Front-end of a non-invasive wireless real-time brain imaging system based on near-infrared spectroreflectometry

In this paper, we present a fully integrated front-end of a portable spectroreflectometry-based brain imaging system dedicated for acquisition of modulated optical signals at a frequency of 50 kHz. The proposed front-end preamplifier is composed of a photodetector, a transimpedance preamplifier, a two-stage voltage amplifier and a mixer. Strict constraints regarding noise thus have to be considered. The preamplifier consists of a transimpedance block featuring a 95 dBΩ gain and an average input current noise density at the frequency of interest of approximately 1 pA/√Hz . Each of the two subsequent voltage amplifiers allows the user to obtain an additional 25 dB gain, which makes it possible to obtain a considerable total gain. Considering the tuning capabilities and the losses due to the filters and the non-ideal buffers, the proposed front-end allows us to obtain a total gain up to 165 dB. The back-end of the amplification chain, is composed of a mixer which is used to produce a continuous voltage proportional to the amplitude of the input optical signals. All those features were integrated using CMOS 0.18 μm technology.