Specific Lock-In Amplifier for Fluxgate Sensor Testing

Characterizing the noise and long-term offset stability of a fluxgate sensor are two of many important tasks during its development and manufacturing. For this purpose, it is advantageous to use synchronous demodulation realized by a lock-in amplifier. Commercial lock-in amplifiers are very complex, expensive, bulky, and frequently generate unpleasant acoustic noise. Additionally, these instruments are often heavily utilized in the lab. We, thus, present the development and testing of a specific instrument tailored for fluxgate sensor measurements. It combines a low-noise data acquisition front-end module with an excitation unit featuring variable parameters (frequency, H-bridge voltage) and a precise feedback current source module for possible closed-loop (CL) operation. With this configuration, it forms a single-axis digital magnetometer with up to 26-bit resolution. Furthermore, a Cyclone V FPGA development kit controls all the modules and performs the necessary signal processing. The presented lock-in amplifier uses only internal signal reference coupled with excitation unit signals—a solution that greatly simplifies the design. The user interface is based on LabVIEW software and provides real-time signal and spectrum charts. The results of the instrument’s characterization as well as the testing of a race-track fluxgate sensor are presented herein.