Dual Electrochemical Gas Sensor with Temperature …

1 Circuit NoteCN-0396 Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve today s analog, mixed-signal, and RF design challenges. For more information and/or support, visit Devices Connected/Referenced AD7798 3-Channel, Low Noise, Low Power, 16-Bit, Sigma-Delta ADC with On-Chip In-Amp ADA4528-1/ADA4528-2 V, Ultralow Noise, Zero Drift, RRIO, Single/Dual Op Amp AD5270-20 1024-Position,1% Resistor Tolerance Error, 50-TP Memory Digital Rheostat ADT7310 C Accurate, 16-Bit Digital SPI Temperature Sensor ADP7102 20 V, 300 mA, Low Noise, CMOS LDO ADR3412 Micropower, Accurate, V Voltage Reference Dual Electrochemical Gas Sensor with Temperature Compensation Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room Temperature .

2 However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, Box 9106, Norwood, MA 02062-9106, : Fax: 2016 Analog Devices, Inc. All rights reserved. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0396 Circuit Evaluation Board (EVAL-CN0396-ARDZ) ADICUP360 Development Board (EVAL-ADICUP360) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a portable gas detector, using a 4-electrode Electrochemical Sensor , for simultaneous detection of two distinct gases.

3 The potentiostatic circuit uses an optimum combination of components designed to provide single-supply, low power, and low noise performance, while offering a high degree of programmability to accommodate a variety of sensors for different types of gases. Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of many toxic gases. Most sensors are gas specific and have usable resolutions under one part per million (ppm) of gas concentration. The Alphasense COH-A2 Sensor , which detects carbon monoxide (CO) and hydrogen sulfide (H2S), is used in this example. The EVAL-CN0396-ARDZ printed circuit board (PCB) is designed in an Arduino-compatible shield form factor and interfaces to the EVAL-ADICUP360 Arduino-compatible platform board for rapid prototyping. CN-0396 Circuit Note Rev. 0 | Page 2 of 7 Figure 1. Dual Gas Sensor Simplified Schematic (All Connections and Decoupling Not Shown) CIRCUIT DESCRIPTION Figure 2 shows a simplified schematic of an Electrochemical Sensor measurement circuit.

4 Figure 2. Simplified Potentiostatic Electrochemical Sensor Circuit Electrochemical sensors work by allowing gas to diffuse into the Sensor through a membrane and by interacting with the working electrode (WE). The Sensor reference electrode (RE) provides feedback to Amplifier U2-A, which maintains a constant potential with the WE terminal by varying the voltage at the counter electrode (CE). The direction of the current at the WE terminal depends on whether the reaction occurring within the Sensor is oxidation or reduction. For a carbon monoxide Sensor , oxidation takes place; therefore, the current flows into the working electrode, which requires the counter electrode to be at a negative voltage (typically 300 mV to 400 mV) with respect to the working electrode. The op amp driving the CE terminal must have an output voltage range of approximately 1 V with respect to VREF to provide sufficient headroom for operation with different types of sensors (Alphasense Application Note AAN-105-03, Designing a Potentiostatic Circuit).

5 For 4-electrode Electrochemical gas sensors, there are two working electrodes (shown as WE1 and WE2 in Figure 1). Each WE responds to a certain gas. In this example, the Alphasense COH-A2 Sensor is used. One working electrode responds to CO, while the other responds to H2S. The sensors are designed such that special chemical filters prevent one gas from affecting the other electrode. The two working electrodes share a common RE and CE electrode. The current into the WE terminal is less than 100 nA per ppm of gas concentration for CO and less than 1000 nA per ppm for H2S; therefore, converting this current into an output voltage requires a transimpedance amplifier with a very low input bias current. The ADA4528-1/ADA4528-2 op amp has CMOS inputs with a maximum input bias current of 220 pA at room Temperature , making it a very good fit for this application. The ADA4528-1/ADA4528-2 is an auto-zero amplifier, which has a maximum offset voltage of V at room Temperature and an industry leading V/ Hz of voltage noise density.

