A charge-redistribution based controller for keeping charge balance in neural stimulation

In this article, we present a circuit solution for sensing and controlling functional electrical stimulation (FES) currents. The objective is to keep both charge balance and residual electrode voltage under a safe limit throughout stimulation. In the proposed circuit, the stimulation current is set through an ultra-low-power charge-redistribution digital-to-analog converter (CR-DAC). Each stimulation pulse is copied by high-ratio current mirror into switched capacitors which integrate the residual direct current (DC) resulting from FES imbalances. As residual electrode voltage may buildup even for perfectly balanced current pulses, alternated cathodic-first and anodic-first stimulation is adopted to complement the charge-balancing mechanism. Simulation results suggest that the circuit is able to keep both residual charge and electrode voltage under safe limits, smaller than 0.05% and 64 mV respectively.