Functional Electrical Stimulation (FES) has been explored in order to restore the capabilities of the nervous system in patients that suffer from paralysis. This area of research and of clinical practice greatly benefits from any technological improvement yielding miniaturization. In this regard, we recently proposed and demonstrated an innovative electrical stimulation method based on implanted microstimulators that operate as rectifiers of bursts of innocuous high frequency current supplied by skin electrodes, generating low frequency currents that are capable of stimulating excitable tissues. We envision flexible ultrathin implants (diameters < 300
m) containing ASICs that have advanced capabilities, such as addressability and current control. As miniaturization is the main aim of this method, the use of bulky DC-blocking capacitors (e.g. 10
F) to accomplish zero net charge injection and avoid electrochemical tissue and electrode damage is highly inconvenient. As an alternative, here we present an active charge-balance method based on the use of a digital charge quantifier, whose operation is inspired in the functioning of the tipping bucket rain gauge. The system monitors the charge injection, matching the charge injected in the cathodal phase, with the charge injected in the anodal phase, generating a biphasic current waveform that adapts itself to possible current source mismatches. We have implemented a prototype built with discrete components which uses a capacitor of only 100 pF for the charge counter.