Please see below about PSD:
https://www.onosokki.co.jp/English/hp_e/c_support/faq/fft_common/fft_general_5.htm

 

In the OS-2000 series, the power spectral density (PSD) displayed by FFT analysis is calculated with （m/s²）/√ Hz.

 

Use the following method to calculate the squared power, [ (m/s²)² / Hz].
On the analysis screen, right-click to display "CSV Export" and run it.

Open a CSV file in Excel and square the value for each frequency.

 

[To display “PSD”]
1. After performing FFT, click “Analysis setting” to convert “PSD”.

 

2. When the setting dialog box is opened, switch to “Analysis condition” and set the density calculation to “PSD”.。

 

3. The unit of the vertical axis is changed to “PSD”.