expert 1345 points
Part Number: ADS1258


Hi,

      My customer have almost  50% crystal not strat up issue. This issue have probability not start up when power cycle.

1.We had tried to change cap c1/c2 from 5pF to 18pF then this issue was gone. But last year my customer just

change the cap value from 18pf to 5pf. 

2.We had tried to parallel resistor 1M ohm to xtal1/xtal2 but even worse.

3.We had tried to change the -2.5v to 0v  the issue was gone.

Could you help to check the schematic or the cap value.

Thanks

9ht7.pdf

  • 0
    TI__Mastermind 44720 points

    Hi Jeff,

    From the schematic, the only problem I can see is the capacitor value used for the PLLCAP capacitor (C82). For the ADS1258, this capacitor MUST be 22 nF as specified in the datasheet for the internal PLL circuit to operate as expected. This is probably the most critical issue that should be addressed first; however, I'm not sure if this alone will resolve the issue, but it should be tested first...

    After resolving the PLLCAP capacitor value, I guess my next question would be why the customer changed the crystal load capacitance values from 18pF to 5pF, when 18 pF was working? For a given crystal, the customer will need to test the crystal, capacitors, and ADC together to ensure compatibility. There is a safety margin test described in application note SLAA322 that will help determine the most appropriate capacitor values.

    Apart from that, my only other recommendations would be to make sure the PCB layout places the crystal close to the ADC XTAL1 and XTAL2 pins. ALWAYS clean the PCB after assembling it, as any solder flux residue around the crystal may cause leakage and prevent the crystal from oscillating. And finally, if possible, try to use a crystal with a lower ESR resistance; the 65kOhm ESR of the  9H T7 crystal is a little high. I've seen crystals with comparable ESR work with the ADS1258, but I believe the ADS1258 was designed to be used with lower ESR crystals, preferably below 50kOhm.

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Expert 1345 points
    Hi Chris,
    Thanks for your reply.
    Because they also had crystal not start up issue in last year, so they change the cap from 18pf to 5pf. We will follow your suggestions tomorrow.
    Thanks
  • 0 in reply to Jeff Chen71
    TI__Mastermind 44720 points

    Hi Jeff,

    Ok, I understand better now...

    Generally, lowering the load capacitance values does help with crystal startup issues; however, I have had one customer report back to me that increasing the load capacitor values improved their situation. I think that just goes to show that the correct capacitor value will be very specific to that particular design...

    If your customer decides that trying to deal with crystal and IC compatibility is too much of a hassle, another alternative solution would be to recommend a crystal oscillator with a 15-16 MHz clock output to clock the ADC, instead of trying to use a crystal. The compatibility between the crystal and crystal driver circuitry is then no longer an issue as both circuits are designed by the same manufacturer and integrated into the crystal oscillator, providing much better reliability. Since I see a lot of issues between the ADS1x58 and many 32.7 kHz watch crystals, I would recommend the crystal oscillator option as well.

    Best regards,
    Chris