Design Electronic Sensors Incorporating Electrochemical Gas Sensors

This article describes how to design electronic circuits to incorporate SGX Sensortech electrochemical gas sensors. This guidance helps users to achieve outstanding performance with SGX Sensortech electrochemical gas sensors. However, care must be exercised to adapt the electronic circuits to the requirements of the specific application.

Gas Detection System

A block diagram of a gas detection system featuring an electrochemical gas sensor is presented in Figure 1. A bias circuit (Potentiostat) is required by the electrochemical gas sensor so that the appropriate bias potential can be maintained between the reference and the sensing electrodes. The output current produced by the gas sensor is in proportion to the gas concentration. The small currents generated from the electrochemical cell are converted into measureable voltages by a transimpedance amplifier.

Block diagram of typical gas detection system using an electrochemical gas sensor; R is reference electrode, C is counter electrode, and S is sensing electrode.

Figure 1. Block diagram of typical gas detection system using an electrochemical gas sensor; R is reference electrode, C is counter electrode, and S is sensing electrode.

The transimpedance amplifier output is sampled by the analog to digital converter (ADC), generating a digital reading for the level of voltage. The microprocessor uses this value for computing the concentration of the target gas. A zero setting and a gain setting correction will be required at some point of stage in the system. This could be done at the transimpedance amplifier or within the microprocessor. An analog reading of gas concentration can be directly obtained from the voltage output of the transimpedance amplifier. Ambient pressure and temperature need to be compensated for more critical applications.

Unbiased Sensor Circuit

Figure 2 shows unbiased sensor circuit with split power rails. The reference electrode (VREF)’s potential is monitored by operational amplifier IC1, from which the counter electrode gets an appropriate potential (VCOUNT) to maintain VREF equal to VSET. This voltage changes with changing gas concentration due to current supply into the counter electrode to offset the current released by the sensing electrode. VSET is zero for most electrochemical sensors as they are unbiased.

Unbiased sensor circuit with split power rails

Figure 2. Unbiased sensor circuit with split power rails

TR1 is a P-channel FET that keeps the bias between the reference and sensing electrodes as zero during power down. The absence of a shorting FET will cause the sensor to take a few hours for re-stabilization after turned on.

Electrochemical sensors generate an electrical current as an output in proportion to the gas concentration. Electron flow is from the sensing electrode for most gases, resulting in a positive output voltage. Gases such as oxygen, chlorine, and nitrogen dioxide are reduced in the cell, resulting in a negative voltage due to electron flow into the sensing electrode (Table 1).

Table 1. Bias Potential for SGX Sensortech Sensors

Sensor Applied bias (VSENSE – VREF) VOUT Polarity
EC4-1-ClO2 0 V Negative
EC4-50-ClO2 0 V Negative
EC4-50-Cl2 0 V Negative
EC4-500-CO 0 V Positive
EC4-2000-CO 0 V Positive
EC4-100-ETO +300 mV Positive
EC4-200-ETO +300 mV Positive
EC4-1000-ETO +300 mV Positive
EC4-1000-H2 0 V Positive
EC4-100-H2S 0 V Positive
EC4-1000-H2S 0 V Positive
EC4-250-NO +300 mV Positive
EC4-2000-NO +300 mV Positive
EC4-20-NO2 0 V Negative
EC4-20-PH3 0 V Positive
EC4-1000-PH3 0 V Positive
EC4-20-SO2 0 V Positive
EC4-2000-SO2 0 V Positive
EC410 (O2) -600 mV Negative

The transimpedance amplifier converts the current from the electrochemical sensor into a voltage. It is recommended to use a 10R load resistor with the sense electrodes in series. This resistor along with the intrinsic sensor capacitance creates an RC smoothing filter. The response time can be improved by lowering the load resistor value. However, this will also lead to higher output noise. The noise on the output can be reduced through a high frequency cut-off provided by a combination of a capacitor and the gain resistor connected in parallel.

Biased Sensor Circuit

A potentiostat circuit for a biased sensor is illustrated in Figure 3. The sensing electrode voltage is maintained at 0V by the biasing effect of the output circuit, resulting in a +300 mV bias between VSENSE and VREF. This +300mV biasing configuration is ideal for NO and ETO sensors. The VSET value should be +600mV for oxygen sensors, for which the connection of the VSET supply would be between +V and ground.

Biased sensor circuit with split power rails

Figure 3. Biased sensor circuit with split power rails

In some cases, the bias circuit power supply is kept on even after the equipment is turned off so that the bias across the sensor is maintained, which, in turn, allows the sensor to be used immediately after being turned on. This configuration does not require a shorting transistor but needs a coin cell as a backup supply.

Biased Sensor Circuit with Single Supply

A potentiostat circuit for a biased sensor with a 5V supply is shown in Figure 4. A virtual ground of 50% of the supply voltage (2.5V) needs to be generated with the help of a stable voltage reference. It is possible to adapt the circuit for even lower supply voltages, but the virtual earth voltage needs to be changed to obtain adequate voltage swing.

Biased Sensor Circuit with Single Power Rail

Figure 4. Biased Sensor Circuit with Single Power Rail

Selection of Operational Amplifier

The following parameters need to be considered in the selection of the operational amplifier for use in the bias circuit:

  • Input bias current – An op-amp with an input bias current of below 5nA is generally selected.
  • Input offset voltage – The high sensitivity of electrochemical sensors even for slight variations in bias voltages can result in a high level of current flow due to the large capacitance of the sensors. Stabilization time for the sensors subsequent to a change in bias could extend to several hours. It is recommended to choose an op-amp with an input offset voltage less than 100µV.
  • Input offset voltage temperature drift – It is recommended to select an op-amp with low input offset voltage temperature drift.
  • Output voltage swing – care must be exercised to the required output swing for the operational amplifier especially in low voltage systems.
  • Output current drive – It is necessary to ascertain the capability of the selected op-amp to source or sink the required current.

Calibration

It is essential to set the zero point during the application of clean air or zero gas to the sensor. It is possible to change the sensitivity of the circuit (mV/ppm) by fine-tuning RGAIN through the application of a gas of known concentration to the sensor. The impact of ambient pressure and temperature needs to be compensated for to improve the accuracy of sensors.

Conclusion

Very stable bias voltage is crucial as even slight variations can significantly impact the output of the sensor for many hours. Examples of bias circuit for -300 mV and +600 mV are shown in Figures 5 and 6. In all these cases, it is necessary to check the circuit operation for the specific supply voltage and the chosen reference device. Figure 7 demonstrates the application of an operational amplifier to produce a virtual ground from a single supply.

Example bias circuit for -300mV

Figure 5. Example bias circuit for -300mV

Example bias circuit for +600mV

Figure 6. Example bias circuit for +600mV

Example circuit to generate virtual ground

Figure 7. Example circuit to generate virtual ground

Having very short PCB track lengths is recommended, particularly in the potentiostat and transimpedance amplifier circuits. It is necessary to decouple operational amplifiers in the proximity of the IC. Output signal oversampling and data averaging help achieving further noise reduction.

About SGX Sensortech (IS)

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:-

  • Mining
  • 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.

For more information on this source, please visit SGX Sensortech (IS) Ltd.

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