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Original question:

DAC39J84EVM: Unable to replicate the pattern sent from DAC on ADC/Oscilloscope and ways to achieve integer sampling rate

DAC39J84EVM: Dodgy DAC response

Vaibhav Jain
Prodigy 180 points
Part Number: DAC39J84EVM
Other Parts Discussed in Thread: ADC12DJ3200, , ADC12DJ3200EVM

Hi All,

Happy new year to all and hope you are doing great.100I working with DAC39J84EVM for an experiment and while connecting DAC to an oscilloscope and ADC for monitoring its response, I am getting dodgy response from the DAC even when no signal is sent through the DAC. The screenshot for the frequency spectrum received on the ADC is attached with the message (similar readings were observed on oscilloscope) (fig. 1). I also tried to send in a sinusoidal pulse centred at 100 MHz from the DAC, the settings of the DAC used and the screenshot for sinusoidal pulse are attached below (fig. 2 and 3). The response received on ADC12DJ3200 is attached as well (fig. 4 and 5). Is it possible to ascertain any reason for such response from the screenshots attached or has the DAC gone completely awry and need to be replaced?

Thanks for your consideration and waiting for your response. Slight smile

frequency response on ADC when no signal on DACDAC settingssinusoidal pulse 100 MHzfrequency response on ADC for sinusodial pulse 100 MHz sent from DACtime domain response on ADC for sinusodial pulse 100 MHz sent from DAC

  • 0
    TI__Mastermind 23760 points

    Hi Vaibhav,

    Does this same noisy output occur on more than multiple DAC outputs? Can you post an image of the tone as captured on the oscilloscope like you mention? Is it just as noisy as the capture via the ADC12DJ3200EVM? Have you modified the DAC EVM in any way? Are you aware of if this board has ever output cleanly? I am wondering if this is an assembly issue, testing another DAC output should help us narrow down what could be the issue. 

    Regards, Chase

  • 0 in reply to Chase W
    Prodigy 180 points

    Hi Chase,

    Thanks for your response. Just answering the questions in sequence.

    1) Yes, the same noisy output occurs on all 4 DAC outputs.

    2) Yes, The capture by oscilloscope of  the 100 MHz sinusoidal pulse generated on DAC is just as noisy as the ADC capture

    3) Yes, there have been some changes made on the DAC board. One set of changes was to remove the BALUN associated with the the SMA output of DAC, in order to obtain differential output. After making these set of changes, the  DAC output was good and was not generating noisy responses. However, after performing the second set of changes, the DAC started giving above attached noisy outputs. These changes were made to make the DAC EVM frequency reference as 100 MHz which is by default 122.8 MHz. To achieve this, we have physically modified the DAC39J84EVM and placed CVHD-950-100.000 instead of Y1, which is currently CVHD-950-122.880. After this change, however, we started seeing the noisy response.

    Are there any other accompanying changes to the hardware/software (GUI changes as well) which should be made, without which this noisy response is being generated?

    Thanks for your help with this issue. Please ask for any other relevant information which could help in narrowing down the issue. 

    Thanks and Regards,

    Vaibhav Jain

  • 0 in reply to Vaibhav Jain
    TI__Mastermind 23760 points

    Hi Vaibhav,

    Interesting. Thanks for clarifying that the output was fine with the initial modifications and that it wasn't until the crystek oscillator was swapped that you started seeing this noisy output... I will replicate what you've done this Monday but I don't believe there is any software setting which should affect this on any of the LMK tabs.

    Regards, Chase

  • 0 in reply to Chase W
    Prodigy 180 points

    Thanks a lot, Chase. Looking forward to your findings Slight smile

  • 0 in reply to Vaibhav Jain
    TI__Mastermind 23760 points

    Hi Vaibhav,

    I've tried all that comes to mind and can't replicate this. Can you try the config file and procedure which I sent in the other E2E thread? I wonder if there are some settings which were off.

    My DAC output stays clean regardless of using the onboard crystek VCXO (mine is stock so it will be 122.88MHz) or if modifying the board to use an external 100MHz from a signal generator.  

    Regards, Chase