High accurate and wideband current excitation for bioimpedance health monitoring systems

Current sources are critical for the performance of bioimpedance health monitoring device. The challenging features for a high accuracy and wide bandwidth voltage controlled current source are stable and safe current magnitudes. Corresponding to the typical measurement requirements, it should allow typical load impedances in the range from 100 Ω to 10 kΩ and have high output impedance of approximately 10 MΩ at 5 kHz and 1 MΩ at high frequencies up to 1 MHz without harmonics and influence of stray capacitances. Generalized impedance converters and negative impedance converters help to reduce the influence of stray capacitances. In this study, we compare between two types of voltage controlled current sources for grounded loads: Howland and Tietze circuits. Based on this study, we suggest an improved Howland circuit in inverting dual configuration with a negative feedback using the high bandwidth amplifier AD8021. With this configuration, a better accuracy is reached at higher frequencies up to 1 MHz. With the addition of compensation capacitor to the operational amplifier, this circuit configuration is capable to maintain approximately constant current amplitude within an accuracy of less than 0.022% at both low and high frequencies up to 1 MHz.