random vibration PSD analysis

I am performing a random vibration analysis using data I captured with my smartphones accelerometer to develop a PSD chart. The data I have is in .CSV format and I have developed a PSD curve using the following code:

filename = 'RV.CSV'; randvib = importdata(filename); freq=abs(randvib(:,1)); time=randvib(:,2); Fs=time(end)/length(freq); Hpsd=dspdata.psd(freq,'Fs',Fs); plot(Hpsd)

The curve seems to be reasonable (although there is looks messy) but it gives the y axis in dB/Hz, when I need in g^2/Hz. Is there a way to convert between these different units for the y axis?

Alternatively, is there a more efficient way of doing my analysis? The spectrum.welch or spectrum.periodogram seem more what I require but I'm not sure of a way to use data as opposed to a function.

Cheers