Electrode–Tissue Impedance Measurement CMOS ASIC for Functional Electrical Stimulation Neuroprostheses

The design of a complementary metal-oxide-semiconductor (CMOS) integrated circuit that has the capability of evaluating the electrode-tissue contact and the lead functionality of implanted electrodes in neuroprostheses for functional electrical stimulation applications is presented. The system allows in a quite simple way the verification of the state of the electrode by the evaluation of its impedance. By using a 1-bit analog-to-digital converter, the voltage drop between the anode and cathode of the stimulating electrode is measured when a controllable amplitude current is delivered. The impedance that is associated with the electrode will be obtained from the ratio between the anode-to-cathode voltage drop and the delivered current. Final resolution for the impedance measurement system will depend on the range of available current amplitudes. Based on the same impedance measurement method, two different systems for two different CMOS integrated neuroprostheses have been implemented. Both measurement systems allow an impedance characterization range from a few ohms to several kilo-ohms. This resistance range is enough to evaluate if the electrode is working inside normal conditions or not; however, some damage has taken place, making the electrical stimulation using that electrode impossible. In vitro results of the impedance measurement systems are provided.