This will be a mostly visual guide to FFT size and effect on frequency resolution and bin residual noise. Each of the following graphs are the exact same measurement of an ADI-2 Pro DAC output looped back to ADC input using HpW Works. Test conditions: DAC reference level 0dBFS = +19 dBu DAC filter is SD Sharp, DAC output adjusted to -20 dBFS which translates to -1 dBu DAC output looped back to ADC input ADC reference level = +19 dBu Sample rate is 44.1 KHz The only change between each graph is the FFT size and thus bin frequency resolution. They represent the same measurement with different FFT resolutions. Notice how the SNR, THD and THD+N are relatively consistent in all graphs. The residual energy per bin lowers as the FFT size increases. Visually it becomes easier to see harmonics and distinguish from noise spurs at higher resolutions. By 8M points it becomes debatable if there is any further information to be gained by increased resolution for this particular measurement. Look carefully at all the information on each graph. Outside of my tongue-in-cheek labels, the following information can be seen: 1. Y axis units 2. Number of measurements averaged together 3. FFT size in number of points 4. Date and Time of measurement 5. X axis units 6. SNR 7. THD 8. THD+N 9. DC level 10. RMS level of measurement signal 11. Sample rate 12. Channel displayed 13. FFT window used 14. Bandwidth filter if any 15. Weighting filter if any FFT = 4K FFT = 16K FFT = 65K FFT = 262K FFT = 1M FFT = 8M FFT = 16M FFT = 33M This last graph is an overlay consisting of the 4K, 262K and 16M FFT measurements.