# "Noise Spectral Density" a.k.a "Power Spectral Density" in datasheets

In the case of statistics, there are two terms: "Variance" and "Standard Deviation" to - for in the latter case - have one to note the case in which the units are without squares.

Am I correct to understand, that there is no such differentiation in signal processing, and in both cases - taken root or not - it's always termed "Power Spectral Density"?

I am asking as on page 3 of Analog Devices AD8307 there's the unit of $\frac{nV}{\sqrt{Hz}}$ for the "Input Noise Spectral Density".

Is there a specific reason as to why manufacturers are taking the square-root?

Power spectral density is given in power per $Hz$. "Input Noise Spectral Density" is given in units of volts per $\sqrt{Hz}$. These are related:
$$PSD = \frac{W}{Hz}$$
$$W = \frac{V^{2}}{R}$$
$$Input\ noise\ spectral\ density\ = \frac{V}{\sqrt{Hz}}$$
The $\sqrt{Hz}$ is from the conversion power to volts. For example if you want the noise ($nV$) in $1kHz$ $BW$ you must multiply $\frac{nV}{\sqrt{Hz}}\sqrt{1000}$