CMOS-Based Impedance Analyzer for Biomedical Applications

This thesis presents a device which can be used to monitor the hydration level of patients suffering from renal diseases and who are forced to undergo regular dialysis sessions. The system comprises a skin interface and a stimulation and readout circuit. A dimensionless hydration index obtained by performing a bio impedance analysis has been proposed. The proposed device combines the tetra polar multifrequency bio impedance analysis with the 4-electrodes focused impedance measurement approach. Moreover, it combines the lock-in structure with the dual step super-heterodyne demodulation scheme. In contrast to the full analog approach, a mixed analog/digital solution is adopted. In particular, the proposed solution performs a first frequency down conversion in the analog domain and shifts the I,Q demodulation in the digital domain. This solution allows removing any sensible dual path from the analog domain with important benefits in terms of complexity, precision and power consumption. Moreover, with the adopted solution the digital demodulation step is achieved in a very simple and efficient way, without the need of high complexity digital multipliers. We compared the results obtained with a high precision LRC meter with the impedance measurement performed with the new chip and showed a relative error of less than 0.8%. In addition, a FEM model of the thorax tissue has been developed and simulated with the EIDORS toolbox. Simulated results have been compared with in-vivo measurements and we found good accordance between them.