A CMOS optical sensor array for tissue absorption and scattering parameter extraction using multi-distance near infrared spectroscopy

An optical sensor array has been integrated on a 2.25mm², 180nm CMOS chip for Frequency Domain Near Infrared Spectroscopy (fdNIRS). The individually addressable sensor array enables concurrent measurements as required in multi-distance fdNIRS. The changes in amplitude and phase at multiple distances from the source are measured to specifically calculate the absorption and scattering coefficients of the tissue. Each sensor in the 4-channel array consists of a 22THzΩ transimpedance amplifier to detect light modulated at 110MHz followed by a heterodyned architecture along with amplitude and phase detection circuitry based on pulse width modulation and zero-crossing detection. The sensor draws less than 40mA of current from a 1:8V power supply. An FPGA based counter is used to resolve time delays with a 20ns resolution corresponding to a 0.288° phase resolution. The 0 - 360° phase detector has a maximum measured linearity error of <;2.5%. The amplitude detector has a maximum measured linearity error of <;6.5% for modulation depths between 20% and 80%. Sensor performance is measured using a homogeneous tissue phantom to observe the change in phase and amplitude with maximum linearity error of <;60;8%.