Method and apparatus for applying electrical stimulus pulses to a subject

Abstract

A method and apparatus for stimulating evoked potentials in a human subject maintains the common mode voltage at the stimulation electrodes substantially constant both before and during application of a stimulus pulse of constant current. A pair of resistors are connected between the stimulation electrodes and are joined at a reference node. One stimulation electrode (e.g., the anode) is connected through a field effect transistor to the power supply voltage and the other electrode (e.g., the cathode) is connected to the supply common return through a transistor which, when turned on, is controlled to conduct a constant current. The voltage at the reference node is supplied to the gate of a second field effect transistor which has its source clamped to a reference voltage. The voltage at the drain of the second field effect transistor is suppled back to the gate of the first field effect transistor. The two field effect transistors cooperate to supply a voltage to the anode electrode which is controlled such that the voltage at the reference node remains substantially constant before, during and after the switching transistor switches to provide a stimulation pulse. The common mode voltage at the anode and cathode is thus maintained substantially constant during stimulation pulses to minimize stimulus artifacts at the sensing electrodes.