Five valuable functions of blocking capacitors in stimulators

Blocking capacitors, (known as coupling capacitors in some literatures), are extensively used in neural stimulators. Usually, a blocking capacitor is connected in series with a stimulation electrodes and the other end of the capacitor goes to the stimulation circuitry which supplies the current. Known for the “pass AC, block DC” characteristic, blocking capacitors are important for safety in chronically implanted stimulators. They have five functions: they help to correct charge imbalance; they prevent direct current passing under fault conditions; they limit maximum net charge and charge per phase; they provide larger electromotive force for discharging and therefore faster passive discharge; and finally they automatically adjust the resting potential of the electrode to accommodate more charge injection. For high-intensity stimulation, the blocking capacitors are large in volume, which means that designers would like to avoid their use for applications with many channels. Various approaches have been proposed for their elimination, but some of them come at the expense of reduced safety. The authors believe that the blocking capacitor should not be eliminated from the stimulator output stage design unless the alternative passes a stringent safety analysis.