Spurious Free Dynamic Range (SFDR) is the ratio of the level of the input signal to the level of the largest distortion component in the FFT spectrum. This specification is important because it determines the minimum signal level that can be distinguished from distortion components. Label B in the figure below shows the SFDR ADC measurement data.

The FFT spectrum obtained from the A/D converter will have a noise floor that is a function of N, data resolution in bits, and M, the number of points in the FFT data. For a set of M-point FFT data, the level of the FFT noise floor can be determined using the equation below, where N is the resolution of the ADC in bits.

(1)
$$FFT Noise Floor = 6.02 • N +1.76 dB - log (M/2)$$