A SAR based calibration scheme for ISFET sensing arrays

This paper presents an automatic calibration system for ISFET chemical sensing arrays to tune out any mismatch in sensitivity. Through exploitation of the high frequency spectrum of the sensed signal which does not contain any chemical reaction information, local sensitivity is examined and then calibrated through a successive approximation control mechanism and an 8-bit variable gain amplifier to provide high accuracy tuning. The total calibration process contains three phases, peak detection, AGC through a SAR and finally sensor readout of the detected chemical signal. Designed in a typical 0.35μm CMOS process, the system is capable of compensating a sensitivity deviation of up to 24% in an ISFET array, constraining the error to just 1.5%.