Noise and Interference basics (Raymond Schouten) txtpage4
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              NOISEFLOOR ESTIMATION                   


To create an optimal measurement situation it is good 
practice to compare an estimated noisefloor to the 
measured noise & interference. This gives information 
about the need to improve the setup.


To estimate the noisefloor a few parameters are needed:

 -Thermal noise from the measured device Rs
  resistance & temperature -> sqrt(4KTRs) [V/sqrt(Hz)]

 -Input noise specifications from the amplifier
  (only the first amplifier stage is important)
  For a voltage measurement:
  voltage & current noise  En [V/sqrt(Hz)],In [A/sqrt(Hz)]
  For a current measurement : In [A/sqrt(Hz)]                  
  
 -Bandwidth of the total setup [Hz]
  input part:
  (for volt.meas:) Fs =1/(2*Pi*Rs*Cs)   
                   Cs =total capacitance input circuit
                       (cable=100pF/m)
  (for curr.meas:) Fs =1(2*Pi*Rin*Cs)
                   Rin=Input impedance current amplifier          
                   Cs =total capacitance input circuit
  amplifier part:  Fa =(spec's)
  measure part:    Fm =bandwidth of interest
  Fs sets the maximum bandwidth.
  Then: Fm <= Fs  (filter on measure part ) 
  Also: Fa >= Fm  (for all gain settings)


Noisefloor (input) = sqrt(Bandwidth) * Input noise   

For a voltage measurement:         
Eni = sqrt(Fm) * sqrt( (4KTRs+(En)^2+(In*Rs)^2))   [Vrms]   (1)
For a current measurement:
(when using a feedback resistor Rf and Rs>Rf) 
Ini = sqrt(Fm) * sqrt(4KT/Rf)                  [Arms]   (1)

This gives the lowest value that can be measured
in a perfect setup without interference.

Multiply this value by the overall gain G to get
the equivalent output signal

Noisefloor (output): Eno = G * Eni

This value can be compared to a measured value.
 


(1)  This is only exact for a very steep filter on Fm,
     for a first order (R-C) filter multiply Fm by Pi/2