Differential read-out architecture for CMOS ISFET microsystems

The operation of a pH (Hþ ion concentration) sensitive ion-sensitive field-effect transistor (ISFET) microsystem with a sensitivity of 40– 45 mV=pH is demonstrated. This system has two identical ISFETs as the inputs to a pair of ISFET operational transconductance amplifiers (IOTAs) arranged in a novel differential architecture. The IOTAs have different sized p-MOSFET load transistors that enable pH sensitive operation without any post-fabrication processing or material deposition. The CMOS ISFET chip is fabricated in an unmodified 1.5 mm commercial process.