Electrical signals in plants are minute and surrounded by a lot of electrical noise. There was need therefore to design an instrumentation that detects, amplifies and filters the signal from the noise. This chapter reports on the design of an innovative instrumentation, starting with circuit design and implementation of the general purpose of the op-amp. The analysis of the op-amp schematic by simulation was done using LT spice software. Simulation results enabled the extraction of useful characteristics of the signal i.e. the frequency and the magnitude. The electrical signal obtained from the plants, was about 25 mV in magnitude and 1Hz in frequency. The signal was subsequently amplified to 200 mV and filtered for further analysis. This was made possible by incorporating a digital lock-in amplifier, connected to the developed amplifier kit (with the op amp). The extra-cellular recordings were performed by inserting Ag/AgCl microelectrodes, fabricated by this study, into the stem and leaf petiole parts of the spinach plant. The use of the lock-in amplifier made this work unique and also improved on sensitivity and noise filtration of the potential signal of the spinach plant.