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Can I clock 3 ADS1255 using the same CLKOUT signal?

user1827693
Prodigy 20 points
Other Parts Discussed in Thread: ADS1255

Hi,

I´m making a design consisting of 4 ADS1255 (high resolution converter). The 4 ADC shall be synchronized why I need to feed them with the same clock signal. One idea was to connect one crystal to the first ADC and use the output signal from this ADC (CLKOUT) and connect it directly to CLKIN ports of the other ADC´s.

Would this solution work?

-CLKOUT signal quality good enough to feed CLKIN?

-Does CLKOUT generate enough current to power the CLKIN of all 3?

If the answer is no, do you have any suggestion of suitable external clock generator for the ADS1255?

Thanks for your help!

  • 0
    TI__Mastermind 44550 points
    Hi,

    Welcome to the TI E2E Forums!

    You should be able to drive multiple CLKIN ports from the CLKOUT signal. The only downside of doing this is that the first ADC will add a small phase delay to the clock signal, so it won't be perfectly in phase with the other ADC's. I'll see if I can provide data on this phase delay...

    What are you using for the SPI communication? Most MCUs should be able to provide a clock output, which you could then route to all four ADCs; all ADCS will be synchronized if the clock trace lengths are matched.

    Note: Clock jitter does not have as much impact on a delta-sigma ADC as it would with a SAR ADC. Depending on the application, most clock sources should work fine.

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Prodigy 20 points

    Thank you Chris for your reply. For this application a phase shift is ok. More important they are not drifting from each other. 

    Would you then connect the CLKOUT signal directly to the other three ADC´s? An option would be, if it´s possible- to use the CLKOUT from the next ADC, connect to the CLKIN and so on (which would perhaps generate a phase shift for every ADC). Would this give better clock signal quality you think?

    In the technical description it´s written:

    "Make sure the external clock
    generator supplies a clean clock waveform. Overshoot
    and glitches on the clock will degrade overall performance"

    Any other design advice? I will try to keep the length of the traces equal. 

    Thanks,

    Chris

  • 0 in reply to user1827693
    TI__Mastermind 44550 points

    Hi Chris,

    Stringing the clock from CLKOUT to CLKIN to connect to all ADCs is also a valid option...I would use whichever option provides a better PCB layout for you.

    In order to maintain good signal integrity, I would recommending routing the clock traces first to keep them short, and also ensure that they are routed directly above a ground plane.

    If you have split analog and digital grounds planes, do not route any traces over the split otherwise they become very inductive. The analog and digital grounds either need to be bridged at the ADCs, or a single ground should be used while maintaining analog and digital partitions (keeping analog and digital circuitry separate, but still sharing a ground plane without a splits). I personally prefer the later method because it avoids ground loops when having to connect the analog and digital grounds together again near the power circuitry.

    If you follow good layout practices, particularity when it comes to grounding, then you should be able to achieve good performance with the ADS1255.

    Let me know if you have any additional questions!

    Best Regards,
    Chris