# Calculating estimated ADC Noise Floor from given specifications

As per my current understanding, the ADC datasheets for modern high speed ADCs give the noise level measurements in terms of Noise Spectral Density (NSD) in unit of dBFS/Hz. Now to convert that value to the noise floor first the dBFS/Hz value is converted to dBm/Hz by using the value of full scale power in dBm for the respective device. Then the resulting value is added to $$10\log 10(\text{Bandwidth})$$ to get the noise floor for SNR and Dynamic Range calculations. The values should correspond to what is measured when analyzing the noise using FFT.

1. If there is anything wrong in these steps then correction is most welcome.

2. When integrating the noise over the total bandwidth, how should that value be chosen? As we are taking about thermal noise, shouldn't the value of the ADC analog bandwidth be used rather than the bandwidth of the filter prior to the ADC, even if Nyquist or oversampling is used? (Considering the thermal noise in ADC circuitry)

3. For the case of sub-sampling over multiple Nyquist zones and using IQ data, what bandwidth value should be considered?

3. Subsampling does not change the bandwidth that is chosen as given in 2 above. With regards to IQ data, typically we are concerned with the signal and noise over the range of $$-f_s/2$$ to $$+f_s/2$$ and then the occupied bandwidth of our signal within that. If the receiver was designed properly when undersampling from a higher Nyquist zone, bandpass filtering would be provided such that the noise from only one Nyquist zone where the signal originates passes through.