EVAL-CN0357-ARDZ

Circuit Note CN-0357 Devices Connected/Referenced 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 www.analog.com/CN0357. ADA4528-2 5.0 V, Ultralow Noise, Zero Drift, RRIO, Dual Op Amp AD5270-20 1024-Position,1% Resistor Tolerance Error, 50-TP Memory Digital Rheostat ADR3412 Micropower, 0.1% Accurate,1.2 Voltage 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 EVALUATION AND DESIGN SUPPORT The circuit shown in Figure 1 uses the ADA4528-2, dual auto zero amplifier, which has a maximum offset voltage of 2.5 µV at room temperature and an industry leading 5.6 µV/√Hz of voltage noise density. In addition, the AD5270-20 programmable rheostat is used rather than a fixed transimpedance resistor, allowing for rapid prototyping of different gas sensor systems, without changing the bill of materials. Circuit Evaluation Boards CN-0357 Circuit Evaluation Board (EVAL-CN0357-PMDZ) SDP to Pmod Interposer Board (PMD-SDP-IB1Z) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, and Bill of Materials The ADR3412 precision, low noise, micropower reference establishes the 1.2 V common-mode, pseudo ground reference voltage with 0.1% accuracy and 8 ppm/°C drift. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a single-supply, low noise, portable gas detector, using an electrochemical sensor. The Alphasense CO-AX carbon monoxide sensor is used in this example. For applications where measuring fractions of ppm gas concentration is important, using the ADA4528-2 and the ADR3412 makes the circuit performance suitable for interfacing with a 16-bit ADC, such as the AD7790. 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. 3.3V VREF U4 ADR3412 VIN VOUT C10 0.1µF GND R4 1.2V 12.4kΩ C8 0.1µF AVCC R3 12.4kΩ U2-A U1 ADA4528-2 CO-AX CE WE C3 0.02µF RE C4 0.02µF Q1 MMBFJ270 D S R6 12.4kΩ C9 10µF C2 0.02µF G W A VDD REF(+) U5 1.2V AD8500 R12 150Ω AIN1(–) DOUT/RDY AIN1(+) U8 R9 3.3kΩ C14 5.6nF SCLK AD7790 CS GND AGND TO PROCESSOR DIN REF(–) AGND U3-B R1 1MΩ R10 3.3kΩ R8 100kΩ R2 33Ω 3.3V R5 12.4kΩ U2-B ADA4528-2 AD5270-20 12332-001 AVCC 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 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2014 Analog Devices, Inc. All rights reserved. CN-0357 Circuit Note CIRCUIT DESCRIPTION Table 1. Typical Carbon Monoxide Sensor Specifications Figure 2 shows a simplified schematic of an electrochemical sensor measurement 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. In the case of 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 should 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, Alphasense, Ltd.). Parameter Sensitivity IWE + RE VREF CE RF Overrange Limit (Specifications Not Guaranteed) The output voltage of the transimpedance amplifier is VO = 1.2 V + IWE × RF IWE The maximum response of the CO-AX sensor is 100 nA/ppm, and its maximum input range is 2000 ppm of carbon monoxide. These values result in a maximum output current of 200 μA and a maximum output voltage determined by the transimpedance resistor, as shown in Equation 2. VO = 1.2 V + 2000 ppm × 100 IWE VOUT SENSOR Figure 2. Simplified Electrochemical Sensor Circuit The current into the WE terminal is less than 100 nA per ppm of gas concentration; therefore, converting this current into an output voltage requires a transimpedance amplifier with a very low input bias current. The 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 ADR3412 establishes the pseudo ground reference for the circuit, which allows for single-supply operation while consuming very little quiescent current (100 µA maximum). Amplifier U2-A sinks enough current from the CE terminal to maintain a 0 V potential between the WE terminal and the RE terminal on the sensor. The RE terminal is connected to the inverting input of Amplifier U2-A; 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. Transimpedance Amplifier U2-B converts the sensor current into a voltage proportional to the gas concentration. The sensor selected for this circuit is an Alphasense CO-AX carbon monoxide sensor. Table 1 shows the typical specifications associated with carbon monoxide sensors of this general type. (1) where IWE is the current into the WE terminal, and RF is the transimpedance feedback resistor (shown as the AD5270-20 U3-B rheostat in Figure 1). WE 12332-002 – VREF Response Time (t90 from 0 ppm to 400 ppm CO) Range (ppm) CO, Guaranteed Performance) Value 55 nA/ppm to 100 nA/ppm (65 nA/ppm typical)