This paper presents a wideband frequency-shift keying (FSK) demodulator which is suitable for a digital data transmission chain of wireless neural stimulation microsystems such as cochlear implants and retinal prostheses. The demodulator circuit derives a constant frequency clock directly from a FSK carrier, and uses this clock to sample the data bits. The circuit occupies 0.03 mm2 in a 0.6 μm, 2M/2P, standard CMOS process, and consumes 0.25mW at 5V. This demodulator circuit is experimentally tested at the transmission speed up to 2.5Mbps while receiving a 5/10 MHz FSK carrier signal in a cochlear implant system.