An Automatic Gain Control System for ISFET Array Compensation

This paper presents an automatic gain calibration system for ion-sensitive field effect transistors (ISFETs) sensing arrays which compensates for mismatch in the gain of the pixels. The system utilizes the high frequency spectrum of the sensed signal to superimpose a reference sine wave through the reference electrode, which allows identification and calibration of the local gain. The complete system has been fabricated in a 0.35 μm CMOS process and has been designed to achieve very high accuracy and a small settling time to allow fast pixel switching. Measured results confirm good performance with only a 1.15% deviation in gain caused by noise and harmonic distortion and a rapid settling time of 70.5 μs which is suitable for CMOS-based ISFET arrays.