An Active Quasi-Circulator Technique for MHz Impedance Measurement for Wearables

A unique circuit technique utilizing the active quasi-circulator (AQC) for impedance measurement is presented. Overcoming limitations of size, frequency, and sensitivity, the technique enables sensitive MHz impedance measurements for wearable applications. The AQC measures the impedance of the device-under-test (DUT) at MHz excitation frequencies through nulling the output at the DUT match point while offering enhanced sensitivity. A circuit analysis presents the theory of operation and models the AQC to extract the DUT impedance. Fabricated in a 180-nm CMOS process, the circuit occupies an active area of 0.012 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and demonstrates impedance measurement at excitation frequencies up to 25 MHz. The proposed circuit is attractive for measuring living tissues that exhibit strong bioimpedance response at MHz frequencies.