Low-Frequency Impedance Measurements for Biosensing Applications

In this paper, we present a cost-effective, compact, and versatile circuit designed for impedance measurements at ultra-low frequencies. The core of the circuit is a voltage-to-current converter with high output resistance, based on a Howland current pump derived from a highly reliable integrated circuit device. A precision current divider is cascaded to the current generator, and it is used to drive extremely low signals, allowing for high impedance measurements. A buffer output stage allows for the independent measures of load voltage and current. The circuit features selectable voltage-to-current conversion coefficient from 200 nA/V to 200 μA/V thanks to three different configurations enabled by two switches, ensuring modularity and versatility for different applications. More than characterizing the circuit through resistive and capacitive measurements, its performances have been validated through the measurement of the RC-network emulating a diamond biosensor designed for Electrochemical Impedance Spectroscopy, comparing the results with those reported in the literature. The proposed method shows excellent performances down to the mHz range, for resistances up to 100 MΩ, demonstrating the effectiveness in measuring signals from biosensors.