Design and Modeling of an Interrelated System Towards a Fully Optimized Electrochemical Impedance Spectroscopy

Abstract

Typically, electrochemical impedance spectroscopy (EIS) is operated by applying a stimulus voltage to an electrochemical cell. Then, the sensed signal is analyzed by an analog front-end (AFE) chain, followed by an analog-to-digital converter (ADC) for impedance characterization. Therefore, a bulky analog filter is required after the excitation mechanism to achieve a high-resolution AC stimulus voltage. This paper proposes an interrelated analog/digital EIS system to eliminate the need for this bulky analog filter. Such a filter is replaced by the already existing ADC digital filter. A direct digital synthesizer (DDS) is implemented to generate the AC stimulus voltage over a wide frequency band, from 10 Hz to 100 kHz. In addition, an ultra low- noise AFE is introduced in two modes of operation to relax the ADC design. These modes assure an optimized integrated system with the former DDS, which has a total harmonic distortion (THD) of less than 0.02 %. The AFE achieves an input-referred current noise of 8.3 fA/ √Hz, while its resolution reaches 0.48 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$p$</tex>A over a bandwidth of 2.5 kHz. The attained resolution complies with the sensitivity requirements of EIS applications. Finally, the entire EIS system is validated using a combination of MATLAB modeling and spice simulations in 180nm CMOS technology.