A CMOS turbidity to frequency converter with calibration circuits for detecting turbidity applications

In this paper, a CMOS turbidity to frequency converter with calibration circuits for detecting turbidity applications is newly proposed. A calibration technique is proposed in this work. Another innovation is that the outputs of the proposed chip are directly digitized, they could be easily sent over a wide range of transmission media, such as PSN, radio, optical, IR, ultrasonic, and etc. Before performing the proposed calibration circuits, the maximum linear error is 25.82%. After calibration, the maximum linear error is reduced to 0.89%. The turbidity ranges from 1.5 to 992 Nephelometric Turbidity Units (NTU), and its corresponding output frequency range is 24.875 to 18.214 kHz. The chip area is 1.01 × 1.01 mm². The proposed chip is suitable for devices of measuring water turbidity.