A New Single-Chip Analog Lock-In Amplifier with Automatic Phase and Frequency Tuning for Physical/Chemical Noisy Phenomena Detection

A new fully-analog integrated lock-in amplifier for the accurate detection and measurement of noisy physical/chemical phenomena in sensor applications is here presented. The proposed system, implemented as a single-chip in a 0.35 mu m standard CMOS technology, performs an automatic alignment of the relative phase between input and reference signals as well as a self-tuning, at the reference operating frequency, both at power-on and for any variation during its working time, so allowing to reveal, accurately and in a continuous way, the mean value of weak signals (also buried in noise) coming from sensors. The circuit has been optimized to operate in a low frequency range, which has been set to [2.5Hz-25Hz]. Experimental results have confirmed the correct functionality of the system in the measurement of both noisy signal amplitudes as low as 60nV and reduced ethylene glycol concentrations. With respect to a simpler resistive voltage divider used as a basic gas sensor interface, the improvement experimentally given by the proposed automatic lock-in amplifier, in terms of resolution, allows a theoretical gas detection in the order of tens of ppb.