A Low-phase-error FD-fNIRS Readout Circuit with Sub-1V Transimpedance Amplifier and LC-ADC-based Amplitude Control Loop

Frequency-domain functional near-infrared spectroscopy (FD-fNIRS) ultilizes the propagation time and amplitude attenuation of light in tissue to quantify blood oxygen concentration. This enables more detailed monitoring of human brain activity, providing a potential solution for non-invasive and wearable brain-computer interface (BCI) devices. However, the amplitude-phase coupling effect of the comparator in the phase detector introduces phase errors, which in turn leads to inaccuracies in the measurement of light propagation time. To suppress amplitude-phase coupling effect, an amplitude control loop based on the level-crossing analog-to-digital converter (LC ADC) is employed. The utilization of the LC ADC eliminates the need for a high-speed envelope detector, thereby reducing circuit complexity and power consumption. In addition, a sub-1V transimpedance amplifier (TIA) is employed to improve energy efficiency. The proposed structure achieves a phase error of 0.11° and 12 mW power consumption.