A Simplified Two-Wired Biopotential Active Electrode Topology

Active electrodes include electronic devices allowing to present a very high impedance to the biopotential signal that is to be acquired, and at the same time a very low output impedance, thus achieving robust acquisition systems relative to electromagnetic interference. Their main drawback is that power must be fed to the electrode. The most usual solution employs 3 wires for this purpose: two providing the supply voltage, and the third one the output signal. Another alternative is the use of only 2 wires. In this latter case, the active electrode is fed through a current source and one of the wires has the double function of providing energy and transmitting the signal. In this work, a circuit which replaces the current source with a simple resistor and that is functional with a single supply is presented. The circuit is extremely simple requiring a reduced number of components, and one of its wires is ground-connected thus being suitable for use in shielding. The circuit analysis is presented, as well as its design equations and experimental results through electrocardiographic signal measurements.