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Accurately resolving small fluorescence power variations in presence of noise and high-background tissue autofluorescence from deep brain structures with a fiber photometry system requires highly linear and sensitive photo detectors. This paper presents a high-dynamic range (DR) CMOS biosensor fusing a low-noise photosensing front-end with a high-precision extended counting analog-to-digital converter (ADC) with noise cancellation to detect florescence neural signal fluctuations of very low incident power. The 7 MSBs are resolved by a first order continuous-time resettable $\Sigma \Delta $ ADC, whereas the residue voltage is quantized by a 10-bit single slope ADC for enabling wide-dynamic range and high-precision fluorescence sensing. The reset noise is canceled out by an embedded noise cancellation scheme which is subtracting the reset noise from the signal using a correlated double sampling scheme. Unlike other solutions, the biosensor has a short conversion time of $306.5~\mu \text{s}$ compared to a typical fluoresence sampling period of 10 ms, providing a very low duty cyle of 3%, which is a key to achieve low excitation source power consumption in this application to extend system autonomy, and to avoid photobleaching and phototoxicity in the tissue. The proposed optoelectronic biosensor is implemented in a 0.18- $\mu \text{m}$ CMOS technology, consuming $93~\mu \text{W}$ from a 3.3-V supply voltage while achieving a DR of 104 dB, a minimum detectable current of $1.3~\text {pA}_{\mathrm{ rms}}$ , and a chip area of 0.475 mm2. We present the measured performance of the biosensor using an optical experimental setup including a LED driver, a fiber optic, and a test board. [ABSTRACT FROM AUTHOR]