Integrated Multipurpose Analog Front-End For Electrochemical Isfet Sensors

Ion-Sensitive-Field-Effect-Transistors (ISFETs) are an alternative to the common glass electrode for pH sensing. They promise higher integration, easier manufacturing and are interesting for biological, medical and environmental applications. However, they require a different and slightly more complex readout circuit. We propose a new circuit based on a standard mixed-signal-processor with minimal required external components. The operating point of the ISFET is set with a servo loop in software using only the integrated DACs and opamps and an external resistor. With the internal ADC, feedback is provided for the software loop and the result is read. We compare this method with a fully analog ISFET readout circuit and show their equivalence. The new circuit provides some additional features, such as run time configurable operating point and channel type settings. The integrated MCU can also perform calibration and sensor diagnosis functionality and eases the integration in other systems and parallel operation of many units with its digital interfaces.