This paper presents an implantable bio-impedance measurement system for long-term monitoring devices. The proposed magnitude/phase system features a low-power analog frontend that generates pulse-width-modulated outputs. In addition to the low power consumption, the bio-impedance readout makes use of a Fibonacci DC-DC converter to convert voltage to time for backscattering communications. The proposed IC designed and simulated in a 0.18 µm CMOS process occupies 200µmx170/µm and is capable of measuring the bio-impedance at 2 kHz over a wide dynamic range from 100 Ω to 4 k Ω with 1.25 Ωrms accuracy while consuming just 4 /LA from a 1.8 V supply.