Whats the difference between a DAD and a PDA?

Nothing. Different manufacturers use slightly different acronyms for the same thing. “DAD” = “Diode Array Detector”, and “PDA” = “PhotoDiode Array”. For you pedantic types, the phrase “PDA detector” makes sense (“PhotoDiode Array detector”), while “DAD detector” is redundant (“Diode Array Detector detector”). The older variable wavelength absorbance detector (commonly called a “UV detector”, because the most commonly used wavelengths are in the ultraviolet) differs from the PDA in the arrangement of the optical components.