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Abstract
This work compares two design methodologies, emulating both AgCl electrode and skin tissue Cole models for testing and verification of electrical bio-impedance circuits and systems. The models are based on fractional-order elements, are implemented with active components, and capture bio-impedance behaviors up to 10 kHz. Contrary to passive-elements realizations, both architectures using analog filters coupled with adjustable transconductors offer tunability of the fractional capacitors’ parameters. The main objective is to build a tunable active integrated circuitry block that is able to approximate the models’ behavior and can be utilized as a Subject Under Test (SUT) and electrode equivalent in bio-impedance measurement applications. A tetrapolar impedance setup, typical in bio-impedance measurements, is used to demonstrate the performance and accuracy of the presented architectures via Spectre Monte-Carlo simulation. Circuit and post-layout simulations are carried out in 90-nm CMOS process, using the Cadence IC suite.
Highlights
The electrical properties of tissues are strongly related to their structural characteristics and their functional properties [1,2,3]
The fractional order impedance is formed as a parallel combination of a resistor and a fractional capacitor acting as a Constant Phase Element (CPE), introducing the Cole behavior
To achieve electronic tuning of the CPE’s model characteristics we follow [37] and use operational transconductance amplifier (OTA) and current conveyors of the second generation (CCIIs). Both fractional-order capacitors for skin and electrode models are designed using two cascaded filters, H1 (s) and H2 (s), connected with a multiple-output OTA, which acts as a voltage-to-current (V/I) converter [37,38,39,40,41,42,43,44]
Summary
The electrical properties of tissues are strongly related to their structural characteristics and their functional properties [1,2,3]. Proper modelling of the tissues under test electrical characteristics is a crucial step during the design process of bio-impedance measurement systems. To this end, designers simulate and employ equivalent circuits of the SUT [21,22,23,24], usually implemented using lumped passive elements (capacitors and resistors). The circuit is designed, laid out, and simulated in Cadence using a Taiwan Semiconductor Manufacturing Company (TSMC) 90-nm CMOS process Both electrode and skin Cole IC-design models are validated and compared to the RC approximations (magnitude and phase) for the models’ mean parameter values.
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