Optical brain imaging using functional near infra-red spectroscopy
(fNIRS) offers a portable and noninvasive tool for monitoring of blood oxygenation.
In this paper we have introduced a new miniaturized photodetector front-end on achip
to be applied in a portable fNIRS system. It includes silicon avalanche photodiodes
(SiAPD), Transimpedance amplifier (TIA) front-end and Quench-Reset circuitry to
operate in both linear and Geiger modes. So it can be applied for both continuous-wave
fNIRS (CW-fNIRS) and also single-photon counting. Proposed SiAPD exhibits high-avalanche
gain (>100), low-breakdown voltage (<12 V) and high photon detection
efficiency accompanying with low dark count rates. The proposed TIA front-end offer
a low power consumption (<1 mW), high-transimpedance gain (up to 250 MV/A), tunable
bandwidth (1 kHz - 1 GHz) and very low input and output noise (~few fA/√Hz and few μV/√Hz).
The Geiger-mode photon counting front-end also exhibits a controllable hold-off
and rest time with an ultra fast quench-reset time (few ns). This integrated system
has been implemented using submicron (0.35 μm) standard CMOS technology.