CIRCUIT FOR MEASURING SKIN-ELECTRODE CONTACT IMPEDANCE

The invention relates to a process and a circuit for measuring the contact impedance between skin and an active electrode of non-invasive measurement of bio-signals which allows the impedance measurement by using the same equipment used for measuring the sampled bio-signals. According to the invention, the measuring circuit comprises an active electrode () consisting of three pins galvanically connected to one another and an auxiliary electrode () also consisting of three pins galvanically connected to one another, the pins being circularly arranged so as the pins of the active electrode () and of the auxiliary electrode () to be shifted from one another, five electronic switches () controlled by a processing unit () consisting of a microprocessor of low complexity provided with a measuring algorithm, an instrumental amplifier () which amplifies and conditions the bio-signals sampled by means of the electrodes (), wherein, in the bio-signals measuring mode, the switches () are positioned so that the signal at the output of the instrumental amplifier () is applied to the differential input of an amplifier () with programmable gain, and in the contact impedance measuring mode, the switches () are positioned so that, at the differential inputs of the amplifier () with programmable gain, the potentials sampled from the active electrode () and the auxiliary electrode (), respectively, are applied, two constant-current sources () of equal value and opposite sense being connected to the active electrode () and to the auxiliary electrode () respectively, the signal at the output of the amplifier () with programmable gain being proportional to the active electrode-skin contact impedance, in both cases the values at the output of the amplifier () with programmable gain being converted into numerical values by means of an analogue-digital converter () and transferred to the processing unit () to be processed.