[D]first order system and second order system

I recommend to read the following references.

1. http://lpsa.swarthmore.edu/Transient/TransInputs/TransStep.html
2. file:///C:/Users/alphacore/Downloads/laboratory_05.pdf
3. http://www.analog.com/library/analogDialogue/archives/43-09/EDCh%208%20filter.pdf
4. http://www.electronics-tutorials.ws/filter/filter_4.html