Lock-in amplifier techniques for low-frequency modulated sensor applications

A flexible design for a low-noise lock-in amplifier for low-level signal and low bandwidth sensors, such as infrared detectors, thermal sensors and strain gauges is presented. This paper attempts to bridge the gap between the use of a high performance commercially available lock-in amplifiers or the design of a dedicated high performance lock-in amplifier for a sensor application from scratch. The presented system is dedicated to low-level signal sensor applications, operating at modulation frequencies up to a few kHz. The input amplifier can be flexibly adapted to different input sensors and signals. A tunable low-pass filter allows easy control of the system bandwidth. Compensation techniques, such as DC filtering, phase compensation and differential signal adjustments have been implemented. The circuit can be placed near the sensor on a small printed circuit board and uses low-cost commercially available components. The input noise is typically.. nV/Hz and the bandwidth.. kHz at 80 dB overall gain.