Units of 6.02*N + 1.76 as an FFT noise floor

1. How to calculate noise floor from FFT?
2. What is FFT noise floor?

How to calculate noise floor from FFT?

Average FFT Noise Floor (dB) = 6.02n + 1.76dB + 10 log(3*M / (pi * ENBW)), where M is the number of data points in the FFT. A reasonable number of samples for the FFT of a 12-bit converter is 4096.

What is FFT noise floor?

The FFT noise floor is not the SNR of the ADC, because the FFT acts like an analog spectrum analyzer with a bandwidth of fs/M, where M is the number of points in the FFT. The theoretical FFT noise floor is therefore 10log10(M/2) dB below the quantization noise floor due to the processing gain of the FFT.