Design of a versatile voltage based output stage for implantable neural stimulators

Neural stimulators have the potential of becoming very important devices for the treatment of a wide variety of diseases. One of the major problems with existing stimulators is the limited waveform adjustability. This precludes the use of sophisticated stimulation programs and thereby affects the efficacy of the therapy applied. For this reason a new type of stimulator is required. The physical principle underlying stimulation is based on elevating the tissue potential up to a particular level by injecting a particular amount of charge. Furthermore the injected charge needs to be canceled precisely in order to prevent tissue damage. Most existing stimulators use a current based architecture in which the charge is controlled by enabling the stimulator for a particular amount of time. Voltage based stimulation however yields a much higher power efficiency. A novel type of voltage based architecture using indirect current feedback of the tissue current is proposed. Using a current integrator with a very high dynamic range the injected charge can be controlled very precisely, while any arbitrary voltage waveform can be used for stimulation. Circuit simulations prove the feasibility of the approach and show a charge mismatch in the order of 0.1% paving the way to full charge balancing. Furthermore, they predict correct functionality over all process corners, including mismatch. The system only uses a single-ended supply and its quiescent power consumption is less than 15μW.