Design of a low power 100 dB dynamic range integrator for an implantable neural stimulator

Neural stimulators are expected to play an important role in the future treatment of a wide range of pathologies. A novel system architecture was presented in which the fundamental quantity for functional electrical stimulation, charge, is controlled by measuring the stimulation current. This sets the need for a current integrator to calculate the injected charge. Existing current integrators cannot cope with the specifications for the neural stimulator, including a very high dynamic range, low power consumption and robust enough against process and power supply variations. Therefore a current integrator design is proposed here, which is able to handle a large dynamic range by converting the output to a periodic signal. For this purpose a novel Schmitt trigger design is presented based on a threshold compensated inverter. The implementation shown here has a dynamic range of 100 dB, while achieving a low static power consumption (171 nW). This makes it suitable for application in an implantable neural stimulator.