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This paper presents a novel ultra low power Gaussian frequency shift keying (GFSK) demodulator with efficient input DC offset cancellation and frequency offset cancellation for wireless body area network (WBAN) application. The proposed demodulator uses a new multi-threshold phase domain analog to digital converter (PDADC) to improve the phase accuracy of the zero crossing detection, hence can achieve smaller modulation index and robust demodulation performance. Measured results show that the minimum detectable modulation index for the demodulator is 0.2. The demodulator symbol rate is up to 1.17 Mbps and the carrier frequency is from 1 MHz to 4 MHz. The measured signal to noise ratio (SNR) for 0.1% bit error rate (BER) with 250 kbps symbol rate and 0.28 modulation index is 16.7 dB. The demodulator is implemented in a 0.18 mum digital CMOS process with only 0.11 mm2 chip area. The demodulator only drains 350 muA current from a 1.8 V power supply.