Time domain CMOS ISFET systems for pH sensing

Abstract

In recent years, there has been a focus on integrating and miniaturising complementary metal-oxide-semiconductor (CMOS) electrochemical sensors, particularly ion-sensitive field-effect transistors (ISFETs), for applications in healthcare and environmental monitoring. However, CMOS-based ISFETs do not always show stable behaviour, and their interface circuits are not suited to supply voltage variation. This work aims to develop a stable platform for sub-pH detection by addressing these challenges. This work presents two front-end architectures for pH signal frequency conversion, suitable for low supply voltage and pH-sensing arrays. These circuits offer nonlinear and linear responses to pH changes, with a focus on the linear version. CMOS post-processing steps are used to further improve the sensitivity in the sensing area. The linear pulse frequency modulator (LPFM) architecture exhibits the best outcome in long-term response against pH variations and a highly stable signal with up to 25% variation in the power supply range. This could be a very significant feature for implantable and other wireless applications. Two data acquisition techniques are employed to increase long-term stability: the standard ‘Static Method’ and a ‘Dynamic Method’ using a pulsed reference signal. The Dynamic Method outperforms conventional techniques, addressing the common issue of the rapid pH signal decay in the time domain measurements evaluated in this work.