Circuit Note CN-0357 Devices Connected/Referenced 5.0 V, Ultralow Noise, Zero Drift, RRIO, ADA4528-2 Dual Op Amp Circuits from the Lab reference designs are engineered and 1024-Position,1% Resistor Tolerance Error, AD5270-20 tested for quick and easy system integration to help solve todays 50-TP Memory Digital Rheostat analog, mixed-signal, and RF design challenges. For more Micropower, 0.1% Accurate,1.2 Voltage information and/or support, visit www.analog.com/CN0357. ADR3412 Reference AD8500 Micropower, RRIO, Op Amp AD7790 Low Power, 16-Bit Sigma-Delta, ADC Low Noise, Single-Supply, Toxic Gas Detector, Using an Electrochemical Sensor with Programmable Gain TIA for Rapid Prototyping The circuit shown in Figure 1 uses the ADA4528-2, dual auto EVALUATION AND DESIGN SUPPORT zero amplifier, which has a maximum offset voltage of 2.5 V at Circuit Evaluation Boards room temperature and an industry leading 5.6 V/Hz of CN-0357 Circuit Evaluation Board (EVAL-CN0357-PMDZ) voltage noise density. In addition, the AD5270-20 programmable SDP to Pmod Interposer Board (PMD-SDP-IB1Z) rheostat is used rather than a fixed transimpedance resistor, System Demonstration Platform (EVAL-SDP-CB1Z) allowing for rapid prototyping of different gas sensor systems, Design and Integration Files without changing the bill of materials. Schematics, Layout Files, and Bill of Materials The ADR3412 precision, low noise, micropower reference CIRCUIT FUNCTION AND BENEFITS establishes the 1.2 V common-mode, pseudo ground reference The circuit shown in Figure 1 is a single-supply, low noise, voltage with 0.1% accuracy and 8 ppm/C drift. portable gas detector, using an electrochemical sensor. The For applications where measuring fractions of ppm gas Alphasense CO-AX carbon monoxide sensor is used in this concentration is important, using the ADA4528-2 and the example. ADR3412 makes the circuit performance suitable for interfacing Electrochemical sensors offer several advantages for with a 16-bit ADC, such as the AD7790. 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. 3.3V VREF R3 U4 U2-B REF(+) V AVCC AVCC DD 12. 4k R10 ADR3412 R4 U1 ADA4528-2 U5 U2-A 1.2V 3. 3k 1.2V 12. 4k AIN1() AD8500 ADA4528-2 DOUT/RDY V V CO-AX TO IN OUT R8 CE WE AIN1(+) PROCESSOR 100k GND R2 R12 DIN C10 C8 RE 33 150 0.1F 0.1F C3 C4 C9 U8 SCLK 0.02F 0.02F Q1 10F C14 AD7790 C2 MMBFJ270 R9 5.6nF 0.02F CS 3. 3k D S 3.3V GND REF() W A AGND R5 R6 AGND G 12. 4k 12. 4k U3-B R1 AD5270-20 1M Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown) 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. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com 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) Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. 12332-001CN-0357 Circuit Note Table 1. Typical Carbon Monoxide Sensor Specifications CIRCUIT DESCRIPTION Parameter Value Figure 2 shows a simplified schematic of an electrochemical Sensitivity 55 nA/ppm to sensor measurement circuit. Electrochemical sensors work by 100 nA/ppm allowing gas to diffuse into the sensor through a membrane and (65 nA/ppm by interacting with the working electrode (WE). The sensor typical) reference electrode (RE) provides feedback to Amplifier U2-A, Response Time (t from 0 ppm to 400 ppm CO) <30 seconds 90 which maintains a constant potential with the WE terminal by Range (ppm) CO, Guaranteed Performance) 0 ppm to varying the voltage at the counter electrode (CE). The direction 2,000 ppm of the current at the WE terminal depends on whether the Overrange Limit (Specifications Not 4,000 ppm Guaranteed) reaction occurring within the sensor is oxidation or reduction. In the case of a carbon monoxide sensor, oxidation takes place The output voltage of the transimpedance amplifier is therefore, the current flows into the working electrode, which VO = 1.2 V + IWE RF (1) requires the counter electrode to be at a negative voltage (typically 300 mV to 400 mV) with respect to the working electrode. The where I is the current into the WE terminal, and R is the WE F transimpedance feedback resistor (shown as the AD5270-20 op amp driving the CE terminal should have an output voltage U3-B rheostat in Figure 1). range of approximately 1 V with respect to VREF to provide sufficient headroom for operation with different types of sensors The maximum response of the CO-AX sensor is 100 nA/ppm, (Alphasense Application Note AAN-105-03, Designing a and its maximum input range is 2000 ppm of carbon monoxide. Potentiostatic Circuit, Alphasense, Ltd.). These values result in a maximum output current of 200 A and I WE a maximum output voltage determined by the transimpedance resistor, as shown in Equation 2. RF V REF + nA RE V = 1.2 V + 2000 ppm 100 R O F ppm CE WE V V REF OUT I I WE WE VO = 1.2 V + 200 A RF (2) SENSOR Applying 1.2 V to VREF of the AD7790 allows a usable range of Figure 2. Simplified Electrochemical Sensor Circuit 1.2 V at the output of the transimpedance amplifier, U2-B. Selecting a 6.0 k resistor for the transimpedance feedback The current into the WE terminal is less than 100 nA per ppm resistor gives a maximum output voltage of 2.4 V. of gas concentration therefore, converting this current into an output voltage requires a transimpedance amplifier with a very Equation 3 shows the circuit output voltage as a function of low input bias current. The ADA4528-2 op amp has CMOS inputs ppm of carbon monoxide, using the typical response of the with a maximum input bias current of 220 pA at room sensor of 65 nA/ppm. temperature, making it a very good fit for this application. V (3) V = 1.2 V + 390 O ppm The ADR3412 establishes the pseudo ground reference for the circuit, which allows for single-supply operation while consuming The AD5270-20 has a nominal resistance value of 20 k. There very little quiescent current (100 A maximum). are 1024 resistance positions, resulting in resistance step sizes of 19.5 . The 5 ppm/C resistance temperature coefficient of the Amplifier U2-A sinks enough current from the CE terminal to AD5270-20 is better than that of most discrete resistors, and its maintain a 0 V potential between the WE terminal and the 1 A of supply current is a very small contributor to the overall RE terminal on the sensor. The RE terminal is connected to the power consumption of the system. inverting input of Amplifier U2-A therefore, no current flows in or out of it. This means that the current comes from the Resistor R4 keeps the noise gain at a reasonable level. Selecting WE terminal and it changes linearly with gas concentration. the value of this resistor is a compromise between the magnitude of Transimpedance Amplifier U2-B converts the sensor current into the noise gain and the sensor settling time errors, when exposed a voltage proportional to the gas concentration. to high concentrations of gas. For the example shown in Equation 4, R4 = 33 , which results in a noise gain of 183. The sensor selected for this circuit is an Alphasense CO-AX carbon monoxide sensor. Table 1 shows the typical 6.0 k (4) NG =1 + =183 specifications associated with carbon monoxide sensors of this 33 general type. Warning: carbon monoxide is a toxic gas, and concentrations higher than 250 ppm can be dangerous therefore, take extreme care when testing this circuit. Rev. 0 Page 2 of 5 12332-002