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 circui try on a single silicon substrate. A standard CMOS process has been us ed 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 thei r macroscopic counterparts. The polarization impedance of the electrod es, their noise performance and influences of the planar arrangement o n the field distribution have been investigated. A four-electrode meth od using a square-wave current has been developed to avoid errors resu lting from parasitic polarization voltages. The chip size is 1 x 5 mm and the measuring current is 10 muA. Measurements with a prototype ver sion of the chip in KCl solution show a linearity of +/- 1 %. The syst em can be easily adapted to the desired conductivity range by changing the current or the amplification factor.