Output Noise Spectrum

To determine the output noise spectrum of a filter, FA needs to know the following.

1. The state-space model of the filter network;
2. the capacitance values;
3. the noise factor.

An easy way to specify the capacitance values is to give a value to the variable ctot. The noise factor, stored in the variable noisefactor has a default value of 1.

An onoise plot specification results in a plot of the output noise spectrum of the filter in V²/Hz . Note that this spectrum is double-sided. This means that also negative frequencies must be taken into account when integrating the noise spectrum to obtain the total output noise voltage. Most programs, such as SPICE, determine noise spectra single sided. The double-sided noise spectrum of a resistor R is 2kTR , its single-sided spectrum is 4kTR .

This analysis assumes a single-ended filter. If the filter is differential, the noise voltage is twice the value from the analysis, and the noise power is four times the result from this analysis. Because the differential signal voltages are also twice the single-ended signal voltages, the dynamic range remains unchanged.