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ADS1258: ADS1258 Crystal Oscillator Connection

user3699334
Prodigy 10 points
Part Number: ADS1258
Other Parts Discussed in Thread: ADS1158

Dear Sir:

    I want to know if there are other  capacitors connected to the XTAL1 and XTAL2  inside  of the ads1258  chip?   What is the value of the internal capacitors?

   According to the chip manual recommendation, the XTAL1 and XTAL2 should be connected to 4.7pf .But  in general applications,4.7pf  is too small to use.

   

   Anyone knows why  the datasheet recommend the 4.7pf?

   Thanks!

  • 0
    TI__Mastermind 44680 points

    Hi,

    Welcome to the TI E2E forums!

    There is no internal capacitor between the XTAL1 and XTAL2 pins; however, due to PCB board parasitics you may have a few pF of capacitance between these pins. I provided some additional details on this topic here: https://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/628309/2341477#2341477

    Keep in mind that the crystal you're using may require a different load capacitance then the suggested circuit. It is a good idea to test the IC and crystal together to ensure compatibility. There is an application note for the MSP430 that discusses many design considerations as well as tests that you can perform to test the "Safety factor" of the crystal. Even, though this discussion is centered around using an external crystal the the MSP430, it is still applicable to the ADS1158. You can find this application note here: SLAA322.

    If testing for crystal compatibility is something that you'd prefer not to have to do, then perhaps an easier design approach would be to provide a 15-16 MHz external directly clock to the ADS1158, from a crystal oscillator or MCU, rather than trying to generate this clock from a crystal.

    Best regards,
    Chris