This paper presents a fully differential ultra low power successive approximation (SA) Analog-to-digital converter (ADC) for biomedical application. In order to reduce the system power consumption, the building block components of the SA ADC architecture has been optimised. In addition, the ADC the input voltage swing is scaled down to in order to reduce the slope gain error and the nonlinearity errors. The SA ADC has been implemented in Cadence Analog Design Environment using 0.18-micron CMOS technology. The designed SA ADC operates at a sampling rate of 200S/s at 3V power supply and consumes only 12µW of power at this frequency. The ADC standby power consumption is less than 1µW. The designed 16-bit ADC occupies an area of 0.1 mm2 and is the smallest in size among its 16-bit counter parts reported in the literature. The proposed 16-bit ADC achieves the differential-non-linearity (DNL) and integral-non-linearity errors (INL) of ± 0.5 LSB and ± 0.3 LSB respectively.