A novel voltage-clamped CMOS ISFET sensor interface

This paper presents a novel interface for ion-sensitive field effect transistors (ISFET) in which operation at a fixed electrical bias is achieved by voltage clamping. The chosen topology provides pH-dependent current and voltage output signals to drive an appropriate output stage. The circuit can be operated in either strong or weak inversion, depending on the requirements of the application with a pseudo-differential ISFET-REFET topology to allow use of an on-chip reference electrode. Simulation results are shown herein for single-ended and differential implementations. For a single-ended front-end with 1nA bias current, the strong inversion implementation at 2.5V supply has a power consumption of 0.185mW at pH 7 and the weak inversion implementation at 1.5V supply has a power consumption of 13nW at pH 7. The circuit and ISFET-REFET pair have been fabricated in the UMC 0.25mum CMOS technology.