An ultra low voltage VCO-based ADC was designed and taped out using STM65nm technology. The nonlinear Voltage to phase/Frequency relation of VCO results in a nonlinear output. A background calibration scheme was used to estimate and correct nonlinearity. . ADC operates at 0.4V and giving 8.8bits ENOB, over 10KHz bandwidth with a power consumption of 1.15uW.