Monolithic expandable IC based portable fNIRS system design

Abstract

A custom tranceiver IC is designed to implement a portable brain imaging system based on functional near-infrared-spectroscopy (fNIRS) and the way to select optimum wavelength pairs is analyzed based on wavelength dependent optical properties. The fNIRS IC generates multichannel time-divided spread-spectrum codes and drives light emitting devices in the transmitter (Tx) chain, and performs optimum filtering, quantization, and serialization in the receiver chain. A dual slope ADC subsequent to an operational transconductance amplifier-C (OTA-C) based matched filter shares a capacitor to save area while achieving optimum signal-to-noise ratio (SNR) in the brain channel. The receiver (Rx) chain including an off-chip TIA ensures sufficient electrical SNR irrespective of the brain region for the accurate extraction of hemodynamic response. The minimum detectable light power of the Rx chain is 400 fW. The output power of the Tx is made adjustable by controlling the occurrence rate and the power of the time-divided spread-spectrum code to reduce subject dependent measurement variations. The proposed fNIRS system uses 780nm and 850nm pair to extract hemodynamics from detected signals and the validity of used wavelength pair is proved by comparison of contrast-to-noise ratio (CNR) depends on the input SNR and hemodynamics changes. The proposed fNIRS system measures 3cm separated 48 brain regions simultaneously, and the experimental results clearly verify the validity of the proposed portable fNIRS system.