Constantinou, L; (2014) Novel CMOS Integrated Current Driver Circuits for Bioimpedance Measurements. Doctoral thesis , UCL (University College London).

Abstract

Bioimpedance spectroscopy is a study of the variation of tissue’s electrical properties, both conductive and dielectric, through a frequency spectrum (100Hz to 1MHz). It involves the application of AC signals to the surface of the tissue via electrode pairs, which can either be a current signal or a voltage signal, and the resulting surface signals are recorded via a separate or the same pair. The recorded signals are converted to impedance measurements via a demodulation procedure. The study of tissue’s electrical properties can provide with useful information regarding both its physiology and pathology. The main aim of the work described in the thesis is the design and development of novel CMOS current drivers, for wideband tetrapolar bioimpedance measurements (100Hz- 1MHz). The work presented resulted in the fabrication and experimental validation of two current driver circuits which offer superior performance relative to existing designs. The first design was fabricated using a 0.6 µm CMOS technology and occupies a silicon area of 0.64 mm2. It can deliver a maximum output current of 5 mAp-p operating from a ±9V supply (15V output compliance). The output impedance is 665 kΩ at 100 kHz and 372 kΩ at 500 kHz. The second design was fabricated using a standard 0.35 µm CMOS technology and occupies a silicon area of 0.4 mm2. It can deliver a maximum output current of 1 mAp-p operating from a ±2.5V supply (4V output compliance). The output impedance is higher than 1 MΩ up to 500 kHz reducing to 360 kΩ at 1MHz, thus further improving the performance up to 1MHz. A custom-made bioimpedance measuring device is also presented which incorporates the second current driver that has been tested in both in-vitro and ex-vivo experiments (using post-operative human colon cancer specimens). Experimental results verify a stable and reliable operation of the circuit up to 1MHz. Finally a third current driver, using a standard 0.35 µm HV CMOS technology, is presented (its silicon implementation is a future plan) which aims to improve upon certain performance limitations of the previous designs. Simulated results demonstrate an input/output phase (~2.08º) at 1MHz (better than any reported CMOS current driver), and an output impedance of 1MΩ at 1MHz. The current driver employs a fully automated continuous time common mode feedback (CMFB) loop to accurately set the output DC levels which has not yet been implemented on any existing CMOS design.

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