Development of a System-on-a-Chip Electronic Nose

James Beeley, David R. S. Cumming, Jon Cooper, Andrew Glidle, Paul A. Hammond, Christopher A. Mills, Kevin Timmons, Lei Wang, Kathy Wyse.

Electronic olfaction has wide application in areas such as agriculture, food manufacturing, environmental monitoring and narcotic and explosive detection. A number of successful electronic olfaction instruments have been produced, including a system developed at the University of Glasgow. An electronic nose is formed from three main components. Firstly a sensor, or sensor array, whose electrical properties change in response to the presence of the analyte. Secondly interface electronics, which may be analog, digital or mixed-signal, measure the change in behaviour of the sensor in response to the analyte. Thirdly data logging and signal processing functionality is required to record the data, and determine the presence of specific gases or vapours from the data patterns obtained from the sensors.

An electronic nose system has been successfully developed at The University of Glasgow, which is based on an array of quartz crystal microbalance (QCM) or potentiometric sensor devices, a sensor array being necessary as each individual sensor responds to multiple analytes, along with discrete-component interface electronics. The QCM sensor is formed from a quartz crystal onto which an analyte-sensitive polymer is coated, adsorbtion of analyte onto the polymer causing an increase in mass and corresponding change in resonant frequency. The potentiometric sensors are similarly coated and exhibit a change in volume and resistance in response the analyte adsorbtion.

Current electronic noses are generally assembled from multiple discrete electronic components and sensors, and at very best integrate only a limited part of the system's functionality, such as a sensor array or portions of the interface electronics, onto a single chip. Hence a system-on-a-chip electronic nose offers advantages of significantly reduced power consumption and size in comparison to existing electronic olfaction systems. This is particularly desirable in battery-powered, portable or wearable sensor systems, or indeed remote equipment powered by renewable energy.

The University of Glasgow's System-on-a-Chip Electronic Nose project combines existing sensor technology with new integrated interface electronics.

The first phase of the project involved the development of an all-digital ASIC for measurement of QCM sensor resonant frequency. This provides an interface for a single 10 MHz QCM, with frequency measurement to a resolution of 1Hz and a precision of +/- 1 Hz, with a parallel data output. This chip has been successfully tested and employed in the detection of alcohol vapours.

The second project phase implements a complete sensor array interface and communication system on a single chip. It supports an array of 8 QCM sensors via frequency measurement circuitry as used in the first chip, along with analog measurement electronics and an analog-to-digital converter to support 8 potentiometric sensors. Measurement is controlled by an in-system programmable 6805 embedded processor, and an on-chip serial interface facilitates data transfer to a PC for logging and analysis. The SoC nose has successfully undergone testing, and is currently being used in alcohol vapour detection experiments. Further experimental work will focus on veterinary diagnostic applications.

Fig. 1 Quartz Crystal Microbalance Sensor Interface Chip		Fig. 2 System-on-a-Chip Electronic Nose
Fig. 3 System-on-a-Chip Electronic Nose Chip

PUBLICATION

All-Digital Interface ASIC for a QCM-Based Electronic Nose, James Beeley, Chris Mills, Andrew Glidle, Jon Cooper, David Cumming, Eurosensors XVII, University of Minho, Gumares, Portugal, September 21 - 24, 2003. [LINK THIS TO PDF]