System identification for characterization of the frequency response of an “open-source” biomedical circuit

In this paper, system identification techniques are applied to accurately characterize the frequency behavior of a commercially available but with unknown characteristics of a photoplethysmography (under the commercial name of Pulse Sensor). Although claimed to be open-source, the electronic schematic of this device does not allow for a full characterization of it. Therefore a second order auto-regressive with exogenous input model has been employed to find the frequency response of this system. An optical excitation circuit was developed to send a pseudo-random signal to the Pulse Sensor. Results show that the model is able to reproduce the output with a fitness of 89.7 %. The obtained model is validated by using the inverse of the obtained transfer function to reconstruct the input. Accurate determination of the frequency response of the Pulse Sensor allows for the reconstruction of the real input. The proposed approach can also be applied to teach engineering students about reverse engineering and system identification.