I want to calculate the Noise-equivalent Power (NEP) of a photodetector. Hereto, I have to measure the Amplitude Spectral Density (ASD) of the detector and divide the result by the Responsivity:
The Responsivity calculation is straightforward: I integrate the current between 0.0 s and 0.5 s and divide the result by the duration of 0.5 s. I then divide the result by the incoming illumination power to obtain the responsivity in units of [A/W].
Next, I have to determine the Amplitude Spectral Density without any optical input. Hereto, I perform a Fast Fourier Transform on the detector output in the range between -0.7 s and - 0.2 s. The result is shown below. I have already scaled the data to be consistent with Parsval's theorem.
I do not understand how the calculate the Amplitude Spectrum Density in Units of A/sqrt(Hz) that I need for the NEP calculation from this graph.
My plan was to integrate the Amplitude Spectrum over the frequency range and then take the average, but this approach does not end up with units of A/sqrt(Hz). Where is my mistake and where does the sqrt(Hz) term come from?