A Wireless Fiber Photometry System Based on a High-Precision CMOS Biosensor With Embedded Continuous-Time ΣΔ Modulation

Fluorescence biophotometry measurements require wide dynamic range (DR) and high-sensitivity laboratory apparatus. Indeed, it is often very challenging to accurately resolve the small fluorescence variations in presence of noise and high-background tissue autofluorescence. There is a great need for smaller detectors combining high linearity, high sensitivity, and high-energy efficiency. This paper presents a new biophotometry sensor merging two individual building blocks, namely a low-noise sensing front-end and a 2nd order continuous-time Sigma Delta modulator (CTSDM), into a single module for enabling high-sensitivity and high energy-efficiency photo-sensing. In particular, a differential CMOS photodetector associated with a differential capacitive transimpedance amplifier-based sensing front-end is merged with an incremental 2nd order 1-bit CTSDM to achieve a large DR, low hardware complexity, and high-energy efficiency. The sensor lever-ages a hardware sharing strategy to simplify the implementation and reduce power consumption. The proposed CMOS biosensor is integrated within a miniature wireless head mountable prototype for enabling biophotometry with a single implantable fiber in the brain of live mice. The proposed biophotometry sensor is implemented in a 0.18-mu m CMOS technology, consuming 41 mu W from a 1.8-V supply voltage, while achieving a peak dynamic range of 86 dB over a 50-Hz input bandwidth, a sensitivity of 24 mV/nW, and a minimum detectable current of 2.46-pArms at a 20-kS/s sampling rate.