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

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

An `onoise` plot specification results in a plot of the
output noise spectrum of the filter in
*V*^{2}/*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 2*kTR*
, its single-sided
spectrum is 4*kTR*
.

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.