## 3 System Implementation
### Sensor Interface Circuit  
The proposed sensor interface circuit is depicted in Figure 7. An incremental ADC [13] measures H\({}_{2}\) concentration and temperature at different times. Individual Pd nanowires that have between 7 k\(\Omega\) and 9 k\(\Omega\) resistance should be biased, with a minimum bias voltage of 50 mV for 10 seconds. These nanowires have been represented by NW\({}_{\mathrm{ref}}\) and NW\({}_{\mathrm{sense}}\) in Figure 7. The conductance of NW\({}_{\mathrm{sense}}\) nanowires may change by 20% in comparison with NW\({}_{\mathrm{ref}}\) nanowires as H\({}_{2}\) concentration varies from zero to 30% [7]. The voltage around the reference nanowire, \(V_{R}=(V_{1}-V_{2})\), is used as the reference voltage, while the voltage of the sensing nanowire (V\({}_{2}\)) is used as the input voltage for the following incremental ADC in consecutive non-overlapping clock phases. This ADC can achieve 12 bits resolution by using 200 fF metal-insulator-metal (MIM) capacitors, as C\({}_{3}\), C\({}_{4}\) and C\({}_{5}\), and a conventional two-stage amplifier with at least 75 dB gain for the integrator [9]. As a 1 MHz clock has been used for ADC and since this ADC needs 2\({}^{\mathrm{N}}\) cycles for N-bits conversion, a 12-bit conversion takes nearly 4 ms. After finishing gas sensing, temperature is measured to further increase the accuracy of gas sensing using an integrated temperature sensor. In the proposed integrated temperature sensor, substrate PNP transistors have been used to generate proportional to absolute temperature (V\({}_{\mathrm{p}\mathrm{t}\mathrm{a}}\)) and temperature-independent reference (V\({}_{\mathrm{r}\mathrm{e}\mathrm{f}}\)) voltages, and the ADC converts (V\({}_{\mathrm{p}\mathrm{t}\mathrm{a}}\)/V\({}_{\mathrm{r}\mathrm{e}\mathrm{f}}\)) to a 12-bit digital value. In Figure 7, by using a 50 fF MIM capacitor as C\({}_{1}\) and a 360 fF MIM capacitor as C\({}_{2}\), a temperature-independent reference voltage of approximately 310 mV has been generated. Similar to gas sensing, n-bit digital representation of (V\({}_{\mathrm{p}\mathrm{t}\mathrm{a}}\)/V\({}_{\mathrm{r}\mathrm{e}\mathrm{f}}\)) is stored in the ADC counter and sent to a wireless transceiver after temperature sensing. The accuracy of temperature sensing is mainly limited by mismatch between Q\({}_{1}\) and Q\({}_{2}\) and nonlinearity in temperature dependence of V\({}_{\mathrm{B}\mathrm{e}\mathrm{1}}\) and (V\({}_{\mathrm{B}\mathrm{e}\mathrm{1}}\)\(-\)V\({}_{\mathrm{B}\mathrm{e}\mathrm{2}}\)). Although these errors can be minimized by using dynamic methods presented in [19] to reach \(\pm\)0.1 \({}^{\mathrm{o}}\)C accuracy, such power-consuming techniques are not needed here. The proposed low power temperature sensor can achieve \(\pm\)1 \({}^{\mathrm{o}}\)C accuracy by only calibrating C\({}_{2}\) and I\({}_{\mathrm{T}\mathrm{e}\mathrm{m}}\) at room temperature [9]. When the sensor interface circuit operates at a lower frequency, Q\({}_{1}\) and Q\({}_{2}\) transistors are biased with a lower I\({}_{\mathrm{T}\mathrm{e}\mathrm{m}}\) bias current to reduce the average power consumption of the circuit.  
Figure 7: Circuit diagram of the proposed sensor interface.