Circuit Note CN-0428 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays LTC6078 High Impedance pH Buffer Op Amp analog, mixed-signal, and RF design challenges. For more Precision Analog Microcontroller with information and/or support, visit www.analog.com/CN0428. ADuCM355 Chemical Sensor Interface Water Quality Measurement System important parameters that affect water quality, including EVALUATION AND DESIGN SUPPORT chemical indicators, biological and bacteriological indicators Circuit Evaluation Boards and even some low level contaminants like heavy metals. Many Water Quality Sensor Board (EVAL-CN0428-EBZ) of these indicative measurements are pertinent to determining Arduino Shield Interface Board (EVAL-M355-ARDZ-INT) important quality parameters of the tested analyte. ADICUP3029 Arduino Form Factor Ultra Low Power ARM Cortex-M3 Development Platform (EVAL-ADICUP3029) The circuit shown in Figure 1 is a modular sensing platform Design and Integration Files that allows the user to design a flexible electrochemical water Schematics, Layout Files, Bill of Materials, Software quality measurement solution. Its high level of integration enables an electrochemical measurement platform applicable CIRCUIT FUNCTION AND BENEFITS to a variety of water quality probes including pH, oxidation Many important liquid analyses like pH rely on electrochemistry, reduction potential (ORP), and conductivity cells. a branch of chemistry that characterizes the behavior of The system allows up to four probes to be connected at one reduction-oxidation (redox) reactions by measuring the transfer time for different water quality measurements. of electrons from one reactant to another. Electrochemical techniques can be used directly or indirectly to detect several 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 2019 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) CN-0428 Circuit Note ADuCM355 LOW POWER POTENTIOSTAT ULP CORTEX PROCESSOR RE0 26MHz CORTEX-M3 128kB FLASH, 64kB SRAM HIGH IMPEDANCE ADVANCED SECURITY S1 BNC SENSOR pH BUFFER OP AMP FEATURES SE0 INPUT Z LTC6078 (2/2) PH AIN6 Z PH HIGH PERFORMANCE AFE V ZERO0 <0.1Hz to 200kHz IMPEDANCE MEASUREMENT ENGINE 16-BIT ADC INT S2 FLEXIBLE SWITCH MATRIX +PGA/AAF CONFIGURABLE 16-BIT HI-Z RECEIVE CHANNEL LTC6078 (1/2) SPI, I2C, OR UART COMMS HIGH IMPEDANCE TIA AIN5 TO ADICUP3029 CHANNEL (10M GAIN) LOW POWER POTENTIOSTAT DIGITAL PREPROCESSING FEATURES RE1 V ZERO0 DFT ACCELERATOR RCA/HEADER WAVEFORM GEN SE1 DIGITAL FILTERS (SINC3, SINC2, AVG, 50/60Hz) RESISTIVE TEMPERATURE SENSORS (RTD, THERMISTOR) RCAL0 RCAL1 AIN0 AIN1 AIN3 DE0 AIN2 DE1 200 25.5k 2k 200k 1M EXTERNAL CALIBRATION AND TIA RESISTORS Figure 1. Simplified Circuit Block Diagram as the solution becomes more alkaline (higher pH), the potential of CIRCUIT DESCRIPTION the glass electrode becomes more negative (mV) in comparison Fundamentals of pH Measurements to the reference electrode. The difference between these two The pH value is a measure of the relative amount of hydrogen electrodes is the measured potential. A typical pH probe ideally and hydroxide ions in an aqueous solution. In terms of molar produces 59.154 mV/pH units at 25C. This is expressed in the 7 concentrations, water at 25C contains 1 10 moles/liter of Nernst equation as follows: hydrogen ions and the same concentration of hydroxide ions. A 2.303RT 273.1 neutral solution is one in which the hydrogen ion concentration Ea pHpH ISO exactly equals the hydroxide ion concentration. pH is another nF way of expressing the hydrogen ion concentration and is where: defined as follows: E is the voltage of the hydrogen electrode with unknown activity. + pH = -log (H ) a = 30 mV, zero point tolerance. 10 2 T is the ambient temperature in C. Therefore, if the hydrogen ion concentration is 1.0 10 n = 1 at 25C, valence (number of charges on ion). moles/liter, the pH is 2.00. F = 96485 coulombs/mol, Faraday constant. The pH electrodes are electrochemical sensors used by many R = 8.314 volt-coulombs/K mol, ideal gas constant. industries but are of particular importance to the water and pH is the hydrogen ion concentration of an unknown solution. wastewater industry. The pH probe consists of a glass measuring pH is the reference hydrogen ion concentration. Consult the ISO electrode and a reference electrode, which is analogous to a probe documentation. Typical pH = 7. ISO battery. When the probe is placed in a solution, the measuring The equation shows that the voltage generated is dependent on electrode generates a voltage depending on the hydrogen the acidity or alkalinity of the solution and varies with the activity of the solution, which is compared to the potential of hydrogen ion activity in a known manner. The change in the reference electrode. As the solution becomes more acidic temperature of the solution changes the activity of its hydrogen (lower pH), the potential of the glass electrode becomes more ions. When the solution is heated, the hydrogen ions move positive (+mV) in comparison to the reference electrode. And Rev. 0 Page 2 of 9