An auto-offset-removal circuit for chemical sensing based on the PG-ISFET

This paper presents a novel readout circuit for a pH sensitive programmable-gate ion-sensitive field effect transistor (PG-ISFET) to overcome bias issues due to threshold voltage variation and increase output-referred sensitivity. Compared to other commonly-used ISFET readouts, this circuit uses two extra programmable nodes which are driven by a feedback configuration. Using the device in a source follower configuration, one node is used to evaluate and cancel the offset of the intrinsic device while the other tracks and amplifies changes in pH. A sample and hold protocol has been developed to minimize the leakage effects and improve the pH sensing range. The system has been designed and fabricated in AMS 0.35 mum, to compensate for a threshold voltage variation of plusmn10.5 V and provide a pH sensitivity of 200 mV/pH.