Continuous blood-status monitoring by thoracic impedance or impedance phase measurement  is critical in reducing chronic heart failure (CHF) because the symptoms of deterioration are not recognized by patients, thus preventing timely treatment. Therefore, a wearable/implantable bio-impedance (Bio-Z) sensor is required for ambulatory patients to detect early signs of deterioration for appropriate intervention in proper time. Continuous monitoring requires ultralow-power consumption (<10μW) and it requires the high resolution (<0.1Ω) to detect small impedance variations . Prior Bio-Z sensor ICs [2-4] do not satisfy these requirements simultaneously because the resolution depends inversely on the power consumption. Furthermore, previous Bio-Z sensors have a large phase error that is unable to accurately (<1°) and continuously measure phase due to the square-wave modulation scheme of the current generator , resulting in intolerable measurement errors , or the supply and temperature variations of measurement accuracy due to the limited bandwidth (BW) of readout . Although the pre-demodulation  technique can relax BW requirement, conversion loss of sine to square wave demodulation causes SNR degradation by 2/π times, leading to 2.47× increased power consumption.