Abstract
A low-cost 1 kHz–400 kHz operating frequency fully-active electrode bioimpedance measurement module, based on Howland current source, is presented in this paper. It includes a buffered positive feedback Howland current source, implemented with operational amplifiers, as well as an AD8421 instrumentation amplifier, for the differential voltage measurements. Each active electrode module can be connected to others, assembling a wearable active electrode module array. From this array, 2 electrodes can be selected to be driven from a THS413 fully differential amplifier, activating a mirrored Howland current source. This work performs a complete circuit analysis, verified with MATLAB and SPICE simulations of the current source’s transconductance and output impedance over the frequency range between 1 kHz and 1 MHz. Resistors’ tolerances, possible mismatches, and the operational amplifiers’ non-idealities are considered in both the analysis and simulations. A comparison study between four selected operational amplifiers (ADA4622, OPA2210, AD8034, and AD8672) is additionally performed. The module is also hardware-implemented and tested in the lab for all four operational amplifiers and the transconductance is measured for load resistors of 150 Ω, 660 Ω, and 1200 Ω. Measurements showed that, using the AD8034 operational amplifier, the current source’s transconductance remains constant for frequencies up to 400 KHz for a 150 Ω load and 250 kHz for a 1200 Ω load, while lower performance is achieved with the other 3 operational amplifiers. Finally, transient simulations and measurements are performed at the AD8421 output for bipolar measurements on the 3 aforementioned load resistor values.
Highlights
The two selected Howland current pump (HCP) act as mirrored ones, while differential voltages from two selected channels are measured from the instrumentation amplifier
The THS413 is built on an individual printed circuit boards (PCBs) board, which is connected to the active electrode modules via equallength SMA cables
A bioimpedance measurement module, based on a fully-active electrode configuration, was presented in this paper. It consists of a HCP, an electrode connector, a 1st-order highpass filter, and an AD8421 instrumentation amplifier
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The implementation of HCPs is usually simple and can be done with commercially available active components (operational amplifiers-Opamps), while the implementation of differential-difference and current-mode VCCS topologies necessitates transistor-level fabrication The latter topologies may offer better performance in terms of the main VCCS evaluation characteristics (large and stable transconductance, large output impedance, large bandwidth, and low Total Harmonic Distortion-THD), the availability of discrete active components, makes the HCPs preferable for the implementation of prototype bioimpedance measurement circuits [14]. A fully-differential Opamp (which is not included on the modules) generates a differential voltage signal which is provided as an input to the VCCS of two selected modules In this way, the two selected HCPs act as mirrored ones, while differential voltages from two selected channels are measured from the instrumentation amplifier.
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