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A monolithic pH sensor system has been studied and developed, based on standard CMOS technology. The micro system includes an on-chip integration of differential ISFET/REFET sensing devices, metal constructed pseudo reference electrode (PRE) and front-end measurement electronics. A post CMOS process flow is devised in our laboratory to allow the ISFET to be fabricated in a standard CMOS foundry with minimal modifications. The chip has been realized with Chartered Semiconductor's 0.35μm, 4-metal and 2-poly layer CMOS process and operates at 3.3 V. The total die area is 5mm2. Thanks to the on-chip signal conditioning; the micro sensor system achieves a superior sensitivity of 53.67mV/pH as well as an enhanced linearity. Hence, it demonstrates the possibility of embedding ISFET based micro sensor arrays in advanced digital, analog and mixed signal VLSI for the intelligent measurement required in various chemical, biochemical and biomedical applications.