A low noise current readout architecture for fluorescence detection in living subjects

This paper introduces a readout circuit designed to interface with the fluorescence sensor presented in O´Sullivan et al., 2010., a monolithically integrated GaAs detector and vertical-cavity surface-emitting laser (VCSEL). The detector signals to be measured are low currents in the range of 5pAto15nA, with bandwidths up to 100 Hz. In order to capture binding dynamics of the fluorescent probe, as well as a wide range of possible tumor sizes and depths, the desired current resolution is 5pA. In applications with such low bandwidths and high sensitivity, a capacitive transimpedance amplifier (CTIA) can be used to provide high SNR through noise averaging (Ou and Chin, 203). The input current is integrated onto a capacitor C_int for a period of time T_int. The output of the CTIA is usually sampled and held at the end of T_int, before being digitized as a DC signal. The readout noise power introduced at the sampling instant, σ²_read, limits the maximum achievable SNR of this architecture. If, instead, M samples are taken along the integration ramp and line-fitting is performed, the white noise component of σ²_read is reduced by M/12, as shown in Fowler and Gatley, 1991.