How do I set constraints on curve fitting?

hello,

I have a set of data that I would like to fit with a double exponential function (type 'exp2'). However I have very few points at the beginning and sometimes the fitting is not done properly. See image below for illustration:

On the bottom graph the fitting performed by Matlab is according to me correct. On the top graph however the first points are ignored. I insist on the fact that I have used the SAME function to fit both sets of data (the 'exp2'). If you have a solution to my problem I would be very grateful.

[Matt J]

What StartPoint are you using? Try one with a*b+c*d>0.