Analog VLSI circuits for adaptive chemical discrimination

We have designed and fabricated CMOS circuits that successfully discriminate between a variety of reducing chemicals in a manner resilient to concentration changes and other global variations in sensor response. The circuits described here use only six and seven transistors per chemical sensor to evaluate an array of sensory input based on the mean and median values of these inputs, respectively. Since these analog integrated circuits are constructed within a collective architecture, the mean and median values of the array of inputs are calculated in real-time, and each individual input is compared to the mean or median in parallel. The resulting output is a digital representation of the reducing chemical present in the sensing environment. When tested on a 15-element, heterogeneous array of tin oxide sensors, this hardware has proven sufficient to discriminate between five major reducing chemicals (acetone, butanol, ethanol, methanol and xylene).