An integrated common-mode feedback topology for multi-frequency bioimpedance imaging

One of the key limitations in medical impedance imaging and bio-impedance measurements is common-mode errors. We present an integrated common-mode feedback topology which reduces these errors for use in a bio-imaging system for in-vivo monitoring of neonate lung function (10-200 kHz current injection frequency). A frequency-selective feedback network is described which reduces the common-mode voltage due to electrode impedance mismatch at the input of the differential amplifier. The theory and key design blocks are presented. The circuit was implemented in a 5-V 0.35-mum CMOS technology, occupying an area of 0.75 mm² and dissipating about 20 mW. Experiments were conducted using, an RC model of the electrodes, and ECG electrodes on the forearm to demonstrate the working of the integrated circuit. Measured results show that the common-mode signal is reduced by 85%, 75%, 70% and 65% at 10 kHz, 50 kHz, 100 kHz and 200 kHz, respectively.