Chapter 3 Electronic Circuit for MWCNT Ethylene Sensor

Transcription

1 Chapter Electronic Circuit for MWCNT Ethylene Sensor This chapter deals with design and prototype development of electronic circuits required for MWCNT ethylene sensor application. The customized potentiostat circuit developed in this work has got a transimpedance amplifier and two numbers of difference amplifiers. Potentiostat circuit was first simulated using MULTISIM for design validation. After the simulation, the layout for the potentiostat circuit was developed using ULTIBOARD. The potentiostat PCB is fabricated and prototype type development of potentiostat along with customized power supply is used for measuring the sensor response. 1

2 .1 SCHEMATIC OF THE POTENTIOSTAT CIRCUIT A brief introduction about the potentiostat circuit, literature on potentiostat used in chemical sensors for gas detection application and the strong need for developing a customized potentiostat for the MWCNT ethylene sensor focused in this research work is discussed in Chapter 1. The schematic diagram of potentiostat required for MWCNT ethylene sensor application (present work) is shown in Figure.1. It consists of a trans-impedance amplifier (TIA) circuit (U1) made up of instrumentation op-amp OP0E with feedback resistor R f = 0 Ω and differential amplifier (U) made up of OP0 with closed loop voltage gain of. for amplifying the voltage difference available across the feedback resistor (R f ) in the U1 circuit.

3 0Ω U1.kΩ Gas Test Cell MWCNT Sensor Power Supply (0-5) V Current Voltage Converter (TIA) 1kΩ 1kΩ.kΩ U Data Acquisition Module.kΩ Differential Amplifier-1 Sensor current response converted to Voltage signal 1kΩ U Data Acquisition Module 1kΩ.kΩ Differential Amplifier- Sensor voltage response (Potential difference across sensor) Figure.1 Schematic diagram of potentiostat developed for MWCNT ethylene sensor

4 In order to avoid the source loading effect the differential amplifier circuit (U) is used to amplify the voltage across R f. The difference amplifier (U) is used to amplify only the voltage difference across R f and hence the common mode signals will be attenuated and hence the output of the circuit will be free from error due to noise. The schematic of the potentiostat has also got another differential amplifier circuit (U) made up of instrumentation amplifier (OP0E) used to measure the voltage drop across the sensor and to amplify the voltage signal with a voltage gain of.. The application of this difference amplifier circuit (U) is to amplify only the voltage difference available across the sensor and to attenuate any common mode signals during measurement. Due to this, the sensor output will be free from the error due to noise. The difference amplifier circuit (U) also reduces the source loading effect on the sensor and hence the output will be free from measurement loading error. The TIA circuit and amplifier circuits (U1, U and U) are biased using high precision dual regulated ±olts fabricated by us, which is free from noise, ripple and voltage fluctuations. The input to the non-inverting terminal of U1 is supplied through (0-5) Volts regulated external power supply. The inverting terminal of U1 (TIA) and the circuit ground is connected across the sensor. By means of virtual grounding effect of op-amp, the inverting terminal will be experiencing the sample potential as applied to the non-inverting terminal of U1. The current consumed by the sensor in the presence and absence of analyte will produce a voltage drop across the R f proportional to the sensor current and is measured across the feedback resistor R f as a voltage difference.

5 below: The important features of instrumentation amplifier (OP0E) are listed as 1. Low V OS : 5 μv maximum. Low V OS drift: 1. μv/ C maximum. Stablility vs. time: 1.5 μv per month. Low noise: 0. μv p-p maximum 5. Wide input voltage range: ±1 V typical. Wide supply voltage range: ± V to ±18 V. Low input bias current: ± na 8. Closed loop gain: 00 kv/v 9. Common mode rejection ratio: 10 db Few applications of instrumentation amplifier (OP0E) are as follows: Instrumentation Sensors and controls Thermocouples. SIMULATION AND FABRICATION OF POTENTIOSTAT There is a strong necessity to simulate the potentiostat circuit to verify its performance based on the value of the circuit elements and the specification of ICs considered in the design of potentiostat circuit. The performance (gain, linearity and offset) of the circuit is analysed based on the design considerations and expected value of percentage change in resistance of the MWCNT ethylene sensor. If there is a deviation in the expected output of the circuit then the design of the circuit will be modified accordingly to overcome the deviations. Once after the circuit is validated by means of simulation, it will be better to proceed with fabrication of the potentiostat PCB. The design of high precision and low noise dual regulated DC power supply required to operate the potentiostat is also verified for the expected performance before fabrication by means of simulation in this work. 5

