Automatic common mode electrode-amplifier impedance balance with SPLL synchronization

Power-line interference (PLI) is a major disturbing factor in almost all groundfree biosignal acquisition applications. The main cause of interference is body to amplifier Wheatstone bridge imbalance. The bridge is formed from electrode impedances and the amplifier common mode input impedances. Because the electrode impedances vary with time, the Wheatstone bridge tends to be imbalanced, and produces differential PLI which is amplified together with the useful signal. The interference can be canceled only when the bridge is kept continuously in balanced conditions. It was shown how the Wheatstone bridge can be adjusted to maintain balanced conditions by means of two digital synchronous demodulators. For proper demodulation, an accurate synchronization with PLI is needed. Recently, dedicated software PLL (SPLL) for PLI synchronization was developed implemented and tested. This paper presents a complete impedance balancing approach wherein synchronization to PLI is done with the designed SPLL. The stability of the whole system is proven by Matlab mixed signal simulations. The presented approach is applicable in various two-electrode applications, such as Holter monitors, external defibrillators, ECG monitors and other heart beat sensing biomedical devices.