Biphasic stimulator circuit for a wide range of electrode-tissue impedance dedicated to cochlear implants

This paper presents an implementation of a least voltage drop neural biphasic stimulator circuit applied in cochlear implants. Using a double loop negative feedback topology, the output impedance of the current generator is increased, while requiring only one effective drain-source voltage drop (Veff). This allows the circuit to convey more charge into the tissue. The circuit can provide a biphasic stimulation scheme from a single ended supply with an amplitude range of 10µA up to 1.05mA for a wide range of electrode-tissue impedances, RL=1kΩ∼10kΩ, CL=1nF∼10nF. The stimulation current is set by scaling a reference current using a two stage binary-weighted transistor DAC configuration (3 bits HV transistor DAC and 4 bits LV transistor DAC) to improve the speed of stimulation pulses and minimize the area of the circuit. Simulation results, using the AMS 0.18µm high-voltage CMOS process, show that the charge error within a cycle (600µs) is only 0.02%, equivalent to a DC current error of 3nA at the maximum stimulation current with a load of 10kΩ+10nF.