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Original question:

ADS131A04: crystal oscillator not oscillating

ADS131A04: ADS131A04 Crystal problems

Martin Karlsson
Prodigy 90 points
Part Number: ADS131A04

Hello,

I reed a thread regarding problems with crystal startup. We have discovered that the XTAL input of the ADS131A04 is extremely sensitive.

We used the recommended crystal ABLS-16.384MHZ-L4Q-T, the crystal is placed very close to the ADC.

It works best with no capacitors at all, but it is still enough to putt a metal piece on on terminal to make it stop.

Any suggestions to improve this (on current design)?

What is the load capacitance inside the chip?

Best regards,

Martin

  • 0
    TI__Guru 57306 points

    Hello Martin,

    Thank you for your post.

    I'm looking to see if we can provide more details on the input capacitance of the XTAL pins on the ADS131A04.

    Often times, the load capacitors on the crystal output pins can be reduced from what is suggested in the crystal data sheet. This load capacitance is the total capacitance, including PCB traces and digital input pins on the target device. The crystal on the ADS131A04EVM (Y4) calls for 20 pF load capacitance, but only uses only 8.2 pF on each crystal output in order to ensure proper startup. You can try using a lower value between 4.7 pF and 10 pF in your design.

    Regards,

    Ryan

  • 0 in reply to Martin Karlsson
    Prodigy 90 points

    Hello Ryan,

    Thank you for the quick response.

    Yes, that is probably working when testing on the ones we found a problem with, the problem is that we have done ~1000 boards without problem and recently we found this problem on some boards. After more testing we found out that the ADS131A04 is quite sensitive compared to other components with an externa crystal. We have used 2x 10pF.

    One really strange thing is when removing the 10pF capacitors and test it it starts, then re-soldering the exact same capacitors it still starts.

    Best regards,

    Martin

  • 0 in reply to Martin Karlsson
    TI__Guru 57306 points

    Hi Martin,

    I'm not sure what the issue would be then if it is only a few out of ~1000 boards. I suppose there is some margin in the crystal to be considered as well. I'm still waiting to hear from our design team what is the expected input capacitance of the XTAL pins, but I will update you once I have more information.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    TI__Guru 57306 points

    Hello Martin,

    Based on design simulations, both XTAL pins have approximately 2 pF of input capacitance. The root cause may be more related to the PCB design and assembly perhaps.

    Regards,

    Ryan