High sensitivity, high resolution, uncalibrated phase read-out circuit for optoelectronic detection of chemical substances

In this paper, we present an uncalibrated phase read-out circuit for the detection of chemical substances through optoelectronic devices such as photodiodes and image sensors. The system, based on an automatic analogue circuit operating a phase-to-voltage conversion, shows the following main characteristics: high sensitivity and resolution, high accuracy and precision, low noise, reduced circuitry complexity (minimum required silicon area for the integration on chip), good linearity, capability of phase sign detection, independence from input signal amplitudes and waveforms, setting and zeroing of the output voltage offset and autocalibration. For these reasons, since several sensor applications need an array management performed by a single chip, the developed read-out analogue circuit can be easily integrated on a single chip together with optoelectronic devices, so it can be considered as a suitable solution for signal detection portable systems. Experimental measurements, conducted through the fabricated prototype PCB and employing a single commercial photodiode as light power sensor, have shown a high circuit sensitivity and resolution, in the three considered phase full-scales equal to ±10°, ±1° and ±0.1°, confirming the very good performances of the proposed system to reveal also very small phase shifts. The complete measurement system has been utilised to detect and quantify variations of the molar concentration of methylene blue diluted in distilled water by employing a suitable optical set-up that allowed to achieve resolutions lower than 44 pM.