6 The ADR3412 voltage reference establishes the V pseudo ground reference for the circuit, which allows single-supply operation while consuming very little quiescent current (100 A maximum). The ADR3412 has accuracy and 8 ppm/ C drift. Amplifier A2 sinks enough current from the CE terminal to maintain a 0 V potential between the WE terminals and the RE terminal on the Sensor . The RE terminal is connected to the inverting input of Amplifier A2; therefore, no current flows in or out of it. This means that the current comes from the WE terminal, and it changes linearly with gas concentration. ( )AIN1( )REFIN(+)REFIN( ) FU2AD5270-20U3AD5270-2033 F100k FWA1/2 ADA4528-2 COH-A2M1H2 SCO1M 33 AVDDDVDDAIN1(+)AIN2(+)AIN3( ) 100k INTCTVDDGNDWE1WE2T CRITICALT HIGH/LOWSPIU4 ADT7310 TEMP NOMINAL6k 14372-001CE +SENSORVREFVOUTVREFIWEIWEIWEREWERF14372- 002 Circuit Note CN-0396 Rev.

7 0 | Page 3 of 7 The A1-A and A1-B transimpedance amplifiers convert the Sensor currents into voltages proportional to the gas concentration. The Sensor selected for this circuit is an Alphasense COH-A2 carbon monoxide (CO) and hydrogen sulfide (H2S) Sensor . Table 1 shows the typical specifications associated with this type of Sensor . The gases that can be measured using this circuit and compatible sensors are toxic. Take extreme care when testing this circuit. Take note of the exposure limits and safety precautions when handling gases such as carbon monoxide and hydrogen sulfide. Table 1. Typical Carbon Monoxide Sensor Specifications Parameter Value Carbon Monoxide Sensitivity 50 nA/ppm to 100 nA/ppm Response Time (t90 from 0 ppm to 400 ppm CO) <35 sec Range (ppm) CO (Guaranteed Performance) 0 ppm to 1000 ppm Overrange Limit (Specifications Not Guaranteed) 2000 ppm Hydrogen Sulfide Sensitivity 600 nA/ppm to 1000 nA/ppm Response Time (t90 from 0 ppm to 20 ppm H2S) <30 sec Range (ppm) H2S (Guaranteed Performance) 0 ppm to 200 ppm Overrange Limit (Specifications Not Guaranteed) 400 ppm The output voltage of the transimpedance amplifier is VO = V + IWE RAW (1) where: IWE is the current into the WE terminal.

8 RAW is the transimpedance feedback resistor (shown as the AD5270-20 U2 and U3 digital rheostat in Figure 1). The maximum response of the COH-A2 Sensor for CO gas is 100 nA/ppm, and its maximum input range is 1000 ppm. For the H2S gas, the maximum response is 1000 nA/ppm, and the maximum input range is 200 ppm. These values result in a maximum output current of 100 A and 200 A for CO and H2S electrodes, respectively. The transimpedance amplifier feedback resistors determine the maximum input voltage to the analog-to-digital converter (ADC). Equation 2 shows the calculation for output voltage of the TIA for the CO electrode. VO = V + 1000 ppm 100 nA/ppm RAW VO = V + 100 A RAW (2) Applying V to VREF of the AD7798 allows a usable range of V at the output of the transimpedance amplifiers, A1-A and A1-B. Selecting nominal feedback resistors of 12 k for the CO channel and 6 k for the H2S channel gives a maximum output voltage of approximately V for both Sensor types.

9 The exact values of the resistors are determined by a single-point calibration in a gas of known concentration. The AD5270-20 digital rheostat is used for the feedback resistor in both channels and has a maximum resistance value of 20 k . There are 1024 resistance positions, resulting in resistance step sizes of . The 5 ppm/ C resistance Temperature coefficient of the AD5270-20 is better than that of most discrete resistors, and its 1 A of supply current is a very small contributor to the overall power consumption of the system. Temperature Compensation The ADT7310 is a 16-bit, digital serial peripheral interface (SPI) Temperature Sensor with an accuracy of C. In this circuit, it is used to monitor the Temperature of the environment. The Temperature data can be used to correct for the effects of Temperature changes on the performance of the gas Sensor . It is recommended to position the Temperature Sensor close to the gas Sensor .

10 The sensitivity of gas sensors (expressed as nA/ppm) changes by typically C to C with respect to the room Temperature sensitivity (Alphasense Application Note AAN-110, Environmental Changes: Temperature , Pressure, Humidity). The ADT7310 measures the ambient Temperature and sends the result to a microprocessor over an SPI interface. The corrections are then carried out in the software using a lookup table or an equation. Always consult the manufacturer to obtain the Temperature correction data for the specific Sensor used in a system. Noise Performance The output impedance of the Sensor WE terminals is a parallel combination of a relatively large capacitance and resistance. The capacitance causes the noise gain of the amplifier to increase as the frequency increases, thereby increasing the output noise. The R3 and R10 resistors are in series with the Sensor output impedance and keep the noise gain (NG) of the TIAs at a reasonable level.