Description

Bioimpedance spectroscopy is a non-invasive measurement technique in medical applications. It requires low amplitude current excitation to maintain safety. Designing a current source for this purpose is a challenge as it needs to maintain a stable and safe excitation current, below 0.5 mA, independent of the impedance and frequency.Various topologies of voltage controlled current sources exist in the literature, where the Enhanced Howland Current Source (EHCS) and the Dual HCS (DHCS) show advantages concerning low output current deviation and high output impedance.They are composed of operational amplifiers and resistors, where the operational amplifier's non-ideal characteristics, resistors' tolerance influence the circuit performance. In this thesis, we propose an analytical model to systematically select circuit components.Stray capacitances influence the circuit' performance. Thus, we propose a new structure of the Howland current source to eliminate their effects. We have thereby successfully grounded the measured impedance on a virtual ground and isolated it from stray capacitances. As a result, we achieve a wide working bandwidth of 5 MHz and 7 MHz for EHCS and DHCS, respectively. Moreover, the circuits can deliver precise current for a high range of load reaching 10 kOhm.The developed circuit solves critical requirements in the development of a bioimpedance spectrometers by ensuring the safety and accuracy in a wide frequency range and a wide impedance range.