Neural Stimulation and Charge Balancing Approaches

Abstract

The stimulating electrodes are generally configured in monopolar and bipolar configurations. Common simulation modes are the current mode and voltage mode. The usual stimulation waveforms are either monophasic or biphasic. Charge imbalance occurs by semiconductor failure. Such imbalance may also arise from leakage currents. The main cause is cross talk between adjacent stimulating channels (sites) as well as cable failure. Positive charge balance is provided by a blocking capacitor connected in series with each electrode. This protective mechanism is used for electrical safety against fault conditions. The large capacitance value required for the blocking capacitors (sometimes a few microfarads) is realized through off-chip surface-mount components. In applications, e.g., retinal implants, the large-size blocking capacitors cannot be used. This inability is due to physical size limitations. Then, other methods for active charge balancing are resorted to. A stimulator circuit that is foolproof without off-chip blocking capacitors produces an active stimulation phase by high-frequency current switching. This phase is followed by a succeeding passive discharge phase.