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DAC39J84EVM: change/add reference clock to work in combination with ADC12DJ3200

Francesco Raffaelli
Intellectual 320 points
Part Number: DAC39J84EVM
Other Parts Discussed in Thread: ADC12DJ3200EVM, , LMK04828

Hello,

I am using the DAC39J84EVM in conjunction with the ADC12DJ3200EVM to operate as data converters respectively in transmitter and receiver of a bespoke optical communication system.

Given that the clock reference of the ADC EVM is 100MHz, provided by the CVHD-950-100.000 oscillator, I would like to have the same oscillator as a frequency reference in the DAC.

Looking at the user guide for the DAC it looks that I have potentially two options (maybe more):

1) Use the external reference clock and send in input the signal generated by an external board which should contain the CVHD-950-100.000. This could be done through J17 of DAC39J84EVM. 

2) Physically modify the DAC39J84EVM and place CVHD-950-100.000 instead of Y1, which is currently CVHD-950-122.880. The reason why I would go for this option is because it is cheaper and quicker to achieve, given that I don't have to design a new board to hold the oscillator.

Given that my plan is to use the DAC39J84EVM with the TSW14J56 (or 57) with the HSDC GUI and the DAC GUI, is any of the above solutions more convenient? For example, would a change of Y1 create troubles in the GUIs, given that the reference won't be 122.88MHz? 

Kind regards,

Francesco

  • 0
    TI__Mastermind 19505 points

    Hi Francesco,

    It is probably better to replace the 122.88MHz oscillator with a 100MHz oscillator as you show in option 2.

    Francesco Raffaelli said:
    Given that the clock reference of the ADC EVM is 100MHz, provided by the CVHD-950-100.000 oscillator, I would like to have the same oscillator as a frequency reference in the DAC.

    Regarding what you mention here about having the same oscillator as a frequency reference in the DAC, are you wanting these to have the same phase as well? If so, it may take a few more steps, but this should be do-able by using another reference (call it 10MHz) into both LMK devices and using the PLL1 to lock the new 100MHz oscillator on each board to the common input reference.

    About the GUI, you'll need to calculate new PLL2 M and N dividers for the LMK04828 to lock PLL2 to a new VCO frequency using the new 100MHz oscillator. It may be possible that in some cases the existing dividers will workout, but I worry about the VCO frequency falling out of range from the 122.88MHz configuration VCO cut-off. We can assist you with this whenever the time comes.

    Regards, Chase

  • 0 in reply to Chase W
    Intellectual 320 points

    Hi Chase,

    Thanks for your quick reply.

    We don't actually need them to have the same phase, so that should simplify a bit.

    I'll go ahead with the modification and I will get in touch when/if I have troubles with the locking of PLL2 with the VCO.

    Many thanks,

    Francesco