Design of a CMOS Current-to-Frequency Converter for Optical pH Sensing in a culture medium: towards integration in organ-on-a-chip device

In organ-on-a-chip systems, accurate pH sensing is essential for an effective physiological control of the cells' culture media. In this work, there was developed a current-to-frequency (IF) converter integrated with a CMOS photodiode for pH quantification of cells' culture medium. The selection of the optimal features for the converter configuration, such as input current and resolution, was based on initial culture medium optical transmittance measurements, using a 180 nm technology CMOS photodiode. The core of the IF converter consists of two MOSFETs, one operating as capacitor (M2) and other for discharging (M1). Using the Cadence Tools software, the IF converter was simulated for optimizing their characteristics, such as sensitivity, linearity, output frequency range, resolution, power consumption, and temperature effects. It was concluded that the converter's sensitivity and resolution decreased, while its linearity improved with the increase of M2 width. Additionally, the converter input range increased with higher M1 width. Power consumption and temperature effects did not exhibit notable differences when changing the MOSFETs sizes. This study enabled the design of a CMOS IF converter perfectly suitable for detecting and distinguishing pH levels in a culture medium, paving the way for future integration of these systems into OoC devices.