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Circuit Note

Circuit Note CN-0326. Devices Connected/Referenced 3-Channel, Low Noise, Low Power, Circuits from the Lab reference circuits are engineered and AD7793. 24-Bit Sigma Delta ADC. tested for quick and easy system integration to help solve today's analog, mixed-signal, and RF design challenges. For more Quad-Channel Isolators with ADuM5401. information and/or support, visit Integrated DC/DC Converter MicroPower RRIO Low Noise AD8603. Precision Single CMOS Op Amp Isolated Low Power pH Monitor with Temperature Compensation EVALUATION AND DESIGN SUPPORT The Circuit gives accurate readings for pH values from 0 to Circuit Evaluation Boards 14 with greater than 14-bits of noise-free code resolution and is CN0326 Evaluation Board (EVAL-CN0326-PMDZ) suitable for a variety of industrial applications such as chemical, System Demonstration Platform (EVAL-SDP-CB1Z) food processing, water, and wastewater analysis.

Circuit Note CN-0326 i Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s

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1 Circuit Note CN-0326. Devices Connected/Referenced 3-Channel, Low Noise, Low Power, Circuits from the Lab reference circuits are engineered and AD7793. 24-Bit Sigma Delta ADC. tested for quick and easy system integration to help solve today's analog, mixed-signal, and RF design challenges. For more Quad-Channel Isolators with ADuM5401. information and/or support, visit Integrated DC/DC Converter MicroPower RRIO Low Noise AD8603. Precision Single CMOS Op Amp Isolated Low Power pH Monitor with Temperature Compensation EVALUATION AND DESIGN SUPPORT The Circuit gives accurate readings for pH values from 0 to Circuit Evaluation Boards 14 with greater than 14-bits of noise-free code resolution and is CN0326 Evaluation Board (EVAL-CN0326-PMDZ) suitable for a variety of industrial applications such as chemical, System Demonstration Platform (EVAL-SDP-CB1Z) food processing, water, and wastewater analysis.

2 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 Circuit FUNCTION AND BENEFITS to noise and transient voltages often encountered in harsh 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. FERRITE BEAD: BEAD. MURATA BLM21PG331SN1D. AVDD DVDD. 210 A. IOUT2. 10k VISO VDD1. AIN2(+). 1 F. TO. Pt1000 CS VOA VIA CS. RTD pH SENSOR AD7793. 1M . P1 J1 10k SCLK VOB VIB SCLK. AD8603 AIN1(+). 1 F DIN DIN. VOC VIC. AIN1( ). AIN2( ) DOUT/. VID VOD DOUT/RDY. 10k RDY. RFIN(+)/AIN3(+).

3 5k 1 F. RFIN( )/AIN3( ) GNDISO GND1. GND. ADUM5401. 11821-001. GNDISO. 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 suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices One Technology Way, Box 9106, Norwood, MA 02062-9106, be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause Tel: whatsoever connected to the use of any Circuits from the Lab circuits.

4 (Continued on last page) Fax: 2013 Analog Devices, Inc. All rights reserved. CN-0326 Circuit Note Circuit DESCRIPTION Circuit Details Fundamentals of pH Measurements The design provides a complete solution for pH sensor with The pH value is a measure of the relative amount of hydrogen temperature compensation. The Circuit has three critical stages: and hydroxide ions in an aqueous solution. In terms of molar the pH probe buffer, the ADC, and the digital and power concentrations, water at 25 C contains 1 10 7 moles/liter of isolator as shown in Figure 1. 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.

5 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 pH log( H ). internal resistance. The pH sensing and temperature compensation system is based on Therefore, if the hydrogen ion concentration is 10 2. the AD7793, 24-bit sigma-delta ( - ) with. It has three differential moles/liter, the pH is 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.

6 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 Hz to 470 Hz. solution, which is compared to the potential of the reference electrode. As the solution becomes more acidic (lower pH) the The ADuM5401, quad-channel digital isolator with an 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 becomes more negative ( mV) in comparison to the reference used to isolate the logic signals and the power feedback path in the dc-to-dc converter.

7 Electrode. The difference between these two electrodes is the meas- ured potential. A typical pH probe ideally produces mV/pH Buffer for pH Sensor Interface units at 25oC. This is expressed in the Nernst equation as follows The electrode of a typical pH probe is made up of glass that R T creates an extremely high resistance that can range from 1 M . 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: 210 A. E = voltage of the hydrogen electrode with unknown activity IOUT2. = 30 mV, zero point tolerance pH SENSOR. IBIAS. AD7793. J1. T = ambient temperature in oC 1G . 10k . pH 1M AD8603 AIN1(+). n = 1 at 25 oC, valence (number of charges on ion) VOUT. 1 F. F = 96485 coulombs/mol, Faraday constant AIN1( ). AIN2( ). R = volt-coulombs / K mol, Avogadro's number 10k . RFIN(+)/AIN3(+). + pH = hydrogen ion concentration of an unknown solution 5k.

8 1 F. RFIN( )/AIN3( ). pHISO = 7, reference hydrogen ion concentration GND. 11821-002. 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 solution changes the activity of its hydrogen ions. When the The buffer amplifier bias current flowing through this series solution is heated, the hydrogen ions move faster which result resistance introduces an offset error in the system. To isolate the in an increase in potential difference across the two electrodes. Circuit from this high source resistance, a buffer amplifier with In addition, when the solution is cooled, the hydrogen activity high input impedance and very low input bias current is needed decreases causing a decrease in the potential difference.

9 Electrodes for this application. The AD8603 is used as a buffer amplifier for are designed ideally to produce a zero volt potential when this application as shown in Figure 2. The low input current of placed in a buffer solution with a pH of 7. the AD8603 minimizes the voltage error produced by the bias 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. Circuit Note CN-0326. For 200 fA typical input bias current, the offset error is mV Table 2. Standard RTD Accuracy for DIN-43760. ( pH) for a pH probe that has 1 G series resistance at Class Tolerance 25oC. Even at the maximum input bias current of 1 pA, the DIN 43760 Class A @ 0 C. error is only 1 mV. DIN 43760 Class B @ 0 C. The cut-off frequency of the 10 k /1 F low pass noise filter for the buffer amplifier output is given by f= 1/2 RC, or 16 Hz.

10 Table 3. Standard RTD Accuracy for ASTM E-1137. Guarding, shielding, high insulation resistance standoffs, and Grade Tolerance other such standard picoamp methods must be used to minimize ASTM E-1137 Grade A @ 0 C. leakage at the high impedance input of the AD8603 buffer. ASTM E-1137 Grade B @ 0 C. ADC Channel 1 Configuration, pH sensor The RTD resistance value can be computed as This stage involves measuring the small voltage generated by the pH electrode. Table 1 shows the specifications of a typical RTD Resistance = RTD 0 (1 + T ). pH probe. Based on the Nernst equation, the full range voltage where: from the probe can range from 414 mV ( mV/pH) at RTD Resistance = Resistance value at T. 25 C to 490 mV ( 70 mV/pH) at 80 C. RTD0 = Resistance value at 0 C. Table 1. Specifications of a Typical pH Probe T = ambient temperature = / / C, temperature coefficient defined by DIN.


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