A composite ISFET readout circuit employing current feedback

This paper presents the concept of a current mode readout circuit for the ion-sensitive field effect transistor (ISFET) to be used as a pH sensor. An ISFET in conjunction with a current mirror with global feedback to the source terminal yields linearity between a wide dynamic pH range and output. Operating the ISFET and current source in the weak inversion region reduces major problems arising from channel-length modulation. Our experiment using discrete components demonstrates very high accuracy readout with total power consumption including the ISFET of early microwatts.