Novel CMOS integrated current drivers for wideband bioimpedance measurements.
Doctoral thesis, UCL (University College London).
Bioelectrical impedance measurements are routinely used in many clinical applications such as lung function monitoring, brain imaging, body segment monitoring and tissue characterization. Advantages of the method include low-cost, simplicity and non-invasiveness. One bioimpedance imaging technique of particular interest is Electrical Impedance Tomography (EIT). EIT works by reconstructing the differences in electrical conductivity inside a body. The design of instrumentation for bioimpedance measurements is very challenging, particularly for applications requiring wideband operation (e.g., imaging of cancer biomarkers). Such applications require wideband current drivers that can deliver accurate and stable sinusoidal current (AC) to the electrode-tissue load. Many current drivers for bioimpedance measurements are based on discrete electronics, mostly using the modified Howland circuit (MHC). However, the accuracy and performance of the MHC depends on the performance of the operational amplifiers used and on the degree of matching of its resistors, making the approach not attractive for integration. In addition, because it uses both positive and negative feedback it can suffer from stability problems and since it employs voltage feedback it suffers from bandwidth limitations. This thesis is concerned with the design implementation and experimental evaluation of novel current drivers for wideband, fully-integrated, bioimpedance instrumentation for EIT applications. Three new current driver circuits are introduced based on the operational transconductance amplifier (OTA) approach, and implemented in sub-micron CMOS technology. The described integrated current drivers overcome many of the limitations of discrete current drivers, i.e., limited precision at high frequencies, stability, and low-bandwidth. Two new open-loop current drivers implemented in 0.35-μm CMOS technology are presented and their performance is analyzed and simulated. One of the current drivers was fabricated as an integrated circuit and its performance was experimentally verified using RC loads, a saline tank electrode phantom and electrodes on the forearm. In order to further improve its performance, a novel negative impedance converter (NIC) circuit is introduced which compensates for the stray capacitance at the output node due to the cable connecting the current driver to the electrodes. Finally, a new current driver in 0.6-μm CMOS technology is presented using feedback to provide high output impedance from DC to over1 MHz. Simulation results including Monte-Carlo analysis are presented to show the operation of the circuit.
|Title:||Novel CMOS integrated current drivers for wideband bioimpedance measurements|
|Additional information:||Permission for digitisation not received|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Electronic and Electrical Engineering|
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