In this paper we present a novel all digital binary phase shift keying (BPSK) demodulator dedicated implantable biosensor. This demodulator offers the advantages of ultra-low power and low complexity structure which are very essential to develop wireless miniaturized implantable devices. As the continuation of our research approach to implement a glucose sensor implanted under skin, herein, we address the design and analysis of a demodulator integrated in a CMOS chip along with other required building blocks. In order to minimize the effect of transmitter frequency changes and to enhance the circuit robustness a digital self-calibration technique is also proposed. Simulation results show that the demodulator can tolerate a relatively large frequency shift of at least ±80% around the centre frequency in all process corners. The power consumption of the demodulator at a data transmission rate of 16 Mbps and a supply voltage of 1.8 V is as low as 27μW.