This is not correct. The second input to snr must be a real-valued row or column vector that must have the same dimensions as the first input. It is the estimate of noise of your signal. You are entering a single number which is the range of amplitudes of your signal.
How are you quantifying noise? What does your signal look like? You may want to browse the examples on the document page for snr to see if you can get an estimate of the noise that way. Otherwise, you'll have to provide more information here, perhaps in the form of a figure showing the frequency spectrum of your signal.