Surface Acoustic Wave (saw) Gas Sensor Array Signal Acquisition Circuit Design

With the development of industry, toxic and polluting gases are disposed randomly which poses a great threat to human health, so many countries are stepping up their research quickly and accurately in gas sensors that can detect such gases. Compared to traditional semiconductor gas sensors, surface acoustic wave (SAW) gas sensors have the advantages of high sensitivity, small size, and low power consumption, and it becomes one of the hotspots of the domestic and foreign researchers in recent years.When SAW gas sensor is working, the dielectric properties which the surface acoustic wave transmits on change if the sensitive material coated on the sensor surface adsorb gases. Then it makes the resonant frequency of the SAW sensor change corresponding to the gas concentration, and if we take a detection of this change frequency we can know the relative gas concentration, then the real gas concentration can be know by a conversion. Throughout the process, the electromagnetic compatibility of the detection circuit is directly related to the accuracy of the sensor which plays a decisive role. Therefore, a high-performance detection circuit is very important in application of SAW gas sensor.By the frequency detection method and based on the working principle of feedback oscillator, we designed an oscillation which have a RF amplifier, SAW device and matching circuit elements by detecting and adjusting the phase and gain of the open-loop. Then the final detection circuit implementation was decided by selecting appropriate integrated mixer and comparator and designing low-pass filter. First, we designed a2layer circuit board which has a reference channel and a test channel. In the stability test, this circuit’s frequency fluctuation is less than20Hz within30minutes and the standard deviation is about3.92. This circuit has a high medium-term stability. In gas detection, the sensitive material was coated on the SAW sensor, and the detection circuit showed excellent performance.On the basis of two-layer circuit board, we designed a SAW gas sensor array circuit which has three test channels and three reference channels. In the stability test, the stability of the array circuit is slightly worse than the single test channel circuit, but it absolutely can be used to detect more then one gas at the same time. And it makes a foundation to enhance the sensor’s selectivity, reduce the false positive rate and improve the sensor’s sensitivity.