Light-to-frequency converter using integrating mode photodiodes

An optical sensor for operation in the visible spectrum with integrated electronic readout circuits has been realized in a BIFET process. The output signal is a pulse series with a frequency proportional to the intensity of the incident light. Furthermore, the duty-cycle of the output pulses depends on the spectral distribution of the incident light, enabling the measurement of color. The electronic circuits have been designed for operating with a large dynamic range, while using only a relatively small chip area. No internal or external capacitor is needed for the current-to-frequency conversion, since the photodiode operates in the charge integrating mode. This enables fabrication of arrays of smart image detectors where a large area can be used for the photodetector.