Help with noise bandwidth

[nhkch]

I wonder what is the noise bandwidth NBW=0.00018 mean in this graph? is the noise bandwith related to SNR?

 

[nhkch]

Can anyone teach me how to analize the graph given above?

 

[RCinFLA]

It appears to be the noise spectrum of a sigma delta ADC. Everything is normalized to sample rate frequency so NBW 0.00018 of the sample frequency.

An absolute graph would normally show dbC/Hz or db relative to carrier where the scale is the noise spectrum amplitude at a given frequency on the plot is noise measured in a theoretical 1 Hz bandwidth. If measured in a moving 10 Hz bandwidth window on a spectrum analyser each noise amplitude point along the frequency axis would be about 20 db higher. (more bandwidth means more noise power collected in the 10x wider moving window).

dbFS means db relative to full scale. In case of ADC, instead of dbC (db carrier), typically the reference for carrier is the full scale level of the ADC. What the maximum carrier signal the ADC can read before it runs out of high end range. Low end range is not alway determined by lowest digital output word of ADC but the noise floor of the ADC.

Sigma delta ADC push the noise spectrum to higher, out of desired input band frequencies, where noise can be attenuated by decimation and digital low pass filter following of ADC.

 

[nhkch]

Sorry, i still not get it. May i know what information can i get from this graph. For example the graph increaser from -120db to around -40db when the sampling frequency increase, what message it bring? higher frequncy will get lower noise rejection?
The sampling frequncy is normalise, if 1=sampling frequency fs, fs=48kHz, then 0.5 = 24kHz?
Can the SNR=82.5db obtain just by looking at the graph or additional calculation is needed?
The NBW 0.00018, i still can understand what it mean in the graph?

Hope you can explain to me...thanks in advance

 

[RCinFLA]

https://en.wikipedia.org/wiki/Delta-sigma_modulation

see figure 5

 

[nhkch]

Ic, so let the sampling frequency is 48kHz and up sample 128 time to 6.144MHz. My desire signal bandwidth is 20kHz. After nomalize where 1=fs=6.144Mhz. My desire signal bandwidth will be at 20k/6.155M = 0.003. So from the graph given, the sigma delta modulator can supress the noise up to -120 db in my desired bandwith. This is what i understand, is that correct?