Circuit Note CN-0326 Devices Connected/Referenced 3-Channel, Low Noise, Low Power, AD7793 Circuits from the Lab reference circuits are engineered and 24-Bit Sigma Delta ADC tested for quick and easy system integration to help solve todays Quad-Channel Isolators with analog, mixed-signal, and RF design challenges. For more ADuM5401 Integrated DC/DC Converter information and/or support, visit www.analog.com/CN0326. MicroPower RRIO Low Noise AD8603 Precision Single CMOS Op Amp Isolated Low Power pH Monitor with Temperature Compensation The circuit gives 0.5% accurate readings for pH values from 0 to EVALUATION AND DESIGN SUPPORT 14 with greater than 14-bits of noise-free code resolution and is Circuit Evaluation Boards suitable for a variety of industrial applications such as chemical, CN0326 Evaluation Board (EVAL-CN0326-PMDZ) food processing, water, and wastewater analysis. System Demonstration Platform (EVAL-SDP-CB1Z) SDP PMOD Interposer Board (SDP-PMD-IB1Z) This circuit supports a wide variety of pH sensors that have very Design and Integration Files high internal resistance that can range from 1 M to several Schematics, Layout Files, Bill of Materials G, and digital signal and power isolation provides immunity to noise and transient voltages often encountered in harsh CIRCUIT FUNCTION AND BENEFITS industrial environments. The circuit shown in Figure 1 is a completely isolated low power pH sensor signal conditioner and digitizer with automatic temperature compensation for high accuracy. 3.3V ISO FERRITE BEAD: BEAD MURATA BLM21PG331SN1D 3.3V 3.3V ISO AV DV DD DD 210A IOUT2 10k V V ISO DD1 AIN2(+) 1F TO Pt1000 3.3V ISO CS V V CS OA IA RTD pH SENSOR AD7793 1M P1 J1 10k SCLK V V SCLK OB IB AIN1(+) AD8603 1F DIN DIN V V OC IC AIN1() DOUT/ AIN2() V V DOUT/RDY ID OD RDY 10k RFIN(+)/AIN3(+) 1F 5k GND GND RFIN()/AIN3() ISO 1 GND ADUM5401 GND ISO Figure 1. pH Sensor Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. 0 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 2013 Analog Devices, Inc. All rights reserved. whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 11821-001CN-0326 Circuit Note Circuit Details CIRCUIT DESCRIPTION Fundamentals of pH Measurements The design provides a complete solution for pH sensor with temperature compensation. The circuit has three critical stages: The pH value is a measure of the relative amount of hydrogen the pH probe buffer, the ADC, and the digital and power and hydroxide ions in an aqueous solution. In terms of molar 7 isolator as shown in Figure 1. concentrations, water at 25C contains 1 10 moles/liter of hydrogen ions and the same concentration of hydroxide ions. A The AD8603, a precision micro power (50 A maximum) and neutral solution is one in which the hydrogen ion concentration low noise (22 nV/Hz) CMOS operational amplifier configured exactly equals the hydroxide ion concentration. pH is another as a buffer to the input of one of the channels of the AD7793. way of expressing the hydrogen ion concentration and is The AD8603 has a typical input bias current of 200 fA that defined as follows: provides an effective solution to the pH probe that has high internal resistance. pH log(H ) The pH sensing and temperature compensation system is based on 2 Therefore, if the hydrogen ion concentration is 1.0 10 the AD7793, 24-bit sigma-delta (-) with. It has three differential moles/liter, the pH is 2.00. analog inputs and has an on-chip, low noise, programmable gain The pH electrodes are electrochemical sensors used by many amplifier (PGA) that ranges from unity gain to 128. The AD7793 industries but are of particular importance to the water and waste- consumes only a maximum of 500 A making it suitable for any water industry. The pH probe consists of a glass measuring low power applications. It has a low noise, low drift internal band electrode and a reference electrode, which is analogous to a battery. gap reference and can accept external differential reference. The When the probe is place in a solution, the measuring electrode output data rate from the part is software programmable and generates a voltage depending on the hydrogen activity of the can be varied from 4.17 Hz to 470 Hz. solution, which is compared to the potential of the reference The ADuM5401, quad-channel digital isolator with an electrode. As the solution becomes more acidic (lower pH) the integrated dc-to-dc converter provides the digital signal and potential of the glass electrode becomes more positive (+mV) in power isolation between the microcontroller and the AD7793 comparison to the reference electrode and as the solution becomes digital lines. The iCoupler chip-scale transformer technology is more alkaline (higher pH) the potential of the glass electrode used to isolate the logic signals and the power feedback path in becomes more negative (mV) in comparison to the reference the dc-to-dc converter. electrode. The difference between these two electrodes is the meas- Buffer for pH Sensor Interface ured potential. A typical pH probe ideally produces 59.154 mV/pH o units at 25 C. This is expressed in the Nernst equation as follows The electrode of a typical pH probe is made up of glass that creates an extremely high resistance that can range from 1 M 2.303RT 273.1 E a pH pH ISO to 1 G and acts as a resistance in series with the pH voltage nF source as shown in Figure 2. where: 210A IOUT2 E = voltage of the hydrogen electrode with unknown activity pH SENSOR = 30 mV, zero point tolerance 3.3V AD7793 ISO I BIAS J1 o 1G T = ambient temperature in C 10k 1M pH AIN1(+) o AD8603 n = 1 at 25 C, valence (number of charges on ion) V OUT 1F AIN1() F = 96485 coulombs/mol, Faraday constant AIN2() 10k R = 8.314 volt-coulombs /K mol, Avogadro s number RFIN(+)/AIN3(+) +1.05V 1F pH = hydrogen ion concentration of an unknown solution RFIN()/AIN3() 5k GND pH = 7, reference hydrogen ion concentration ISO The equation shows that the voltage generated is dependent on Figure 2. pH Sensor and Buffer Interface to ADC (Simplified Schematic: All the acidity or alkalinity of the solution and varies with the hydrogen Connections, RTD, and Decoupling Not Shown.) ion activity in a known manner. The change in temperature of The buffer amplifier bias current flowing through this series the solution changes the activity of its hydrogen ions. When the resistance introduces an offset error in the system. To isolate the solution is heated, the hydrogen ions move faster which result circuit from this high source resistance, a buffer amplifier with in an increase in potential difference across the two electrodes. high input impedance and very low input bias current is needed In addition, when the solution is cooled, the hydrogen activity for this application. The AD8603 is used as a buffer amplifier for decreases causing a decrease in the potential difference. Electrodes this application as shown in Figure 2. The low input current of are designed ideally to produce a zero volt potential when the AD8603 minimizes the voltage error produced by the bias placed in a buffer solution with a pH of 7. current flowing through the electrode resistance. A good reference on the theory of pH is pH Theory and Practice, Radiometer Analytical SAS, Villeurbanne Cedex, France. Rev. 0 Page 2 of 7 11821-002