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!

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