A CMOS vector lock-in amplifier for sensor applications

The design of an integrated lock-in amplifier is discussed, specifically conceived for the detection of low-level signals at a harmonic of the drive frequency in magnetically excited resonant structures. The circuit includes in-phase and quadrature analogue signal processing channels, whose outputs feed an integrated ΣΔΣΔ analogue to digital converter. The circuit can be operated in different configurations, depending on the application requirements: in particular, by combining the digitized outputs of the two channels, vector operation can be obtained. The entire analogue chain, including the ΣΔΣΔ modulator, was designed using fully differential elaboration. The circuit was developed in a 0.35μm, dual poly-Si, four metal layers analogue CMOS technology with high resistivity poly-Si option. Circuit performance is discussed on the basis of transistor-level simulations and measurement results.