ISFET Digital Readout Circuit With An On-Chip MIPS Processor

In this paper, the design of an integrated CMOS pH sensor with an on-chip 8-bit MIPS processor is presented. The front end pH sensor is based on a voltage-clamped ISFET design whose output is converted to bitstream data. The average of the bistream is proportional to the carrier frequency over a given time period. An on-chip MIPS processor is used for processing the aforementioned bitstream data and further calibration. Simulations of the integrated sensor show 206 μW for the analog front end and 36 mW for the 8-bit MIPS processor and uses a total area of 0.666 mm
²
, with dimensions of 697.2 μm × 955.2 μm. The front end ISFET to bitstream was fabricated in a standard 0.5 μm CMOS technology. The sensitivity of the front end transducer circuit is approximately 50 mV/pH with a 0.05 pH resolution. The overall sensitivity is 440 Hz/pH with a linear pH range of 1-10.