Analog automatic lock-in amplifier for very low gas concentration detection

Abstract

A novel automatic analog lock-in amplifier for low quantity detection of gas is here presented. When compared to similar commercial apparatus in the literature, the proposed scheme shows the self-alignment of the relative phase between input and reference signals, both at power-on and for any variation of the input noisy signal phase and amplitude during the working time, so allowing to detect, in a continuous way, the mean value of the input signal (buried into noise). The circuit has been optimized to operate at a specified reference frequency (77 Hz), whose value is suitable for gas sensor application, avoiding any kind of interferences at 50 Hz net frequency and its harmonics. Experimental results have confirmed the correct functionality of the system, tested for 10, 20 and 30 ppm of CO concentrations. With respect to the simple resistive gas sensor interface implemented by a resistive voltage divider, the sensitivity improvement given by the proposed lock-in amplifier is of a factor of about 80, while the resolution, starting from about 5ppm, has been enhanced to a theoretical value of about 0.05ppm.