An Analog Automatic Lock-In Amplifier for the Accurate Detection of Very Low Gas Concentrations

摘要

We propose here a new analog lock-in amplifier to be utilized in sensor interfaces for the detection of very low quantity of dangerous gases. When compared to other commercial systems in the literature, the proposed scheme shows an automatic operation, consisting in the self-alignment of the relative phase between input and reference signals. This functionality is continuously guaranteed, both at power-on and for any variation of the input noisy signal phase and amplitude during the working time. The proposed lock-in has been designed to work at a specified reference frequency (77 Hz), suitable for gas sensor applications and that avoids interferences with 50 Hz net frequency and its harmonics. The system has been tested using the carbon monoxide as gas to be revealed. With respect to the simple resistive gas sensor interface implemented by a resistive voltage divider, the improvement given by the proposed lock-in amplifier for the system sensitivity is of a factor of about 80, while the resolution, starting from about 5 ppm, has been enhanced to a theoretical value of about 0.05 ppm.