Affordable IoT-Enabled Acquisition System Targeting Bioimpedance Measurement

Many different sensors transduce the change of the measurand in terms of output impedance variation and diverse estimation techniques have been developed, each one offering pros and cons. In the recent past, the need for low cost, but accurate, solutions emerged under the pressure of IoT applications, that demand for large number of measuring points. Additionally constraints related to physical dimensions and power consumption also exist, further advising for effective approaches. Fortunately, in many application scenarios, the frequency of interest is limited, and microcontroller-based implementations can be employed. An example is the estimation of bioimpedance for the development of remotely-accessed Points of Care; in this case, the excitation frequency can be limited to the 0.1 to 1.0 kHz range. Often, static or quasi static impedance estimation is needed, thus admitting longer measurement time. The aim of this work is to discuss an impedance evaluation solution, designed around a microcontroller, aiming at minimizing the number of external components. The synchronous sampling technique is considered for trading off the number of samples per excitation signal period (i.e., only two) and the accuracy of the sampling instants. A prototype has been developed using an ATmega328P 8-bit microcontroller from Atmel, which features most of the required processing blocks, including a multi-channel analog to digital converter and an analog comparator for detecting input signal threshold crossings (required for synchronizing the sampling process). Indeed, only an external transimpedance amplifier is required for impedance current acquisition. Experiments carried out using a set of impedances, emulated by discrete resistors and capacitors connected in parallel, demonstrated a relative error (with respect to component nominal values) below 5% for both admittance magnitude estimation in a range of 0.1-1.0 mS and admittance phase estimation, in a range 0-90°.