The Design of CMOS Electrode-Tissue Impedance Measurement Circuit Using Differential Current Switch with CMFB Bias for Implantable Neuro-Modulation SoCs

A new electrode-tissue impedance measurement circuit integrated with a System-on-Chip (SoC) is proposed and designed for implantable medical devices to monitor the electrode-tissue contact status. In the proposed circuit, a differential step current is injected into the electrode-tissue structure to generate a voltage. A differential current switch with a common-mode feedback (CMFB) bias circuit and shaped control signals are designed to generate accurate step currents and stable common-mode bias voltage. Moreover, the measurement circuit shares part of the bio-signal acquisition circuit in the SoC. Thus only an extra power dissipation of 4.3μW is needed at the operating frequency of 15.625Hz. The proposed circuit is fabricated by 0.18 um CMOS technology. The measurement results have shown that the resolution is 17 Ωpp and the maximum error is 3.4% for an impedance of 46kΩ. The proposed circuit is also verified in an in-vitro test by measuring the electrode-tissue impedance of a subdural grid electrode sunk in the phosphate buffered saline (PBS) buffer.

I. Introduction

In the design of implantable neuro-modulation system-on-chips (SoC) like cochlear implant [1], closed-loop epilepsy control [2], deep brain stimulation (DBS) [3], etc., it is required to monitor the impedance of implanted electrodes to ensure the electrode-tissue interface is under the suitable condition for electrical stimulation or bio-signal measurement. The measured electrode impedance can be transmitted wirelessly out of the body for doctor’s clinical check. Generally, the electrode impedance measurement circuit for implantable SoCs requires low power dissipation, simple circuit structure, and high integration capability with the front-end bio-signal acquisition unit.