## 3 Results
### Design of the Conditioning Sections for Electrochemical Gas Sensors
\[\mathrm{R_{feedback}}=\frac{\mathrm{V_{o,Max}-V_{o,Min}}}{\mathrm{I_{sense}}}= \frac{3.3\ \mathrm{V}\ -\ 0.3\ \mathrm{V}}{0.18\ \upmu\mathrm{A}}=16.67\ \mathrm{M}\Omega\ \approx\ 16\ \mathrm{M}\Omega \tag{2}\]  
Since electrochemical oxygen sensors base their operation on a reduction reaction, they will provide a negative output voltage at the output to a potentiostatic circuit if connected as previously described. Thus, to obtain a positive output voltage, it is necessary to invert the positions of CE and WE. A galvanometric circuit was chosen to perform the conditioning of the signal supplied by the ME3-O2 sensor; the sensor current is dropped across a load resistor, and the consequent voltage is amplified with a non-inverting stage. Figure 10 shows the conditioning scheme designed for the ME2-O2 sensor to guarantee a positive output voltage. Figure 9: Schematic of potentiostatic stage used for conditioning the current signal provided by the ME2-CO and ME2-CH2O sensors. As illustrated in the schematic in Figure 10, the ME2-O2 electrodes are connected to a 100 \(\Omega\) load resistance (R1); the voltage generated by the sensor current is applied to the non-inverting input of the operational U1, configured as a non-inverting amplifier, characterized by a gain equal to \(\left(1+\frac{\text{R3}}{\text{R2}}\right)\). The operational amplifier is powered by the supply terminals to +3.3 V, filtered through capacitor C2 located towards the ground. The gain of the conditioning circuit was calculated taking into account the sensor's sensitivity (0.3 mA/Vol) and the maximum detectable oxygen concentration (23% Vol), at which the current supplied in the sensor output (I\({}_{\text{sense}}\)) is equal to 70 \(\mu\)A (0.3 mA/Vol \(\times\) 0.23 Vol). This current generates a d.d.p. equal to 7 mV on the load resistance (R1). To obtain a maximum voltage of +3.3 V at the output, the gain of the non-inverting stage must be equal to:  
\[\left(1+\frac{\text{R3}}{\text{R2}}\right)=\frac{+3.3\text{ V}}{7\text{ mV}}\ \approx\ 471 \tag{3}\]  
Figure 11a presents the circuit scheme of the conditioning board for the ME2 family sensors, which detects the environmental concentrations of CO, CH\({}_{2}\)O, and O\({}_{2}\). Furthermore, on the two potentiostatic stages used for the ME2-CO (green box) and ME2-CH2O (blue box) sensors, a capacitor was added in parallel to each feedback chain of the transimpedance stage (red box in Figure 11a), which determines the circuit's time constant. Furthermore, RC filters were placed in series with the outputs of the transimpedance stages to remove the noise generated by the potentiostatic stage in the output signals. Additionally, a galvanometric circuit based on the SGM8042 opamp was used for the ME2-O2 sensor. The galvanometric section gain was determined considering a 100 \(\Omega\) sensor load resistance and 23% Vol maximum detectable concentration, at which the output of the non-inverting amplifier must be equal to 3.3 V. From (3), the gain of the non-inverting amplifier is equal to 471; therefore, the two resistances R8 and R10 were set at 100 \(\Omega\) and 47 k\(\Omega\), respectively. The resulting board has dimensions equal to 43 mm \(\times\) 54 mm (Figure 11b). A conditioning board for the ME3-H2S sensor was designed for detecting the H\({}_{2}\)S concentration. Figure 10: Circuit scheme of the conditioning section for the ME2-O2 gas sensor. Specifically, the circuit solution used for conditioning the current signal provided by the H\({}_{2}\)S sensor is depicted in Figure 12, which is essentially a potentiostatic circuit for three-electrode sensors. The conditioning circuit comprises a transimpedance stage (based on Ut opamp), which converts the current signal into a voltage signal, and a further operational amplifier (Up) in a potentiostatic configuration to fix the d.d.p. between the RE and WE. Through negative feedback, the opamp Up guarantees that the d.d.p.