2.4. Signal conditioning of the sensing element for CO2 detection with mixed binary oxide CeO2-Nb2O5 sensitive material

The Analog Devices AD620 operational amplifier is used to build the signal conditioning electronic module, provided by the sensing element. A preamp section comprised of Q1 and Q2, Figure 6, provides additional gain up front. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains a constant collector current through the input devices Q1 and Q2, thereby impressing the input voltage across the external gain setting resistor, RG.

This creates a differential gain from the inputs to the A1/A2 outputs given by Eq. (4):

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 67

Figure 6. A simplified schematic of the AD620 [39].

Electrical Engineering ICPE-CA. The voltage measurements were effected by testing module, in automated process mode. A control panel provides a lot of measuring values, at rate 1/10 s. The bench of testing for the gas sensor consists in an enclosure where there are set of testing conditions for the sensor as well as in connected equipment. The whole process of testing is automated, being controlled by a programmable automaton. The gas for testing is introduced in a controlled way in the testing enclosure, through a mass debit meter. In the testing,

Figure 5. Variation of the voltage depending on time for CO2 sensor, made with mixed oxides CeO2-Nb2O5 sensitive

The gas testing was done in concentration of 10,000 ppm CO2 at the 25, 50 and 70C chamber test temperature. The sensor was developed the voltages values of 48, 50 and 770 mV (Figure 5) [39]. The experimental data shows a good sensor response for CO2 detection with increasing

The Analog Devices AD620 operational amplifier is used to build the signal conditioning electronic module, provided by the sensing element. A preamp section comprised of Q1 and Q2, Figure 6, provides additional gain up front. Feedback through the Q1-A1-R1 loop and the Q2-A2-R2 loop maintains a constant collector current through the input devices Q1 and Q2,

enclosure is set a constant temperature, controlled by a temperature regulator.

2.4. Signal conditioning of the sensing element for CO2 detection with mixed

thereby impressing the input voltage across the external gain setting resistor, RG.

This creates a differential gain from the inputs to the A1/A2 outputs given by Eq. (4):

binary oxide CeO2-Nb2O5 sensitive material

temperature.

material.

66 Cerium Oxide - Applications and Attributes

Figure 7. AD620 closed-loop gain versus frequency [39].

$$G = \frac{R1 + R2}{R\_G} + 1\tag{4}$$

<sup>G</sup> <sup>¼</sup> <sup>49</sup>:4k<sup>Ω</sup> RG

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems

RG <sup>¼</sup> <sup>49</sup>:4k<sup>Ω</sup>

The value of 24.7 kΩ was chosen so that standard 1% resistor values could be used to set the most popular gains. For the input resistors, R1a and R1b were used, capacitor C2p approximately five times to 0.047 μF to provide adequate RF attenuation (Figure 8). With the values shown, the circuit �3 dB bandwidth is approximately 400 Hz and noise levels 12 nV/√Hz. It requires the circuitry preceding the in-amp to drive a lower impedance load and results in somewhat less input overload protection. The output signal VOUT (Figure 8) is a common mode voltage, picked at the output of the operational amplifier. The capacitor groups, 0.01 μF and 0.33 μF make a decoupling of the supply voltage (Figure 8) in the immediate closeness of the operational amplifiers. The supply voltage +Vcc and -Vee, respectively, stabilized is differ-

3. Sensor for CO2 detection with Y2O3-doped CeO2 sensitive material

as hydrothermal [41], electrospinning [23], thermolysis [42] and sol gel [43].

The ion conductivity of CeO2 can be significantly improved upon substitution with some trivalent oxides of lanthanides like Y2O3, Sm2O3 and Gd2O3, because the number of oxygen vacancy will be considerably increased for charge compensation. The electrical conductivity in doped ceria is influenced by factors such as: the dopant ion, the dopant concentration, the oxygen vacancy concentration and the defect association enthalpy. An example is constituted by combination Y2O3-doped CeO2 which has been used usually as the solid electrolyte for moderate temperature solid oxide fuel cells [40]. In our case, we used the Y2O3-doped CeO2 as sensitive material for CO2 detection. For Y2O3-CeO2 synthesis, it utilizes several methods such