6 ..1 CONSTRUCTION OF POTENTIOSTAT CIRCUIT IN MULTISIM SOFTWARE Multisim is the simulation software which is widely used in industry, research and academic organization for the simulation of any newly developed customized circuits to verify its expected performance. The deviation in the performance of the circuit can be compensated by doing appropriate compensations in the circuit design. Mutisim software 11.0 version is used in this research work for the simulation of newly developed customized potentiostat circuit. The values of the circuit elements (discrete components and ICs) in developing the potentiostat circuit were selected based on the requirement its availability in the market. The measurement devices were connected in the circuit layout to measure the current, voltage outputs of the circuit. In place of chemoresistive gas sensor, a variable resistor which is analogous to a chemoresistive sensor was considered and its value can be controlled precisely... SIMULATION OF POTENTIOSTAT UNDER VARYING CONDITION OF SENSOR RESISTANCE The customized potentiostat circuit developed for the MWCNT ethylene sensor subjected for simulation using Multisim. The simulation was carried out by increasing the percentage change in resistance of the sensor assuming that sensor resistance increases linearly upon exposure to analyse. The simulation picture of potentiostat at zero percentage change in sensor resistance is shown in Figure..

7 MWCNT Sensor Rsensor 11.9Ω Key=A 0% R 1kΩ R 1kΩ R9.kΩ - Ammeter +.05m A Sensor Current R8.kΩ U OP0E 1 1 V Power supply 8 R1 0Ω U1 OP0E 1 8 Voltmeter V Sensor Response- (Sensor Voltage) Differential Amplifier- Current to Voltage Converter (TIA) R 1kΩ R 1kΩ R5.kΩ Voltmeter V Voltage across R1 (Feedback Resistor) R.kΩ U OP0E V Voltmeter Sensor Response-1 (I-V) Converted Differential Amplifier-1 Figure. Simulation of potentiostat under zero percentage change in sensor resistance (absence of analyte).

8 .. SIMULATION RESULT OF CUSTOMIZED POTENTIOSTAT The sensor current and potentiostat output voltage based on the current response of the sensor when its percentage resistance assumed to change linearly from (0-50)% upon exposure to analyte is shown in the Figure.. Figure. Sensor current and potentiostat output voltage vs. percentage change in sensor resistance.. FABRICATION OF POTENTIOSTAT The performance (gain and linearity and offset) of the potentiostat was analysed by varying the input value of the circuit by means of simulation. The simulation results were used to finalize the design parameters (values of circuit elements) of the circuit and the PCB layout was developed using the Ultiboard software. The layout developed using Ultiboard was used to fabricate the potentiostat PCB and it is mounted with the circuit elements per the design as shown in the Figure.. 8

9 Figure. Picture of potentiostat PCB developed for MWCNT ethylene sensor. 9

10 . SUMMARY The customized potentiostat schematic is developed for the detection of ethylene using MWCNT sensor for the first time. The performance of the customized potentiostat was verified by means of simulation using Multisim. The design of potentiostat is validated based on simulation performance analysis and Potentiostat PCB is fabricated based on layout generated using Ultiboard. Prototype development of potentiostat (which can be interfaced to LAbVIEW through Data Acquisition Module for acquiring the continuous response of the sensor) along with high precision power supply circuit is developed and used for measuring the sensor response. 80