A Fast and Precise Digital Lock-In Amplifier for Accessible in-situ Fluorometry

The in-situ fluorometer is a ubiquitous tool for monitoring marine and freshwater environments, from detection of algal blooms to performing dye-tracing studies for Ocean Alkalinity Enhancement for Marine Carbon Dioxide Removal. Low-cost open-source instrumentation has been identified as a potential solution to both the demand for data and the need for greater accessibility. In this paper, we present a Digital Lock-In Amplifier designed for the Texas Instruments MSP430 platform, in the context of its central role in a new open-source in-situ fluorometer, the PIXIE. The amplifier is implemented digitally, using the mixed signal peripherals of the TI MSP430FG6626 microcontroller to minimize PCB complexity. DSP is performed using a Lattice Wave Digital Filter and a Q3.28 fixed-point arithmetic designed to maximize use of the MSP430's hardware multiplier for speed, precision, and numerical stability. The design and implementation of the lock-in amplifier, along with its performance metrics and example data collected by the PIXIE, are provided.