A1.3 - Electrical Impedance Spectroscopy Combined with Temperature Cycled Operation for Semiconductor Gas Sensors

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A1 - Impedance Spectroscopy
A. Schütze, G. Reimann, S. Darsch, M. Schüler, T. Gillen - Saarland Universität, Saarbrücken (Germany)
40 - 45


Applications like fuel cells, batteries or food analysis are typical fields of interest for electrical impedance spectroscopy (EIS). Another field with promising results are EIS measurements in semiconductor (sc) gas sensor applications. Due to their high sensitivity and low cost, sc gas sensors are ideally suited for different applications, especially in the field of safety and security. The major drawbacks of sc gas sensors, i.e. poor selectivity and sensor baseline drift, can be overcome by using temperature cycled operation (TCO) or EIS, both in combination with intelligent signal processing. Currently, EIS for sc gas sensors is only possible in laboratory environments using commercial impedance analyzers, which are characterized by high cost, poor mobility and long acquisition time. However, for efficient development of application specific sc gas sensor systems and/or a self-monitoring strategy extensive field tests are necessary. Therefore, a new hardware concept for EIS of sc gas sensors based on FPGA-technology was developed. This concept also allows a combination of EIS with TCO for the first time. Additionally, state-of-the-art feature extraction methods are applied to sc gas sensor data and new features were tested.
By stimulating the gas sensor with a high frequency signal and recording both stimulation and response in the time domain the impedance spectrum can be calculated from the division of the Fourier transforms (FT) of both signals. For sensor stimulation a maximum length sequence (MLS) is used, which is based on digital signals. The hardware of the EIS system is combined with a heater control for temperature cycled operation. MLS signal generation is performed by an FPGA combined with a separate circuit for signal shaping and adjustment of the MLS amplitude. Data acquisition in the time domain is achieved with a high speed ADC and subsequently transformed to the frequency domain by the FPGA, e.g. by implementing algorithms like FFT. In first tests signal generation was realized using a signal generator and data acquisition using a digital oscilloscope. The overall setup allows much faster data acquisition (approx. 1 sec for the complete spectrum) than the commercial impedance analyzer previously used, allowing combined TCO/EIS operation. In the TCO/EIS mode the sensor is heated with a defined temperature cycle and the impedance is recorded once per second. First chcracterization results will be presented.