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Original question:

DAC38RF80EVM: Verifying DAC/CLK interface without FPGA

DAC38RF80EVM: NCOs Operation Levels

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

Hey all, 

So after receiving previous direction on how to test the NCOs and subsequent output of the DAC before interfacing with the FPGA(see: e2e.ti.com/.../776799), I began testing and playing around with the capabilities of the NCOs. While there appears to be no constraint on their frequency (other than the max frequency output being half that of the frequency of the sampling clock), I've ran into an issue where they aren't outputting anything when the sampling frequency is above the default value of 4915.2 MHz. 

Our clock, the LMX2595, is capable of producing the max sampling clock frequency of 9GHz and below. I don't believe it's a power issue of the clock input signal. We also have made the modifications for to use a differential clock on the DAC EVM board.

Do you have any ideas what might be causing the issue?

Thanks,

Jared

  • 0
    TI__Genius 12780 points
    Hi Jared,

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

    Best Regards,

    Dan
  • 0
    TI__Guru* 91947 points

    Jared,

    This should not be an issue. When you try this test, are using the constant input mode and the data format set to Offset binary? Is the mixer enabled and coarse mixer disabled? Please send your register settings and we will take a look at them. Can you verify the clock going to the DAC is the correct frequency? I am wondering if you have a setting wrong with the LMX when you go above the default value. You may also want to consult wit the high speed clock group about setting the LMX registers properly.

    Regards,

    Jim

  • 0 in reply to jim s
    Intellectual 280 points

    Jim,

    Yeah I have Constant Input, Offset Binary, the Mixer Enabled, and NCO Enabled all checked on. Here is a screenshot of my settings:

    Yeah I've been able to definitely verify that the clock is at the right frequency (I've been trying 8000 MHz). My biggest worry would be with the power output of the clock as I don't currently have an adequate way of measuring it, but based off the datasheet, I believe the power output should be sufficient. 

    At one point, I do believe I had it working properly at this frequency, but I am unsure what has changed since then to have it no longer work. 

    Jared

  • 0 in reply to Jared Emerson
    TI__Guru* 91947 points

    Jared,

    Do you have the single-ended clock option checked as shown below? How much current can your 5V supply provide to the EVM?

    Regards,

    Jim

  • 0 in reply to Jared Emerson
    TI__Guru* 91947 points
    Did you click on "UPDATE NCO" after making all of the changes?
  • 0 in reply to jim s
    Intellectual 280 points
    I've unchecked the single clock option so that the DAC is configured for a differential clock, I've been sure to click UPDATE NCO after making changes, and the DC power supply we are using is rated from 0-3 A and I haven't seen the current limit LED flash so I don't think it's a current issue.
  • 0 in reply to Jared Emerson
    Intellectual 280 points

    Jim,

    Any new thoughts on this matter?

    Jared

  • 0 in reply to Jared Emerson
    TI__Guru* 91947 points

    Jared,

    I just ran a test with these setups. The 5V supply is drawing about 1.6A and if the 8GHz clock source falls below 9dBm, the output becomes unstable.

    Regards,

    Jim

  • 0 in reply to jim s
    Intellectual 280 points

    Jim,

    That being the case, I would advise updating the documentation on the DAC to state as much. As is, the user guide specifies a 3-7 dBm input signal for the clock and based on a quick review of the one we purchased the max. output power is 7 dBm. 

    Jared