AD9251 Evaluation: the minimum signal frequency

I bought a HSC-ADC-EVALCZ capture board plus a AD9251 EVL. In my project, the signal frequency which need to be sampled is below 30KHz.

Now, I have a problem. It is found that the signal frequency below 76KHz could not be sampled accurately; I have attaced some results. The signal frequency is 26 KHz, 66KHz and 76KHz. In this case,the sampling rate is set to be 20MHz.

So, I want to know what is the minimum signal frequency that could be sampled accurately by AD9251 EVL. Meanwhile, if AD9251 EVL could not satify me, could you please reconmend another A/D product. 

Thank you so much!

  • 0
    •  Analog Employees 
    on Aug 16, 2016 1:35 AM over 4 years ago

    Hi Yunqi,

    The analog input path on the AD9251 evaluation board has RF baluns to convert the single-ended signal to differential. As you have discovered, these will not pass a low frequency signal. I believe your experiment has shown the minimum frequency of the board as-is. I would have normally said that it would not work below a few hundred kilohertz, but your setup worked at a lower frequency.

    There are board modifications you can make to bypass the RF baluns, if you are interested. Performance will be degraded compared to the numbers in the datasheet, but it will work at low frequency.

    Please let me know if this is of interest to you.

    Thank you.

    Doug

  • Hi Doug,

    Thank you for you reply!

    In order to let AD9251 evaluation board work at low frequency, what modifications should I make to realize it ?

    At the same time, could you please let me know what specifications will be degraded in this way?

    Yunqi

  • 0
    •  Analog Employees 
    on Aug 16, 2016 10:12 PM over 4 years ago

    Hi Yunqi,

    An example of how to modify the board is attached. The original schematic can be found in UG-003 on www.analog.com .

    Here is a very incomplete discussion on the possible effect on performance.

    A portion of noise coupling to a differential signal will affect both components of the signal, which is common mode noise. A large portion of this noise is naturally rejected by the ADC. This modification requires going to a single-ended signal at the ADC inputs. Noise coupled to a single-ended signal will not be rejected in the same way.

    Also, this modification causes what previously were the inverting and non-inverting sides not to match with each other, looking back from the ADC inputs. This could also affect ADC performance.

    The board was originally designed for differential signaling, which is best for AC performance. Changing to single-ended could affect SNR, SINAD, SFDR and linearity, as well as other parameters.

    Thanks,

    Doug

    AD9251_DC_SingleEnded_input_mods.pdf
  • Excuse me,sir!Ihave the same question.The target signal to be sampled is at the frequency of 15 kHz,but as you have said,the balun mode suggested RFtransformer,which is aimed at 0.4~800MHz.The problem is that the PCB layout has been fixed.Therefore,it's necessary to find another transformer which is packaged in the same way and can deliver signals whose frenquencies are ranging from 50 to 15kHz.Can you give me some advice?I would appreciate it very much if you can spare your precious time and give me some advice.

    Yours sincerely.

  • AD9251.pdf

    The pdf above is the input design.I will appreciate it a lot if you can give me some tips on modification.

    Thanks a lot.