A readout circuit realizing electrochemical impedance spectroscopy for FET-based biosensors

Electrochemical impedance spectroscopy (EIS) is a useful approach for modeling the equivalent circuit of biosensors such as field-effect transistor (FET)-based biosensors. During the process of sensor development, laboratory potentiostats are mainly used to realize the EIS. However, those devices are normally not applicable for real use-cases outside the laboratory, so miniaturized and optimized instrumentations are needed. Various integrated circuits (IC) are available that provide EIS, but these make developed systems highly dependent on semiconductor manufacturers, including component availability. In addition, these generally do not meet the instrumentation requirements for FET-based biosensors, thus external circuitry is necessary as well. In this work, an instrumentation is presented that performs EIS between 10 Hz and 100 kHz for FET-based biosensors. The instrumentation includes the generation of the excitation signal, the configuration of the semiconductor and the readout circuit. The readout circuit consists of a transimpedance amplifier with automatic gain adjustment, filter stages, a magnitude and a phase detection circuit. Since magnitude and phase are converted to a DC signal, digitization of the results is trivial without further signal processing steps, minimizing the computational load on the microcontroller. The transmission behavior of the magnitude and phase measurement circuits shows a high linearity for sinusoidal signals. Furthermore, the overall system was tested with resistors, whereby the magnitude measurement error (1.7%) and the phase shift error (1.6 degrees) were determined within the working range of the instrumentation. The functionality of the instrumentation is demonstrated using pH-sensitive field-effect transistors (ISFET) in various solutions.