A Sub-0.01° Phase Resolution 6.8-mW fNIRS Readout Circuit Employing a Mixer-First Frequency-Domain Architecture

This paper presents a frequency-domain functional near-infrared spectroscopy readout circuit for absolute value measurement of tissue optical characteristics. A mixer-first analog front-end (AFE) structure and a 1-bit $\Sigma-\Delta$ phase-to-digital converter (PDC) are proposed to lower both the required circuit bandwidth and the laser modulation frequency to save the power while maintain high resolution. The IC achieves sub-0.01° phase resolution and 6.8-mW power consumption. 9 optical solid phantoms are produced to evaluate the chip. Compared to a self-built high-precision measurement platform which combines a network analyzer with an APD module, the maximum measuring errors of absorption coefficient and reduced scattering coefficient are 14.2% and 16.5%, respectively.