The implementation of a fully-differential (FD) instrumentation amplifier (IA), based on indirect current feedback (ICF) and aimed to electrical impedance measurements in an Internet of Things (IoT) biomedical scenario, is presented. The IA consists of two FD transconductors, to process the input signal and feed back the output signal, a summing stage, used to add both contributions and generate the correcting current feedback signal, and a common-mode feedback network, which controls the DC level at the output nodes of the circuit. The transconductors are formed by a voltage-to-current conversion resistor and two voltage buffers, which are based on a super source follower cell in order to improve the overall response of the circuit. As a result, a compact single-stage structure, suitable for achieving a high bandwidth and a low power consumption, is obtained. The FD ICF IA has been designed and fabricated in 180 nm CMOS technology to operate with a 1.8-V supply and provide a nominal gain of 4 V/V. Experimental results show a voltage gain of 3.78 ± 0.06 V/V, a BW of 5.83 MHz, a CMRR at DC around 70 dB, a DC current consumption of 266.4 $\mathsf{\mu }$A and a silicon area occupation of 0.0304 mm${}^2$.

The design of an instrumentation amplifier (IA), based on indirect current feedback and suited to electrical bioimpedance spectroscopy, is presented. The IA consists of two transconductors and a summing stage, featuring a single-stage configuration process that allows the maximum achievable bandwidth to be extended. The transconductors are linearized by means of resistive source degeneration, whereas the use of super source followers allows a reduction in the values of the source degeneration resistors. This fact leads to a decrease in the overall noise and the silicon area, thus resulting in a compact implementation. A thorough analysis of the proposed solution, accompanied by a design procedure and verified by means of electrical simulations, is also provided. Two versions of the IA, i.e., a single-ended (SE) and a pseudo-differential (PD) structure, were designed and fabricated using 180 nm CMOS technology to operate with a 1.8 V supply. The experimental results, including a BW of 5.2 MHz/8.0 MHz, a CMRR higher than 72 dB/80 dB, a DC current consumption of 139.0 $\mathsf{\mu }$A/219.3 $\mathsf{\mu }$A and a silicon area equal to 0.0173 mm${}^2$/0.0291 mm${}^2$ for the SE/PD implementation, validate the suitability of the approach.

A CMOS widely tunable second-order $G_m$-C bandpass filter (BPF), intended to be used in multi-sine bioimpedance applications, is presented. The filter incorporates a tunable transconductor in which the responses of two linearized voltage-to-current converters are subtracted. As a result, the effective transconductance can be continuously adjusted over nearly three decades, which allows a corresponding programmability of the center frequency of the BPF. The circuit was designed and fabricated in 180 nm CMOS technology to operate with a 1.8 V supply, and the experimental characterization was carried out over eight samples of the silicon prototype. The simulated transconductance of the cell can be tuned from 5.3 nA/V up to 19.60 $\mathsf{\mu }$A/V. The measured range of the experimental transconductance varied, however, between 1.42 $\mathsf{\mu }$A/V and 20.57 $\mathsf{\mu }$A/V. Similarly, the center frequency of the BPF, which in the simulations ranged from 500 Hz to 342 kHz, can be programmed in the silicon prototypes from 22.4 kHz to 290 kHz. Monte Carlo and corner simulations were carried out to ascertain the origin of this deviation. Besides, the extensive simulation and experimental characterization of the standalone transconductor and the complete BPF are provided.

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 $\mu$ 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.

An instrumentation amplifier (IA), aimed at wideband bioimpedance analysis in the low‐voltage low‐power scenario of internet of medical things (IoMT), is presented. The operation principle is based on the indirect current feedback technique, where an input and a feedback transconductor determine the voltage gain of the preamplifier. The required transconductors consist of two bulk‐driven flipped‐voltage‐follower cells and an active pseudo‐resistor, thus leading to a linear and compact implementation. The circuit has been designed and fabricated in 180 nm CMOS technology to operate with a 0.6‐V supply. Experimental results obtained from measurements on eight samples of the silicon prototype show that when the IA is programmed to have a nominal voltage gain of 11 V/V, the bandwidth is 316.2 kHz, the CMRR exceeds 63 dB, and the maximum output voltage that can be processed with a THD below –40 dB is 555 mV .