A multiple‐ISFET integrated with CMOS interface circuits

To achieve the simultaneous measurements of ion concentrations with high accuracy using multiple ISFETs, the integration of multiple ISFETs with signal processing circuits has been attempted. To achieve process matching in the fabrication of the ISFETs and additional circuits, and to lower the power consumption, CMOS devices have been used for the signal circuits. The integrated circuits were composed of four ISFETs, a constant current source, a buffer amplifier, and a multiplexer. To measure ion concentration at high speed with a high degree of accuracy, each ISFET contained a source follower and a buffer amplifier to achieve an independent operation. Only the output signal was switched by a multiplexer. The average sensitivity of each ISFET was 54.4 mV/pH for the range of pH 1 to 10. The response time for an increase of the pH by 1 was about 1 s. The time needed to switch the output signal was 200 μs, which is three orders of magnitude faster than ion response speed. As a result, it was found that this integrated sensor structure was suitable for measuring the response of multiple ISFETs at high speed. This paper describes the sensor structure, the circuit design and its response characteristics.