A Comparison of Front-End Amplifiers for Tetrapolar Bioimpedance Measurements

Many commercial benchtop impedance analyzers are incapable of acquiring accurate tetrapolar measurements, when large electrode contact impedances are present, as in bio-impedance measurements using electrodes with micrometer-sized features. External front-end amplifiers can help overcome this issue and provide high common-mode rejection ratio (CMRR) and input impedance. Several discrete component-based topologies are proposed in the literature. In this article, these are compared with new alternatives with regard to their performance in measuring known loads in the presence of electrode contact impedance models, to emulate tetrapolar bioimpedance measurements. These models are derived from bipolar impedance measurements taken from the electrodes of a tetrapolar bioimpedance sensor. Comparison with other electrode models used in the literature established that this is a good and challenging model for bioimpedance front-end amplifier evaluation. Among the examined amplifiers, one of the best performances is achieved with one of the proposed topologies based on a custom front-end with no external resistors (AD8066/AD8130). Under the specific testing conditions, it achieved an uncalibrated worst-case absolute measurement deviation of 4.4% magnitude and 4 degrees at 20 Hz, and 2.2% and 7 degrees at 1 MHz accordingly with loads between 10 Omega and 10 k Omega. Finally, the practical use of the front-end with the impedance analyzer is demonstrated in the characterization of the bioimpedance sensor, in saline solutions of varying conductivities (2.5-20 mS/cm) to obtain its cell constant. This article serves as a guide for evaluating and choosing front-end amplifiers for tetrapolar bioimpedance measurements both with and without impedance analyzers for practical/clinical applications and material/sensor characterization.

Automated summary

This article discusses the challenges with commercial benchtop impedance analyzers in accurately measuring tetrapolar bio-impedance due to large electrode contact impedances, proposing that external front-end amplifiers can help overcome this issue. Comparison is made between various discrete component-based topologies and new alternatives to evaluate their performance in measuring known loads in bio-impedance measurements. Ultimately, one of the best performing amplifiers is identified for use in practical applications and material/sensor characterization.