A Fully-Analog Lock-In Amplifier With Automatic Phase Alignment for Accurate Measurements of ppb Gas Concentrations

A novel fully-analog lock-in amplifier for the accurate measurements of low quantities of gas in sensor applications is here presented. When compared with commercial lock-in and other solutions available in the literature, the proposed system implements an automatic alignment of the relative phase between input and reference signals, both at power-on and for any variation of the input signal phase and amplitude during the working time, so allowing to detect, accurately and in a continuous way, the mean value of the input signal coming from sensors (typically buried into noise). The circuit has been optimized to operate at a specified reference frequency, in particular 77 Hz; this value is suitable for gas sensor applications, since it avoids also any kind of interferences at 50 Hz net frequency and its harmonics. Experimental results have confirmed the correct functionality of the system, also tested for both carbon monoxide
$(CO)$
and ethylene glycol detections. With respect to the simpler resistive gas sensor interface, implemented by a resistive voltage divider, the improvement experimentally given by the proposed automatic lock-in amplifier, in terms of resolution, has been of a factor of about 100 for the
$CO$
and 230 for the ethylene glycol measurements, so allowing a theoretical gas detection in the order of ppb.