sample time arduino-simulink

I'm trying to acquire a single analog signal with Arduino Mega using simulink(Ode 3) from Matlab 2013a. The sample time of the analog block is 0.004 sec and the signal is correctly saved to the workspace. I referred to the doc belo: http://nl.mathworks.com/help/supportpkg/arduino/ug/analoginput.html

Here comes my problem: THe signal saved in the workspace has a real sample time of around 0.03s (sampling frequency of around 30Hz instead of 250Hz) and the time I store with the clock block is the one of the symulink simulation and not the real one. How can I save on the workspace a signal with the proper sample time? Thanks