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ADS1262: No activity seen on crystal pins

Priyank Patil
Prodigy 20 points
Part Number: ADS1262

I have a 7.3728 Mhz crystal and 18pf capacitors connected to crystal pins. On probing the crystal pins I see no activity on them. I was expecting to see 7.3728 Mhz.

The crystal used is: ABMM-7.3728MHZ-B2-T  

The ADC works but it seems it is not using the crystal. Do I need to do something to tell ADS1262 to use the crystal?

  • 0
    TI__Mastermind 44680 points

    Hi Priyank,

    Welcome to the TI E2E Forums!

    The ADS1262 should automatically try to use the external crystal without any additional register configurations changes. If the external crystal fails to start, it can switch to its internal oscillator. To tell which clock source is being used, try enabling the STATUS byte and looking at the "EXTCLK" (bit 5) when reading data.

    Regarding why the crystal may not be starting is difficult to know from the PCB layout screenshot alone; there are many possibles causes including...

    • The PCB may not have been properly cleaned and solder flux residue left on the board is introducing additional parasitics and/or leakage current which is preventing the crystal from oscillating.
    • The load capacitor values might need to be changed. Since the crystal asks for a 18 pF load, try using 30pF capacitors since these capacitors are effectively in series with one another.

    • PCB layout issues might also affect crystal start-up. This could include too much parasitic load capacitance, or not enough load capacitance due to parasitic impedance between the crystal and load capacitors. The latter case might apply here since the load capacitors appear to be connected to the crystal through "stub" traces.

    • The crystal may have also gotten damaged during soldering.

    I would start with cleaning the PCBs, experimenting with different capacitor values, and even replacing the crystal to see if you can resolve the issue.

    Best regards,
    Chris