Noise Fourrier Transform (FFT)

[rjm]

Is the following correct?

IF

1. an op-amp has a listed voltage noise of 3 nV/rqrtHz
2. used with a gain of 100
3. the conditions are such that the voltage noise dominates
4. further ignore 1/f and just consider wideband white noise
5. I take the FFT of the output noise
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   THEN
   
   the noise floor should be observed at 3nV * 100 = 300nV = -130 dB
   
   AND
   
   the rms voltage is equal to 3nV* sqrt(bandwidth)

 

[jcx]

there is an additional BW effect in the fft definition, if the bin size is ~ 1 Hz then your estimate is roughly correct - figure bin size from time duration of data set/#points in fft

also be aware that various math packages use different normalizations; you may see 1/n or sqrt👎 used

input refered noise is en*sqrt(bw) output noise will be 100x larger

it is quite an accomplishment to totaly remove power line fundamental and harmonics at these levels