Microcontroller-based system for multiple ISFET characterization using a single reference electrode

An instrument for multiple ISFET electrical characterization has been developed. The system allows, multiple and simultaneously active, ISFET characterization sharing a single reference electrode. This was possible by using multiple isolated power sources. Isolation amplifiers with embedded isolated power outputs were used instead. The instrument makes use of the constant current biasing mode to ensure ISFET's linearity at differential gate potential variations. An important feature of this system is the capability to determine the stabilization point from the characteristic of measured ISFETs. A stabilization point criterion is evaluated constantly during initial stages of individual operating conditions. Due to the rather low sampling rate, floating point and A/D conversion operations, a microcontroller device was decided to be used as optimal solution. The system uses a real time clock in order to keep a temporal relationship of sampled data. Preprogrammed sampling rate routines can be chosen for long or short evaluation periods. The instrument possesses stand alone features, so it could be used without an attached computer. Results showed linearity, low noise and reliable and easy operation. This measuring system represents a low cost effective solution in experimental chemical laboratories where ISFETs are being tested.