A New CMOS-Integrated Analog Lock-In Amplifier for Automatic Measurement of Very Small Signals

摘要

In sensor applications, sometimes it is important to reveal and measure very low physical/chemical quantities, also with high accuracy and precision [1-3]. This can be achieved through the optimization/maximization of the measurement system sensitivity and resolution. In particular, in the case of small and noisy signals, coming from sensors, the lock-in technique, which is able to extract the signal from noise, can be taken into account. More in general, commercial lock-in amplifiers, as well as ad hoc solutions proposed in the literature also applied in sensor interfacing, are mainly of digital kind and, even if they show good performances and are particularly suitable for multifrequency operations, have high costs, weights, and sizes. On the contrary, in sensor applications, the analog kind of the signal to be revealed suggests, especially when the SNR is less than unity, the use of analog lock-in systems [4-9]. Unfortunately, both analog and digital traditional lock-in amplifiers have the disadvantage of knowing or fixing the operating frequency and requiring, at power on, the initial nulling of the output signal, which corresponds to the "in-quadrature" condition between input and reference signals (this should be also guaranteed continuously during the measurements). Then, the manual activation of switches provides a further 90° phase shift, allowing the reading of the output voltage, proportional to input mean value. In addition, if an operating frequency variation occurs, the system requires an additional calibration or, in worst cases, the redesign of internal blocks (e.g., band-pass filters).