This article is a guide to the design of electronic circuits used with SGX Sensortech electrochemical gas sensors.
The block diagram of a typical gas detection system with an electrochemical gas sensor is shown in Figure 1. The gas sensor operates using a bias circuit for maintaining the appropriate bias potential between the sensing and reference electrodes.
The output current produced by the gas sensor is proportional to the concentration of gas. These small currents are converted into the measurement voltage using a transimpedance amplifier.
The output of the amplifier is sampled through the analog to digital converter (ADC) to produce a digital reading of the voltage level. The reading is used by a microprocessor to measure the actual gas concentration.
Figure 1. Block diagram of typical gas detection system using an electrochemical gas sensor where R: Reference Electrode, C: Counter Electrode, S: Sensing Electrode (sometimes called the ‘Working’ electrode)
Unbiased Sensor Circuit
The main function of the sensor bias circuit, also known as potentiostat is to maintain constant sensing electrode potential with respect to the reference electrode. The required bias level (VSENSE – VREF) differs based on the type of sensor.
A typical potentiostat circuit with a positive and negative supply voltage is shown in Figure 2. The reference electrode potential, VREF can be monitored using an operational amplifier IC1 by applying correct potential VCOUNT to the counter electrode in order to maintain VREF and VSET at the same level. This potential again changes with the change in gas concentration as it supplies current into the counter electrode to maintain the current output from the sensing electrode.
Figure 2. Unbiased sensor circuit with split power rails
The zero bias between the reference and sensing electrodes is maintained by a TR1, a P-channel FET while turning off the supply voltage. The current output from the electrochemical sensors is proportional to the gas concentration. For most gases that undergo a cell reduction, electrons flow into the sensing electrode resulting in the generation of negative voltage from the circuit.
Biased Sensor Circuit
A typical potentiostat circuit for a biased sensor is shown in the Figure 3. The circuit uses a -300mV VSET supply to provide a bias of +300 mV between the sense and reference electrodes. The sensing electrode potential is maintained at 0V by the biasing effect of the output circuit such that a +300mV bias between VSENSE and VREF is generated.
Figure 3. Biased sensor circuit with split power rails
This +300mV biasing arrangement is ideal for ETO and NO sensors. However, it is necessary to use VSET = +600mV for oxygen sensors that require a -600mV bias between VSENSE and VREF. The power to the bias circuit in certain instruments is maintained even while turning off the instrument. This maintains the bias across the sensor stable and is ready to use when it is switched on. This can be achieved using a backup supply such as a coin cell.
Biased Sensor Circuit with Single Supply
A typical potentiostat circuit for biased sensor with a single 5V supply is shown in the Figure 4. It is necessary to generate a virtual ground that is at half the supply voltage, 2.5V. The virtual ground can be generated using a stable voltage.
Figure 4. Biased sensor circuit with single power rail
The virtual ground is used to reference the output circuit such that the sensing electrode can also be at 2.5V. The VSET needs to be set below the virtual earth in order to achieve a +300 mV bias between VSENSE and VREF. Hence, VSET is set at 2.200.
This circuit can be used even for low supply voltages. However, it may be required to alter the virtual earth voltage in order to allow sufficient voltage swing.
Selection of Operational Amplifier
The following parameters should be taken into account while choosing the operational amplifier:
- Input bias current - In general, the op-amp has an ‘input bias current’ which in large quantities will affect the output current from the sensing electrode. It is, therefore, recommended to choose an op-amp with an input bias current of less than 5nA.
- Input offset voltage - The zero bias is clamped through the transistor TR1 when the power is switched off. However, upon turning on the power, zero bias is maintained by the potentiostat circuit. This input offset voltage in the op-amp IC1 will cause a sudden small step in the actual bias. Therefore, it is necessary to choose an op-amp with an input offset voltage below 100mV if possible.
- Input offset voltage temperature drift - When the input offset voltage of the operational amplifier changes with temperature, there is a slight change in the bias voltage. Hence, it is recommended to choose an op-amp with low input offset voltage temperature drift.
- Output voltage swing - Based on the op-amp output, the voltage is supplied to the counter electrode. The voltage supply may vary depending on the concentration and type of gas. Hence, it is advisable to use op-amp that drives at least 1.1V either side of VSET.
- Output current drive - The op-amp output supplies a current into or out off the counter electrode with respect to the current out of or into the sensing output electrode.
It is important to set the zero point while applying zero gas to the sensor. It can be applied by offsetting the voltage at the non-inverting input of IC2 using hardware. However, when the output is supplied to a microprocessor or a digital-to-analog converter (DAC), it will be easy to store a zero point in software as part of a calibration routine.
However, it is not easy to set zero for an oxygen sensor as the normal operating point is near the maximum span. A slight adjustment of RGAIN results in changing the sensitivity of the circuit (mV/ppm). This is achieved by applying a known gas concentration to the sensor. The instruments may also compensate for the effects of ambient temperature and pressure in order to achieve improved accuracy.
It is necessary to maintain stable bias voltage to prevent minute changes that can affect the sensor output. The bias voltage can be produced using a stable reference device such as a series or shunt voltage reference. The reference can be generated with respect to the ground or virtual ground.
Some of the examples of generating -300mV and +600 mV biased sensors using a 1.225 V shunt voltage, reference such as the LM4041 or LM4051 are shown in the Figures 5 and 6.
Figure 7 shows the generation of a virtual ground from a single supply using an operational amplifier.
Figure 5. Example bias circuit for -300mV
Figure 6. Example bias circuit for +600mV
Figure 7. Example circuit to generate virtual ground
The design of the electronics for electrochemical gas sensors is thus described in detail above.
It is necessary to maintain all PCB track lengths very short, particularly in transimpedance amplifier and potentiostat circuits. Operational amplifiers should be properly decoupled with IC. Further, over-sampling the output signal and averaging the data can ensure significant reduction in noise.
About SGX Sensortech (IS) Ltd
SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost- effectiveness.
SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.
As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.
SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.
Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-
- Oil and gas
- Confined space entry
- Indoor air quality
- Industrial area protection
- Leak detection
This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
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