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AD9629 input clock design


I have got a working design with 4 ADCs (AD9629), they have for each an input clock design just like the picture attached.

(I would like to keep 4 times AD9629 for versatility...)

All of my ADC are synchonized to the same clock at 5Mghz.

My question are:

- Is there enough (current/power) for powering 4 Adcs with a single clock differential input?

- Is there any precautions I need to implement?

thanks in advance.

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  • Hi CrazyFrog,

    From a drive strength perspective, it seems to me that your single microcontroller output will have to supply about the same amount of drive power to one transformer that fans out to 4 loads, as it does driving four transformers each driving one clock load.

    So, from a drive strength perspective I think you are OK.

    Try to keep your clock signal path as short as possible to reduce the transmission line effects.

    If there is an output impedance associated with your clock driver pin, you might want to add pads for a possible matching termination resistor, right before the transformer.



  • Hi dougl,

    I have to respawn this post, as am having some issues from EMC testing. 

    It seems that the transformer is ringing too much (at about 500Mgh) even when there is no clock driving it.!!

    So I had to add 2 pulldown resistors at the end of the transformer of 10k, on both CLK+ and CLK-. 

    This has remove completely the EMC disturbance. 

    However, I don't know exactly how... 

    Could you please give me your opinion on this? 

    I know by adding two pulldowb resistors of 10k it does remove the issues but am not sure if it is safe for the AD9629. 

    What value would you recommend? 

    Thanks in advance 

  • Hi crazyfrog,

    Where are you observing the clock ringing? Is it before or after the back-to-back diodes?

    It's safe however those resistors might attenuate the clock amplitude and band-limit the frequency to the ADC clock inputs. 

    Have you tried supplying a clock to the ADC with the 10k pulldown resistors installed? Does it able to source properly the clock and does it able to get a data capture?