System for acquiring signals

The invention relates to a system for the acquisition of potentials deriving, e.g., from electroencephalographic or electrocardiographic signals and comprises a circuit for significantly reducing common mode interferences.

Title of the patent System for acquiring signals
Thematic area Biomedical
Ownership Alma Mater Studiorum - Università di Bologna
Inventors Marco Guermandi
Protection International application
Licensing status Available for licensing agreement
Keywords Medical electric equipment, common mode rejection, common mode voltage, electroencephalography, electrocardiography, active electrodes
Filed on Sep 04, 2014

One of the main issues in designing systems for the acquisition of bioelectrical signals is the common mode interference, in particular at the network frequency of 50 and 60 Hz. The common-mode signal at the input of couples of electrodes can be converted into a differential signal due to mismatches in contact impedances between electrode and skin and limitations in the amplification stage performances. If the common mode rejection ratio (CMRR) is not high enough, this interference can easily have amplitudes which are significantly higher than those of the biological signals to be detected.

The application field of the invention is that of medical electrical equipment. However, the invention can be applied to any kind of electrical measuring device or instrument which requires a high level of common mode rejection.  The present invention is based on the use of two electrodes which reduce the common mode interference which reaches the inputs of the acquisition system. The latter can essentially be a standard instrument for the acquisition of bio-potentials and the invention can easily be integrated as a sub-system of the instrument itself.

With respect to standard techniques for common mode interference reduction (e.g. DRL), the main advantage of the invention is the lack of stability concerns on the feedback loop which would limit the achievable common-mode suppression and rejection. The high level of interference reduction achievable (60 to 80 dB) allows to simplify amplification circuits, for which CMRR specifications are relaxed. Moreover signals from different electrodes can be single-ended amplified before computing their difference. This allows, e.g., to design active electrodes with a gain substantially higher than one without degrading the system’s CMRR.