An Embedded Auto-Calibrated Offset Current Compensation Technique for PPG/fNIRS System

Usually, the current generated by the photodiode proportional to the oxygenated blood in the photoplethysmography (PPG) and functional infrared spectroscopy (fNIRS) based recording systems is small as compared to the offset-current. The offset current is the combination of the dark current of the photodiode, the current due to ambient light, and the current due to the reflected light from fat and skull . The relatively large value of the offset current limits the amplification of the signal current and affects the overall performance of the PPG/fNIRS recording systems. In this paper, we present a mixed-signal auto-calibrated offset current compensation technique for PPG and fNIRS recording systems. The system auto-calibrates the offset current, compensates using a dual discrete loop technique, and amplifies the signal current. Thanks to the amplification, the system provides better sensitivity. A prototype of the system is built and tested for PPG signal recording. The prototype is developed for a 3.3 V single supply. The results show that the proposed system is able to effectively compensate for the offset current

Development and Validation of Offset Current Compensation Technique for Optical Sensors

Optical sensors are widely used in a variety of industrial, scientific and healthcare applications. The offset current due to ambient light affects the overall performance of such sensor system. This is even more critical in biomedical applications such as photoplethysmography (PPG) and functional-near infrared spectroscopy (fNIRS). The continuous offset cancellation technique introduce the delay and affects the shape morphing of such signals. In this paper, we present a mixed-signal based discrete offset cancellation technique for effective compensation of the offset current of the optical sensors for biomedical applications. The system is based on a feedback loop which calibrate the offset and compensate it at the start of recording. A prototype of the system is built and tested for PPG signal recording. The results shows that the proposed system is able to effectively compensate the offset current

Design analysis and experimental validation of relaxation oscillator-based circuit for R–C sensors

Relaxation oscillator-based circuits are widely used for interfacing various resistive and capacitive sensors. The electrical equivalent of most resistive and capacitive sensors is represented using a parallel combination of resistor and capacitor. The relaxation oscillator-based circuits are not suitable for parallel R–C sensors. In this paper, we propose a modified circuit for parallel R–C sensors. The proposed relaxation oscillator-based circuit is based on a dual-slope and charge transfer technique to measure the resistance and capacitance of parallel R–C sensors separately. In addition, the paper provides a detailed analysis and design considerations for the oscillator design by taking into account the various sources of non-idealities. A method to reduce the error by using single-cycle averaging is also introduced. To verify the analyzed design criteria, the circuit is tested with multiple operational amplifiers with different non-idealities. Experimental results verify the performance of the proposed circuit. The circuit is tested for a range from 10 pF to 42 pF and 100 kΩ to 1 MΩ for parallel R–C sensors with an error of less than 1.5%. The circuit is tested with a fabricated water-level sensor. The result confirms the efficacy of the proposed circuit