  Usually, the current generated by the photodiode proportional to the
oxygenated blood in the photoplethysmography (PPG) and functional infrared
spectroscopy (fNIRS) based recording systems is small as compared to the
offset-current. The offset current is the combination of the dark current of
the photodiode, the current due to ambient light, and the current due to the
reflected light from fat and skull . The relatively large value of the offset
current limits the amplification of the signal current and affects the overall
performance of the PPG/fNIRS recording systems. In this paper, we present a
mixed-signal auto-calibrated offset current compensation technique for PPG and
fNIRS recording systems. The system auto-calibrates the offset current,
compensates using a dual discrete loop technique, and amplifies the signal
current. Thanks to the amplification, the system provides better sensitivity. A
prototype of the system is built and tested for PPG signal recording. The
prototype is developed for a 3.3 V single supply. The results show that the
proposed system is able to effectively compensate for the offset current.
