Power spectral density in matlab

[bababui]

I am calculating power spectral density in Matlab of some data I collected. The various power spectral density calculation methods that MATLAB provides return y axis units in db/hz, power/frequency. I need to get a plot where the Y axis is in V/rtHz. My question is, is there a command in matlab that will make that conversion, if not how can I solve this problem?

Any help much appreciated, been stuck on this for a while,

Thanks