Step response model estimation

I have set of datas by solving ODE's using ODE 45 solver. My step input varies like this: [5.7,5.9.6.1,6.3,6.5,6.7] for the entire test process.(5.7 initial condition). Everytime I change the step input, I collect the output data until steady state is reached. My input and output is inversely proportional so a negative gain is expected from the process model. But im not getting such estimates. How exactly should I import input in this case and where could the probable mistake be.

For sample:

Output:160 158 157 156 154 153 152 151

Input:5.7 5.9 5.9 5.9 5.9 5.9 5.9 5.9

Time:0 5 10 15 20 25 30 35