The fully implantable cochlear implant (FICI) interface circuit proposed in this work senses sound harmonics from 8 different piezoelectric cantilever sensors, and generates pulse width modulated biphasic current outputs to stimulate the auditory neurons. Signals from the piezoelectric sensors are amplified, rectified, and sampled. The sampled voltage is held and converted to current by a novel logarithmic voltage-to-current converter. The current is then digitized with a current comparator to determine the width of the generated biphasic current pulses. Continuous interleaved sampling (CIS) is used as the stimulation technique for 8 channels operation. The system is designed and implemented in 0.18 am HV CMOS process. Measurements show that the circuit is able to generate 15 to 62.5 mu s biphasic current pulses with 400 mu A peak amplitude, as the input range varies from 60 dB to 105 dB sound pressure level. The total power consumption of 82 and 513 mu W have been measured at 70 dB input for 1-channel and have been extrapolated for 8-channels configurations, respectively, which are the lowest powers for FICI interface electronics to the best of our knowledge.