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ADS1158: ADS1158 Internal Oscillator Start-up Timing

Joe Martin
Prodigy 70 points
Part Number: ADS1158
Other Parts Discussed in Thread: ADS1258

Hi,

We're trying to diagnose an issue with the ADS1158 that seems to be linked to the crystal oscillator circuit we have and possibly the load caps. We stripped the ADS1158 down to just the analog and digital supplies and we're pulling PWDN high to allow the oscillator drive circuit to start-up. We are monitoring CLKIO pin and the supplies with a logic analyzer to see when and if the 15.7 Mhz from the internal clock outputs. With the recommended 4.7 pf caps we are seeing startup timing around 59 ms and with 10 pf caps we are seeing around 150 ms. My question is - the datasheet says the typical oscillator timing in the ADS1158 datasheet states 150 ms. Is this what I should be seeing when I conduct this test? Is the 59 ms we were seeing with the 4.7 pf caps too far out of tolerance?

Thanks,

Joe Martin

  • 0
    TI__Guru 52066 points

    Hi Joe,

    If you search for "ADS1258 crystal startup issues" on e2e, you will see a lot of questions about this topic, generally due to the finicky nature of these 32kHz watch crystals. Some customers find best results with smaller caps (5pF), some larger (20pF), etc. The point being that these circuits are very hard to tune, and you should not be surprised if they work differently than expected. So, no, I do not believe what you are seeing is too far out of spec - if the oscillator starts up faster, then that is okay. This is actually mentioned in the footnote for Table 6 in the ADS1158 datasheet.

    I will point out that it is our general recommendation to use an external clock oscillator with the ADS1x58 devices as customers have reported fewer startup issues with this configuration.

    -Bryan