ADS5560 Noisy Output Spectrum

Hello,

The signal in the FFT plot does not appear to be coherent. Depending on the number of samples you are using, you may need to set the input frequency to be the appropriate coherent frequency in order to prevent the spectral leakage you are seeing. Another method would be to apply windowing.

Besides using coherent input signal or windowing, you will also need to use bandpass filter on the clock source and input source in order to minimize noise and distortion. 

The typical ADC test setup is attached. In the lab, we use HP8644B signal generators to generate the ADC clock and input signals needed. Even though the generators are low jitter and low distortion, we still use bandpass filters to filter out the harmonics and distortions for best SNR/SFDR performance. The signal generators are phase locked through the 10MHz reference clock. 

The bandpass filters we used are from either TTE or Trilithic

http://www.trilithic.com/RF%20and%20Microwave%20Components/Products/BC%20Series/BC%20Series.html

http://www.tte.com/bandpasschebyshev.php

We used Chebyshev filter topology for its higher roll-off.

For the clock signals, crystal filters may be used to provide narrower passband. We do not recommend using it for ADC input since it may create distortions at high input level. This distortion will degrade the ADC performance.

-KH