Automatic Common Mode Electrode-Amplifier Impedance Balance: Implementation and Results

Power-line interference (PLI) is the major disturbing factor in all ground-free biosignal acquisition applications. The PLI currents picked up by the body must flow through the sensing electrodes, then the electrode cables and finally via the amplifier input impedances they reach the amplifier ground. The electrode impedances and the amplifier input impedances form Wheatstone bridge. Because the electrode impedances vary with time, the bridge tends to be imbalanced and produces differential PLI which is amplified together with the useful signal. An automatic impedance balancing approach, using software PLL for line synchronization, was published. This paper validates the approach through practical results from its implementation.