I have a question about the FFT spectrum of an ADC output enter image description here

I understand SFDR you can measure the difference between the fundamental and the next biggest spur/harmonic.

But how do you find SNDR from the plot? Can I simply just measure the fundamental to noise-floor differene? Similarly, how do I find SNR?

  • \$\begingroup\$ One piece of information missing : the number of FFT bins (or conversely, the width of each one). You can see there's "about" -70dB energy per bin ... you need to sum that across all bins. (Or convert to voltage/rtHz, and intecrate across 90MHz, etc) \$\endgroup\$
    – user16324
    Jan 12, 2020 at 14:42

1 Answer 1


with no obvious spurs, how to proceed?

I'd expect 2*input - Fsample.

Thus 2*76 - fsample of 180 === 152 -180 = 28MHz.

Yet at 28MHz, there is a big dip


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