Dichromatic spectral measurement circuit in vanilla CMOS

The circuit described in this paper uses a "verta-color" stacked two-diode structure to measure relative long and short wavelength spectral content. The p-type source-drain to nwell forms the top diode and the nwell-psubstrate diode forms the bottom diode. The circuit output is a digital PWM signal whose frequency encodes absolute intensity and whose duty cycle encodes the relative photodiode current. This signal is formed by a self-timed circuit that alternately discharges the top and bottom photodiodes. This circuit was fabricated in a standard 3M 2P 0.5 mum CMOS process. Monochromatic stimulation shows that the duty cycle varies between 50% and 7% as the photon wavelength is varied between 400 nm to 750 nm. The output frequency is 150 Hz at incident irradiance of 1.7 W/m². Chip-to-chip variation of PWM duty cycle and frequency is about 1% measured over 5 chips. Power consumption is 20 muW. A modified version of this circuit could form the basis for simple color vision sensors built in widely-available vanilla CMOS.