Abstract

This paper presents a highly linear second-order, tunable bandpass filter (BPF) based on low-Gm operational transconductance amplifiers (OTAs) to be used in a single-frequency bioimpedance analysis (SFBIA) system. The center frequency of the proposed BPF can be set at the target center frequency fc = 1 kHz through a tuning current IC, thus compensating for process variations and temperature changes after fabrication. A low quality factor is established to let the input signal pass through without significant phase shift or gain fluctuation, while reducing the out-of-band noise contribution to the readout circuit. The proposed BPF was implemented in a 0.18 μm CMOS process and consumes 2.6 μW from a 1.8 V supply. The Gm reduction technique enables precise control of the OTA’s transconductance, and therefore of the center frequency of the BPF. For IC ranging from 28 nA to 138 nA, the measured fc varies from 690 Hz to 3 kHz, with a constant Q factor of approximately 0.4. The proposed BPF can handle input signals as high as 132 mVpp with a maximum 1% total harmonic distortion.

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