Self-Calibrating Circuit for Implantable Current Stimulators

In this work, a self-calibrating circuit aimed at balancing biphasic current stimulators for Implantable Medical Devices (IMDs) is presented. Mismatched biphasic pulses cause a charge-unbalanced stimulation leading to tissue damage over time. The proposed module is an integrating, precision current comparator capable of detecting a 0.6% typical charge mismatch between the stimulating phases. The circuit's output can be used by a trimmable current stimulator to protect the tissue by reducing the injected net charge error. The total current consumption of the circuit is 1.5 $\upmu \mathrm{A}$ during a 430 $\upmu\mathrm{S}$ operating cycle. It was designed and fabricated in a 0.18 $\upmu\mathrm{m}$ CMOS-HV technology, occupying a total area of 0.30 mm². The circuit can also be adapted to measure Electrode-Tissue Interface (ETI) impedance.