Sol gel method applied for synthesis of Y2O3-doped CeO2 sensitive material, is in accord with ref. [44] and used as starting reagents Ce(SO4)2 � 4H2O (97% purity, Merck) and Y (NO3)3 � 3H2O (98% purity Karlsrushe GmbH in molar ratio CeO2/Y2O3 = 4:1). The salts were dissolved in deionized water. To 100 ml salt solution, 25 ml solution of 1 M citric acid as chelating agent was added. To obtain gel, the salt solution was heated to 70�C under constant stirring. To this solution, 40 ml ethylene glycol was added to promote citrate polymerization and heated at 90�C. The gel formed was filtered, washed and heat treated in oven at 100�C. The powder obtained was calcined at 800�C for 2 hours. The powder was pressed to disc form

, with dimensions diameter 4 mm, height 1 mm and then sinterized at

So that,

3.1. Synthesis method

using 10 ton force/cm<sup>2</sup>

1100�C for 6 hours [44].

where the resistor RG in kΩ, according to Eq. (6).

entiated, �15Vcc, in comparison with the reference potential bar.

þ 1 (5)

http://dx.doi.org/10.5772/intechopen.80801

69

<sup>G</sup> � <sup>1</sup> (6)

The unity-gain subtractor, A3, removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. The value of RG also determines the transconductance of the preamp stage [34]. As RG is reduced for larger gains, the transconductance increases asymptotically to that of the input transistors. The open-loop gain is boosted for increasing programmed gain, thus reducing gain related errors. Also, the gain bandwidth product (determined by C1, C2 and the preamplifier transconductance, Figure 6) increases with programmed gain, thus optimizing the amplifier's frequency response. In Figure 7, the closed-loop gain of AD620 versus frequency is shown. Finally, the input voltage noise is reduced to 9 nV/√Hz, which is determined mainly by the collector current and base resistance of the input devices. The internal gain resistors, R1 and R2, are laser trimmed to an absolute value of 24.7 kΩ, allowing the gain to be programmed accurately with a single external resistor. The gain equation is Eq. (5).

Figure 8. The electronic module for signal conditioning provided by sensing element, designing with AD620 analog devices.

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 69

$$G = \frac{49.4k\Omega}{R\_{\odot}} + 1\tag{5}$$

So that,

<sup>G</sup> <sup>¼</sup> <sup>R</sup><sup>1</sup> <sup>þ</sup> <sup>R</sup><sup>2</sup> RG

68 Cerium Oxide - Applications and Attributes

The unity-gain subtractor, A3, removes any common-mode signal, yielding a single-ended output referred to the REF pin potential. The value of RG also determines the transconductance of the preamp stage [34]. As RG is reduced for larger gains, the transconductance increases asymptotically to that of the input transistors. The open-loop gain is boosted for increasing programmed gain, thus reducing gain related errors. Also, the gain bandwidth product (determined by C1, C2 and the preamplifier transconductance, Figure 6) increases with programmed gain, thus optimizing the amplifier's frequency response. In Figure 7, the closed-loop gain of AD620 versus frequency is shown. Finally, the input voltage noise is reduced to 9 nV/√Hz, which is determined mainly by the collector current and base resistance of the input devices. The internal gain resistors, R1 and R2, are laser trimmed to an absolute value of 24.7 kΩ, allowing the gain to be programmed accurately with a single external resistor. The gain equation is Eq. (5).

Figure 8. The electronic module for signal conditioning provided by sensing element, designing with AD620 analog

devices.

þ 1 (4)

$$R\_G = \frac{49.4k\Omega}{G - 1} \tag{6}$$

where the resistor RG in kΩ, according to Eq. (6).

The value of 24.7 kΩ was chosen so that standard 1% resistor values could be used to set the most popular gains. For the input resistors, R1a and R1b were used, capacitor C2p approximately five times to 0.047 μF to provide adequate RF attenuation (Figure 8). With the values shown, the circuit �3 dB bandwidth is approximately 400 Hz and noise levels 12 nV/√Hz. It requires the circuitry preceding the in-amp to drive a lower impedance load and results in somewhat less input overload protection. The output signal VOUT (Figure 8) is a common mode voltage, picked at the output of the operational amplifier. The capacitor groups, 0.01 μF and 0.33 μF make a decoupling of the supply voltage (Figure 8) in the immediate closeness of the operational amplifiers. The supply voltage +Vcc and -Vee, respectively, stabilized is differentiated, �15Vcc, in comparison with the reference potential bar.
