A 4-Decade-Tunable High-Selectivity Gm-C Bandpass Filter for Simultaneous Multi-Sine Bioimpedance Analysis

A bandpass filter (BPF), with high quality factor ( ${Q}$ ) and wide tuning range of the centre frequency ( $f_{0}$ ), for signal separation in simultaneous multi-sine bioimpedance analysis, is presented. The cell follows the $G_{m}-{C}$ approach, where a multiple-output transconductor has been incorporated, for the sake of compactness, and a fully-differential structure has been selected, to enhance the general performance of the solution. The linearized transconductors ( $G_{m}$ ) are based on a double asymmetric differential pair and the integrated capacitors ( ${C}$ ) are implemented in such a way that the required transfer function for the differential-mode signal is achieved and the stability of the common-mode component is granted. The BPF has been designed and fabricated in 180 nm CMOS technology to operate with a 1.8-V supply and feature a nominal gain at $f_{0}$ of 0 dB and a ${Q}$ equal to 20, whereas $f_{0}$ is programmed by modifying the biasing current of the transconductors. Experimental results, measured on 8 samples of the silicon prototype, show that the BPF centre frequency can be continuously swept from 102 Hz to 2.06 MHz with a supply current ranging between 981 nA and $686~\mu \text{A}$ . Besides, the maximum output voltage range of the filter, determined according to the–40 dB THD criterion, varies between around 31.5 mV and 64 mV within the frequency range indicated, thus leading to a dynamic range between 42.3 dB and 48.6 dB, which illustrates its suitability for the target application.