EVAL-CN0350-PMDZ

ADI's ADA4528 are zero-drift op amps featuring rail-to-rail input and output swing. With an offset voltage of 2.5 µV, offset voltage drift of 0.015 µV/°C, and typical noise of 97 nVPP (0.1 Hz to 10 Hz, AV = +100), the ADA4528-1 and ADA4528-2 are well suited for applications in which error sources cannot be tolerated. These op amps have a wide operating supply range of 2.2 V to 5.5 V, high gain, and excellent CMRR and PSRR specifications. They come in two configurations: the ADA4528-1 has a single circuit whereas the ADA4528-2 has two. The ADA4528 are designed for pressure sensors, strain gauges, and medical instrumentation that require precision amplification of low level signals.

Devices Connected/Referenced ADA4528-2 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 5.0 V, Ultralow Noise, Zero Drift, RRIO, Dual Op Amp

1024-Position,1% Resistor Tolerance Error, 50-TP Memory Digital Rheostat Micropower, 0.1% Accurate,1.2 Voltage Reference Micropower, RRIO, Op Amp 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

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 circuit shown in Figure 1 uses the ADA4528-2, dual auto zero amplifier, which has a maximum offset voltage �V at room temperature and an industry leading 5.6 �V/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. 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. 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.

The circuit shown in Figure 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. 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.

Figure 1. Low Noise Gas Detector Circuit (Simplified Schematic: all Connections and Decoupling not Shown)

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)

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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 at a negative voltage (typically 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 Value 55 nA/ppm to 100 nA/ppm (65 nA/ppm typical) <30 seconds 0 ppm to 2,000 ppm 4,000 ppm

Response Time (t90 from 0 ppm to 400 ppm CO) Range (ppm) CO, Guaranteed Performance) Overrange Limit (Specifications Not Guaranteed)

The output voltage of the transimpedance amplifier V + IWE � RF 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). 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 200 A and a maximum output voltage determined by the transimpedance resistor, as shown in Equation 2.

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 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 �A maximum). Amplifier U2-A sinks enough current from the CE terminal to maintain 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. Warning: carbon monoxide is a toxic gas, and concentrations higher than 250 ppm can be dangerous; therefore, take extreme care when testing this circuit.

Applying V to VREF of the AD7790 allows a usable range V at the output of the transimpedance amplifier, U2-B. Selecting 6.0 k resistor for the transimpedance feedback resistor gives a maximum output voltage 2.4 V. Equation 3 shows the circuit output voltage as a function of ppm of carbon monoxide, using the typical response of the sensor of 65 nA/ppm.

The AD5270-20 has a nominal resistance value 20 k. There are 1024 resistance positions, resulting in resistance step sizes of 19.5. The 5 ppm/�C resistance temperature coefficient of the AD5270-20 is better than that of most discrete resistors, and its �A of supply current is a very small contributor to the overall power consumption of the system. Resistor R4 keeps the noise gain at a reasonable level. Selecting the value of this resistor is a compromise between the magnitude of the noise gain and the sensor settling time errors, when exposed to high concentrations of gas. For the example shown in Equation 33 , which results in a noise gain of 183.

The input noise of the transimpedance amplifier appears at the output, amplified by the noise gain. For this circuit, only low frequency noise is of interest because the frequency of operation of the sensor is very low. The ADA4528-2 has 10 Hz input voltage noise 97 nV p-p; therefore, the noise at the output 18 �V p-p, as shown in Equation 5. VOUTPUTNOISE = 18 �Vp-p (5) Because this is a very low frequency 1/f noise, the noise is very hard to filter. However, because the sensor response is also very slow, it is possible to use a very low frequency, low-pass filter (R5 and C6) with a cutoff frequency of 0.16 Hz. Even with such a low frequency filter, its effect on the sensor response time is negligible, when compared to the 30 second response time of the sensor. The noise free code resolution of the system is determined from the peak-to-peak output noise. The maximum output of the V, so the noise free number of counts is

The circuit shown in Figure 1 uses the EVAL-CN0357-PMDZ circuit evaluation board, the PMD-SDP-IB1Z interposer board, and the EVAL-SDP-CB1Z system demonstration platform (SDPB) controller board. In addition, the EVAL-CN0357-PMDZ comes in a Pmod form factor, making it possible to connect the board to any Pmod controller board, allowing for rapid prototyping. The CN-0357 Evaluation Software communicates with the SDP board to capture data from the EVAL-CN0357-PMDZ circuit evaluation board.

The following equipment is needed for evaluating the CN-0357 circuit: PC with USB port and Windows� XP, or Windows Vista (32-bit), or Windows (32-bit) EVAL-CN0357-PMDZ evaluation board PMD-SDP-IB1Z interposer Board EVAL-SDP-CB1Z controller board CN-0357 Evaluation Software Calibration gas (less than 250 ppm) EVAL-CFTL-6V-PWRZ or equivalent 6 VDC power supply

The noise free code resolution becomes Noise Free Code Resolution = 15.9 bits (7) To take advantage of the entire ADC range available (�1.2 V), the AD8500 micropower, rail-to-rail input/output amplifier is chosen to drive the input of the AD7790. If the entire range is not necessary, the AD8500 can be removed and the internal buffer of the AD7790 can be used in its place. One important characteristic of electrochemical sensors is their very long time constant. When first powered up, it can take several minutes for the output to settle to its final value. When exposed to a midscale step in concentration of the target gas, the time required for the sensor output to reach 90% of its final value can be in the order of 25 seconds to 40 seconds. If the voltage between the RE terminal and the WE terminal has a sudden change in magnitude, it can take several minutes for the output current of the sensor to settle. This long time constant also applies when cycling power to the sensor. To avoid very long start-up times, P-channel JFET Q1 shorts the RE terminal to the WE terminal when the supply voltage drops below the gate-to-source threshold voltage V) of the JFET.

Load the evaluation software by placing the CN-0357 Evaluation Software CD into the CD drive of the PC. Using My Computer, locate the drive that contains the evaluation software CD and run the setup.exe. Follow the on screen prompts for installing and using the evaluation software.

A functional block diagram of the test setup is shown in Figure 3. There is a complete schematic of the evaluation board contained in the CN-0357 Design Support Package, including printouts of the gerber files, as well as a bill of materials.

Electrochemical sensors operate with very small amounts of current, making them well suited for portable, battery-powered instruments. If lower power is required, the ADA4505-2 amplifier has a maximum input bias current 2 pA and consumes only 10 �A per amplifier. However, the noise of the ADA4505-2 is greater than that of the ADA4528-2. The ADR291 precision voltage reference consumes only 12 �A and can be substituted for the ADR3412, if lower power is required. More information can be found in Circuit Note CN-0234.