_{C}). It is obtained by extending the 1/f and white-noise portions of the noise plot and noting the point at which the lines cross. It is important as a figure of merit. Also, the 1/f corner frequency isn’t necessarily the same for voltage and current noise. Yet it often is only specified for voltage noise.How can I use this information in choosing a low-noise amplifier? Consider the frequency band of interest and relate the rms noise within that bandwidth to your system requirements. Because noise is specified in terms of the square root of frequency, the various noise contributions can be evaluated as the square root of the sum of their squares. Thus, the total rms voltage noise, en,rms, in the bandwidth F

_{L}to F

_{H}, is simply:

where enw is the broadband white noise, F_{C} is the 1/f corner frequency, and F_{L} and F_{H} define the measurement bandwidth of interest.

Generally, any noise component that is four or five times higher than any of the others becomes dominant, and the rest can be disregarded. So at higher frequencies, F_{C} ln(F_{H}/F_{L}) becomes insignificant, and the total rms noise is simply the white noise times the square root of the frequency difference. In fact, if F_{H} is very much higher than F_{L}, the total rms noise is simply the white noise times the square root of F_{H}.

On the other hand, when you’re operating in the 1/f region, the total rms noise is the noise level at the corner frequency (i.e., the white noise level) times the square root of the corner frequency times ln(F_{H}/F_{L}).