A piecewise linear approximating ISFET readout

Abstract

This paper presents a novel readout circuit for ionic concentration change measurement overcoming the non-linear effects of the Ion-sensitive Field-effect Transistor. The readout periodically resets the ISFET biasing and integrates its signal change around a certain operating point utilising a switched current integrator. This enables a linear approximation of the signal change over time which suppresses non-linear effects such as the sensing membrane buffer capacity change, and transistor slope factor variation in weak inversion. We show how the exponential relation of the current with pH change in weak inversion may be linearised, which allows further current mode processing and proton counting for DNA sequencing applications.