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