A CMOS-COMPATIBLE MONOLITHIC CONDUCTIVITY SENSOR WITH INTEGRATED ELECTRODES

A CMOS-compatible monolithic single chip system for the measurement of the electrolytic conductivity of human blood has been developed. The chip combines electrodes, current source, amplifier and digital circuitry on a single silicon substrate. A standard CMOS process has been used with only one additional step for the fabrication of the electrodes, which are made of a thin platinum layer. Due to the small size, the behaviour of the integrated microelectrodes differs strongly from their macroscopic counterparts. The polarization impedance of the electrodes, their noise performance and influences of the planar arrangement on the field distribution have been investigated. A four-electrode method using a square-wave current has been developed to avoid errors resulting from parasitic polarization voltages. The chip size is 1 x 5 mm and the measuring current is 10 muA. Measurements with a prototype version of the chip in KCl solution show a linearity of +/- 1 %. The system can be easily adapted to the desired conductivity range by changing the current or the amplification factor.