CMOS chip with multi junction photo detector for sensing biomedical signals

Optical biomedical signals are of great interest for diagnostic purposes because of their non-invasiveness. Conventionally a single photo detector is used to acquire the optical signal of a single wavelength at a time. In this work we propose a sensor chip which employs a multi junction sensor with three vertically stacked photo diodes with different wavelength dependent responsivities. This enables simultaneous registration of multiple optical biosignals at different wavelengths, e.g., as required for the monitoring of blood oxygenation, a vital biomedical parameter. Thus energy consumption and time of the monitoring can be reduced. The detector and an on chip amplifier are realized in a standard CMOS process (0.35μm, triple well) without any process modifications. The on chip amplifier has a logarithmic transfer characteristic to cope with a high dynamic range of the optical input signals which leave the transilluminated skin region. The realization of this sensor chip is discussed and the resulting photoplethysmograms are demonstrated.