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Original question:

DAC38RF80EVM: System Clock Configuration

DAC38RF80EVM: SYSREF and SYNC

Jared Emerson
Intellectual 280 points
Part Number: DAC38RF80EVM
Other Parts Discussed in Thread: LMX2595EVM, LMK04828, LMX2595

Hey,

In our system we has the DAC38RF80EVM with a LMX2595EVM clock and are following steps in the DAC EVM's guide to set up an external LVDS clock. 

I do have several questions based on looking at this procedure and at the schematic of the DAC EVM. 

First, the last step mentions connecting J24 to the "reference input of the clock". J24 is connected to one of the output clocks of the LMK04828 (DCLKOUT4P) and I'm not entirely sure what is meant by the "reference input" of the clock. I would assume it means the SYSREF of the clock, but I don't believe that the output at J24 is the SYSREF signal of the system. Thoughts or am I missing something? 

Second, are the SYNC signals on the LMX clock and at for the LMK at J8 necessary? They aren't mentioned in documentation at all. I know that JESD204B Subclass 1 devices requires both the SYSREF and SYNC signals for each transceiver and receiver, but I don't know if that includes clocks (or clock jitter cleaners) as well? Could you confirm whether they do and if so, where I would pull those sync signals from as it's unclear by looking in the schematic or guide.

Thanks,

Jared

  • 0
    TI__Genius 12780 points
    Hi Jared,

    One of our experts is looking into your questions, and will be back with you shortly.

    Best Regards,

    Dan
  • 0
    TI__Guru* 91947 points

    Jared,

    The output from J24 can be used as a reference source for an LMX device or other test equipment if they are used to generate the DAC clock. This would be required to get the LMX (or a signal generator) and the LMK synchronized as the LMK is required to generate the SYSREF signal and this has to be synchronized with the DAC sample clock, which would be coming from the LMX in this example. 

    The SYNC signal on the LMX is used if you want to synchronize multiple LMX parts. This is not required if you are only using one LMX. I think the LMX data sheet goes into more detail about this. If you still have questions regarding this, send a post to the clock forum.

    Regards,

    Jim

  • 0 in reply to jim s
    Intellectual 280 points

    Jim,

    When you say as a reference source for the LMX though I'm still a bit confused. The output at J24 is a CLK signal correct, not a SYSREF signal? So where would I attach it to the LMX? It wouldn't be at the OSCin as that's the input for an external oscillator used to generate the main DAC CLK signal through the PLL+VCO on the the LMX, right? So then how would I use this output as a reference/where would it belong on the LMX?

    Additionally, the LMK SYNC signal at (J8) isn't mentioned in the User Guide. Should that be connected to the LMX's sync signal though?


    Thanks and if this is more appropriate in the clock forum, let me know,

    Jared

  • 0 in reply to Jared Emerson
    TI__Guru* 91947 points

    Jared,

    The LMX can use an external reference to lock the internal VCO. This way the VCO can be referenced to another clocking source, such as the LMK04828 in this case. You would have to remove some resistors and install some and use the OSCP/N SMA's on the board in place of the oscillator. See figure below. Another option is to use the RFOUTB/SYSREFOUT output from the LMX2595 and send this to J4 of the DAC38RF80EVM. Then the LMK04828 SYSREF outputs, which will go both to the DAC and FPGA, will be synchronized to the LMX clock.

    The LMX SYNC is only used if you want to synchronize multiple LMX devices.

    Regards,

    Jim

  • 0 in reply to jim s
    Intellectual 280 points

    So then to clarify, we have to do one of the following:
    1) Connect the LMX RFOutBP signal to the J4 of the DAC for the LMK ref. input, operating at 1/4 of the DACCLK frequency produced by the LMX
    2) Connect the LMK J24 output clock to the OSCinP of the LMX to use the LMK's oscillator as a reference to create the DACCLK signal at RFOutAP/N

    But not both as specified in the User's Guide? To do both just seems like circular logic unless it's a means to reference both back to each other. However I would think just referencing one to the other in either of the two options outlined above would be sufficient. Thoughts?

    Jared

  • 0 in reply to Jared Emerson
    TI__Guru* 91947 points

    Jared,

    Correct. Where did you see this in the User's Guide. I did not write it and it appears this may be an error. Which User's Guide are you referring to, the DAC or LMX?

    Jim

  • 0 in reply to jim s
    Intellectual 280 points

    That is in the DAC EVM User Guide (where the snippet of instructions from the original post came from). Under the section discussing when the user is using and external, low voltage clock.