This paper describes a low power asynchronous successive approximation register (SAR) type 12b analog-to-digital converter (ADC) for biomedical applications in a 0.35 ${mu}m$ CMOS technology. The digital-to-analog converter (DAC) uses a capacitive split-arrays consisting of 6-b main array, an attenuation capacitor C and a 5-b sub array for low power consumption and small die area. Moreover, splitting the MSB capacitor into sub-capacitors and an asynchronous SAR reduce power consumption. The measurement results show that the proposed ADC achieved the SNDR of 68.32 dB, the SFDR of 79 dB, and the ENOB (effective number of bits) of 11.05 bits. The measured INL and DNL were 1.9LSB and 1.5LSB, respectively. The power consumption including all the digital circuits is 6.7 ${mu}W$ at the sampling frequency of 100 KHz under 3.3 V supply voltage and the FoM (figure of merit) is 49 fJ/conversion-step.