Circuit Note CN-0234 Devices Connected/Referenced Circuits from the Lab reference circuits are engineered and ADA4505-2 Micropower Rail-to-Rail I/O Dual Op Amp tested for quick and easy system integration to help solve todays ADR291 Micropower 2.5 V Voltage Reference analog, mixed-signal, and RF design challenges. For more ADP2503 2.5 MHz Buck-Boost DC-to-DC Converter information and/or support, visit www.analog.com/CN0234. AD7798 16-Bit Low Power Sigma-Delta ADC Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor Electrochemical sensors offer several advantages for instruments EVALUATION AND DESIGN SUPPORT that detect or measure the concentration of many toxic gases. Circuit Evaluation Boards Most sensors are gas specific and have usable resolutions under CN-0234 Circuit Evaluation Board (EVAL-CN234-SDPZ) one part per million (ppm) of gas concentration. They operate System Demonstration Platform (EVAL-SDP-CB1Z) with very small amounts of current, making them well-suited Design and Integration Files for portable, battery powered instruments. Schematics, Layout Files, and Bill of Materials The circuit shown in Figure 1 uses the ADA4505-2, dual micro- CIRCUIT FUNCTION AND BENEFITS power amplifier, which has a maximum input bias current of 2 pA The circuit shown in Figure 1 is a single-supply, low power at room temperature and consumes only 10 A per amplifier. In battery operated, portable gas detector using an electrochemical addition, the ADR291 precision, low noise, micropower reference sensor. The Alphasense CO-AX Carbon Monoxide sensor is consumes only 12 A and establishes the 2.5 V common-mode used in the example. pseudo-ground reference voltage. 3.3V 5V, AVCC VREF U1 REFIN(+) AV DV DD DD U2-B AVCC AVCC ADR291GR 2.5V U3 U2-A 5 ADA4505-2 AI N1() 2.5V 8 DOUT/RDY TO 2 6 3 ADA4505-2 V V CO-AX IN OUT AIN1(+) SDP 1 2 6 1 R5 CE WE DIN 2 4 100k GND R4 C1 C2 RE AD7798 33 4 0.1F 0.1F SCLK C3 C4 3 C6 0.02F 0.02F Q1 10F C5 MMBFJ177 CS 0.02F D S GND REF I N() AVCC AGND R1 R2 R8 11k 11k G 11. 5k R3 L1 1M 1.5H 1 2 5V 5V 2.5V TO 5.5V ADP2503ACPZ Q2 VCC AVCC EXTERNAL 4 2 NTR2101PT1GOSCT INPUT SW1 SW2 D S 5 1 J2-1 PVIN VOUT L2 8 10 J2-2 VIN FB 1k AT 100MHz 7 3 G SYNC/ PGND C11 C12 C13 C10 MODE R7 0.1F 0.1F 2.2F 22F R6 330k EN AGND 1k 1 DGND 6 9 + B2 C9 2 22F R6 1 36. 5k + B1 2 Figure 1. Low Power Gas Detector Circuit Rev. A Circuits from the Lab circuits 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 10129-001CN-0234 Circuit Note The ADP2503 high efficiency, buck-boost regulator allows single- Amplifier U2-A sinks enough current from the CE terminal to supply operation from two AAA batteries and consumes only maintain a 0 V potential between the WE and RE terminals on 38 A when operating in power-save mode. the sensor. The RE terminal is connected to the inverting input of U2-A therefore, no current flows in or out of it. This means Total power consumption for the circuit shown in Figure 1 that the current comes from the WE terminal, and it changes (excluding the AD7798 ADC) is approximately 110 A under linearly with gas concentration. Transimpedance Amplifier U2-B normal conditions (no gas detected) and 460 A under worst- converts the sensor current into a voltage proportional to gas case conditions (2000 ppm CO detected). The AD7798 consumes concentration. approximately 180 A when operational (G = 1, buffered mode) and only 1 A in the power-save mode. The sensor selected for this circuit note is an Alphasense CO-AX Carbon Monoxide sensor. Table 1 shows typical specifications Because of the circuits extremely low power consumption, two associated with carbon monoxide sensors of this general type. AAA batteries can be a suitable power source. When connected to an ADC and a microcontroller, or a microcontroller with a built-in Warning: Carbon monoxide is a toxic gas, and concentrations ADC, battery life can be from over six months to over one year. higher than 250 ppm can be dangerous therefore, exercise extreme care when testing this circuit. CIRCUIT DESCRIPTION Figure 2 shows a simplified schematic of an electrochemical Table 1. Typical Carbon Monoxide Sensor Specifications sensor measurement circuit. Electrochemical sensors work by Parameter Value allowing gas to diffuse into the sensor through a membrane and Sensitivity 55 nA/ppm to 100 nA/ppm interacting with the working electrode (WE). The sensor reference (65 nA/ppm typ) electrode (RE) provides feedback to maintain a constant potential Response Time (t from 0 ppm to 400 ppm CO) <30 sec 90 with the WE terminal by varying the voltage at the counter Range (ppm CO, Guaranteed Performance) 0 ppm to electrode (CE). The direction of the current at the WE terminal 2,000 ppm depends on whether the reaction occurring is oxidation or Overrange Limit (Specifications Not 4,000 ppm reduction. In the case of carbon monoxide, oxidation takes place Guaranteed) therefore, the current flows into the working electrode, which The output voltage of the transimpedance amplifier is requires the counter electrode to be at a negative voltage (typically 300 mV to 400 mV) with respect to the working electrode. The VO = 2.5 V + IWE RF (1) op amp driving the CE terminal should have an output voltage where IWE is the current into the WE terminal, and RF is the range of approximately 1 V with respect to V to provide REF transimpedance feedback resistor (shown as R8 in Figure 1). 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.). This results in a maximum output current of 200 A and a I WE maximum output voltage determined by the transimpedance RF V resistor, as shown in Equation 2. REF + RE nA CE WE V = 2.5 V + 2000 ppm 100 R V O F V REF OUT I I WE WE ppm SENSOR VO = 2.5 V + 200 A RF (2) Figure 2. Simplified Electrochemical Sensor Circuit Operating the circuit with a 5 V supply results in a usable range of 2.5 V at the output of the transimpedance amplifier, U2-B. The current into the WE terminal is less than 100 nA per ppm Selecting a 11.5 k resistor for the transimpedance feedback of gas concentration therefore, converting this current into an resistor gives a maximum output voltage of 4.8 V, which allows output voltage requires a transimpedance amplifier with a very for approximately 8% overrange capability. low input bias current. The ADA4505-2 op amp has CMOS inputs with maximum input bias current of 2 pA at room temperature, Using the sensors typical response of 65 nA/ppm, Equation 3 making this op amp a very good fit for the application. shows the circuit output voltage as a function of ppm of carbon monoxide. The 2.5 V ADR291 establishes the pseudo-ground reference for the circuit, which allows for single-supply operation while consuming V V = 2.5 V + 748 (3) very little quiescent current. O ppm Rev. A Page 2 of 5 10129-002