A direct-capacitive feedback ISFET interface for pH reaction monitoring

In this paper we propose a low-power, compact ISFET interface capable of overcoming problems of DC offset due to trapped charge and transcoductance reduction due to capacitive division, which commonly exist with implementation in CMOS. Through direct feedback to the floating gate and a low-leakage switching scheme, all the unwanted factors are eliminated while the output is capable of tracking a pH reaction which occurs at the sensing surface. The circuit has been designed to be an inherently constant-voltage-constant-current structure, avoiding any loss of sensitivity due to capacitive division. Additionally, a tradeoff between noise and stability is presented for optimum choice of capacitors. The proposed ISFET and interface has been designed in a typical 0.35 μm process, has an area 60 ×70um² and consumes 231nW of power, which facilitates it´s implementation for large chemical sensing arrays.