A Multi-Channel Interface Circuit with Low Substrate Noise for Surface Acoustic Wave Sensor Array

Electronic nose (E-Nose) is one of the applications for surface acoustic wave (SAW) sensors. In general, the SAW sensor, as a gas sensor, consists of a passive element (LiNbO3 and IDTs) and an oscillator circuit, as shown in Fig. 1. For the E-Nose applications, a SAW sensor array is formed to construct the odor patterns for gas recognition. Because the oscillator frequency of each sensor is so close that the injection current through the substrate seriously affect the performance of the phase noise of the SAW sensors [1] . If the number of the SAW sensors increases, more injection current would raise the output noise level of the SAW sensors, as shown in Fig. 2. The noise will affect the sensitivity of the SAW sensor because the frequency change from the SAW sensor is very small at sensing low concentration gas. In the previous works [2] [3] , asynchronous types SAW sensor array system have been developed to implement a portable E-Nose to reduce the interference between sensors. However, the data acquired from the asynchronous type SAW sensor array system is not continuous, thus it is not suitable for further analysis. In contrast, synchronous type SAW sensor array system has the advantage of data integrity. In this paper, we focus on the design of low substrate noise synchronous type SAW sensor array system. We used deep N-Well to separate the substrates of NMOS transistors to reduce the injection noise from the substrates between sensors. Likewise, the power and the ground of the sensors were also separated. The system was implemented by an application-specific integrated circuit (ASIC) that converted the analog sensor signal into digital. The proposed circuit has been fabricated by TSMC 0.18μm 1P6M CMOS process technology. The chip operated at 1V supply voltage for digital circuits and 1.8V for analog circuits, respectively. ARCHITECTURE The SAW sensor oscillator consists of a CMOS cross coupled pair and a passive element. The cross coupled pair offers negative resistance to the passive element for the oscillator to oscillate continuously. Eq. (1) below shows the relationship between phase noise and jitter, where J represents jitter (fs); fm is the frequency offset from the center frequency; fo is the center frequency; (No/Po)fm is the phase noise magnitude at fm (dbc/Hz) [5] .