This paper presents a functional design and modeling of a successive approximation analog-to-digital converter (ADC) and its application in the conditioning circuit of the vibration energy harvester. The paper published on BMAS'09 highlighted the necessity of a smart digital control to calibrate the system, which requires in turn an ultra low power supplied ADC. The harvester and the ADC are designed using a CMOS 0.35µm High Voltage technology and modelled in a mixed VHDLAMS/ELDO environment. The supply voltage of the ADC is 2.5V. The whole system was simulated using a mixed signal AdvanceMS simulator.