Hi There, I'm not sure about this, but I think that noise voltage was referenced as a RMS value. Since the Root Mean Square equation includes the Square Root of 1/T...and 1/T = f, you have a denominator with sqrt(Hz). You already know that frequency (f) is in Hertz (Hz). I think the 6nV is the voltage during 1 period (T)....maybe. I know this is not a complete explanation, but I hope it helps! .