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information and/or support, visit www.analog.com/CN0326. Fax: 781.461.3113 ©2013 Analog Devices, Inc. All rights reserved. temperature compensation for high accuracy. food processing, water, and wastewater analysis. Figure 1. pH Sensor Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
moles/liter, the pH is 2.00. electrode and a reference electrode, which is analogous to a battery. in an increase in potential difference across the two electrodes. placed in a buffer solution with a pH of 7. Radiometer Analytical SAS, Villeurbanne Cedex, France. isolator as shown in Figure 1. as a buffer to the input of one of the channels of the AD7793. can be varied from 4.17 Hz to 470 Hz. source as shown in Figure 2. Figure 2. pH Sensor and Buffer Interface to ADC (Simplified Schematic: All current flowing through the electrode resistance.
the buffer amplifier output is given by f= 1/2πRC, or 16 Hz. leakage at the high impedance input of the AD8603 buffer. Table 1. Specifications of a Typical pH Probe signal from the pH probe is ±490 mV at 80°C.
1.05 V common-mode bias voltage that also serves as the ADC
combination of the RTD and precision resistor (5 kΩ, 0.1%). The temperature coefficient for pure platinum is 0.003926 Ω/Ω/°C. recognizes two grades as shown in Table 3. Table 2. Standard RTD Accuracy for DIN-43760 Table 3. Standard RTD Accuracy for ASTM E-1137 not affect the accuracy of the system. RTD is less sensitive to wiring resistance errors than a 100 Ω RTD. amplifier, and the associated drift.
actual system results are presented in the next section. source was used to simulate the input of a pH sensor. capture the data according to the user defined calibration option. an equivalent pH reading spread of 0.00053 pH peak-to-peak. Figure 5. Histogram Showing Output Code Spread with AD7793 Input Pins with for a 200 MΩ pH probe impedance. Figure 6. pH Sensor Simulated Output Voltage (with Associated Linearity
1 MΩ, 100 MΩ, and 200 MΩ)
The test dat a was taken using the board shown in Figure 7. CN-0326 Design Support package. Figure 7. Photo of EVAL-CN0326-PMDZ Board is a 16-bit ADC while the AD7785 is a 20-bit ADC. amplifier which is in the same family as the AD8603. which also provides four independent isolation channels. spaced, 25 mil square, right angle pin-header connector.
Rev. 0 | Page 7 of 7 Test Agilent E3631A and Y okogawa GS200 precision voltage supplies were used to simulate the sensor output. The negative terminal of the Yo k o g aw a is connected to the negative terminal of the ADC for the pH sensor. The positive terminal is in series with the resistor, which is connected to the positive terminal of the ADC as shown in Figure 8. The Y okogawa generates the ±420 mV , which then simulates the pH sensor output, and the series resistor is then varied to simulate the impedance of the glass electrode of the pH probe as shown in Figure 8. The CN-0326 Evaluation Software is used to capture the data from the E VA L-CN0326-PMDZ circuit board using the setup seen in Figure 8. Details regarding the use of the software can be found in the CN-0326 Software User Guide. LEARN MORE CN-0326 Design Support Package: www.analog.com/CN0326-DesignSupport MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of ADC Input Noise—Is No Noise Good Noise? Analog Devices. MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC Basics, Analog Devices. MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC Advanced Concepts and Applications, Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND", Analog Devices. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. MT-037 Tutorial, Op Amp Input Offset Voltage. MT-038 Tutorial, Op Amp Input Bias Current MT-040 Tutorial, Op Amp Input Impedance MT-095 Tutorial, EMI, RFI, and Shielding Concepts MT-101 Tutorial, Decoupling Techniques, Analog Devices Kester, Walt. 1999. High Impedance Sensors. Section 5. Analog Devices. Kester, Walt. 1999. Temperature Sensors. Section 7. Analog Devices. Chen, Baoxing. 2006. iCoupler® Products with isoPower® Technology: Signal and Power Transfer Across Isolation Barrier Using Microtransformers. Analog Devices. Wayne, Scott. 2005. “iCoupler® Digital Isolators Protect RS-232, RS-485, and CAN Buses in Industrial, Instrumentation, and Computer Applications. ” Analog Dialogue, Volume 39. Analog Devices (October). Brian Kennedy and Mark Cantrell, Recommendations for Control of Radiated Emissions with iCoupler Devices, Application Note AN-1109, Analog Devices. pH Theory and Practice, Radiometer Analytical, SAS, Villeurbanne Cedex, France. Data Sheets and Evaluation Boards AD7793 Data Sheet AD7793 Evaluation Board ADUM5401 Data Sheet ADuM5401 Evaluation Board AD8603 Data Sheet
REVISION HISTORY
9/13—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of yo ur product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN11821-0-9/13(0)