Noise and Interference basics (Raymond Schouten) txtpage4 --------------------------------------------------------- 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