50nW Opamp-Less ΔΣ-Modulated Bioimpedance Spectrum Analyzer for Electrochemical Brain Interfacing

A fully integrated 130-nm CMOS 12-channel biofouling-resistant potassium-selective brain neurochemistry impedance spectrum analyzer is presented. Each 0.004 mm ² channel is an amperometric readout circuit composed of an opamp-less delta-sigma-modulated analog-to-digital converter (ADC) that consumes only 50 nW of power from a 0.6-V supply. The latter is analytically proven to be approximately equivalent to the former under conditions that are typical for the biochemical microsensors. The circuit also includes a low-power multiplierless frequency response analysis (FRA) unit that performs bioimpedance extraction. The channel achieves 1 pA current sensitivity and attains 50.3-dB signal-to-noise-and-distortion ratio (SNDR). The impedance analyzer achieves a dynamic range of 1 pA to 20 nA and a 5-kHz frequency scan range. The 2 mm $\times \,\,1$ mm die has been validated in vivo in real-time multisite potassium sensing in the rodent brain using gold microelectordes implemented on a polyimide substrate.