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ADC14X250EVM: EXTREF signal

Lizon Maharjan
Intellectual 420 points
Part Number: ADC14X250EVM
Other Parts Discussed in Thread: LMK04828,

Hi,

I am trying to provide EXTREF signal to the LMK04828 on ADC14X250EVM.

The clock reference signal available to me is a 125 MHz AC coupled differential signal with amplitude of 300mV, can I make any modification in the EVM board to make it compatible with such EXTREF signal?

Thank you

Lizon

  • 0
    TI__Expert 3515 points
    EXTREF is used to lock the on-board VCXO using PLL1 of LMK. The LMK is programmed to the usual input of 10MHz.
    you can feed another frequency, but will have to change the PLL1 settings in LMK accordingly.
    Whether you have to modify the EVM or not depends on the components in the signal path (for EXTREF) can support 125MHz. Check the schematics.
  • 0 in reply to Satish Uppathil
    Intellectual 420 points

    Two questions:

    1. What changes do I make in the PLL1?

    2. I already made the changes in the circuit.

    I first used TSW2170 as EXTREF signal, and selected the configuration 1 for LMK04828 in ADC14X250EVM GUI the system worked fine. The outputs of the LMK04828 were all 70MHz following TSW2170 signal.

    I followed the signal and I realized the 2Vp-p signal of TSW2170 was reduced to 640mV signal using resistor dividers,  since the signal that I an putting in is only 300mVp-p I tried to take this resistor divider out. From the circtui included below, I removed R34 with a 0 ohm resistor and tested the output of LMK04828. Then I removed R36, and then I removed R35 -  none of these scenarios worked. LMK04828 did not have any output. I put the R35 back because I believe this is the termination resistor.

  • 0 in reply to Lizon Maharjan
    TI__Expert 3515 points
    I reviewed the ADC14X250EVM schematics (page 3) and don't see the on-board VCXOs installed. The default clocking option for the EVM is an external clock source. Have you reviewed section 5 of the User's guide (SLAU5625)? The alternative clocking options and the changes needed are explained in there. Let me know if you have any further questions.
  • 0 in reply to Satish Uppathil
    Intellectual 420 points

    Hi Satish,

    I am providing external clock through EXTREF, not trying to use the on-board VCXO. I have already made all the changes suggested in section 5. I have got the system working with TSW2170 EVM as clock, which produces 2.4 V(0-pk) analog clock signal.

    The problem I am facing is: TSW2170 is not the source that I can use for my system, the source that I am limited to is a LVDS clock source that produces AC swing of about 300mV per channel of a differential signal. The EXTREF input is a single ended input and has attenuation (may be targeted towards similar source as TSW2170). I have removed the attenuating resistors and have connected one of the channels of the differential signal to the EXTREF signal - I could not get the LMK04828 to work when I did this (it would not spit out anything on the SYSREF and ADC clk outputs).

    So my question is: How can I make the LMK04828 mounted on the ADC14X250EVM board work with differential signal of 300mV amplitude each (or differential amplitude of 600mV)? What do I need to do with the signal and/or what can I do on the board to get this working? I have also referred to LMK04828 data sheet, it should be able to read signals ranging from 0.125V.

  • 0 in reply to Lizon Maharjan
    TI__Guru 70144 points
    Hello Lizon,

    Satish will be out of office so I can help. The issue with the ADC14X250EVM is that the external clock that you are trying to provide is only single-ended, and by connecting only one leg of the LVDS driver that you have, you lose 1/2 of the swing already from 300mV peak to 150mV peak.

    To maintain the swing, I recommend that you connect the LVDS clock driver that you have with a 1:1 balun to convert differential to single-ended. You can maintain the swing going into the LMK04828. There are many EVMs out there from different balun vendors that sell SMA input/output balun EVMs for your connections.

    For the LMK04828 input, set it to single-ended configuration so it accept as low as 0.25Vp of input swing.

    I think this is as far as our group in the data converters can help you. If you need more advanced assistance, I suggest you place a dedicated post with topics specifically on the LMK04828 in the clocking forum. The experts on the clocking circuits will be able to help better than us.

    -Kang
  • 0 in reply to Kang Hsia
    Intellectual 420 points
    Hi Kang,
    I will try the solution with baluns. If this does not work, I will start a new request in clocking forum.
    Thank you very much for your help.