Circuit Note CN-0349 Devices Connected/Referenced 250 kSPS, 12-Bit Impedance Converter AD5934 Network Analyzer Precision, Low Noise, Rail- to- Rail AD8606 Circuits from the Lab reference designs are engineered and Input/Output, CMOS, Op Amp (Dual) tested for quick and easy system integration to help solve todays 2 analog, mixed-signal, and RF design challenges. For more CMOS, Low Voltage, I C Controlled, ADG715 information and/or support, visit www.analog.com/CN0349. Octal SPST Switches 2 ADuM1250 Dual-Channel I C Digital Isolator ADuM5000 2.5 kV, Isolated DC-to-DC Converter Fully Isolated Conductivity Measurement Data Acquisition System This design solution is optimized for high precision and low EVALUATION AND DESIGN SUPPORT cost, and it uses only five active devices. The circuit has a total Circuit Evaluation Boards error of less than 1% FSR after calibration. The small footprints CN-0349 Circuit Evaluation Board (EVAL-CN0349-PMDZ) of all the components make the circuit ideal for applications SDP-I-PMOD Interposer Board (SDP-PMD-IB1Z) where printed circuit board (PCB) real estate is a premium. The System Demonstration Platform, SDP-B (EVAL-SDP-CB1Z) digital output of the circuit is fully isolated therefore, the ground Design and Integration Files loop interference is eliminated, making it ideal for harsh Schematics, Layout Files, Bill of Materials industrial environments. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 provides a complete, robust data acquisition solution for processing conductivity cell outputs, including temperature correction. This circuit is ideal for measuring the ionic content of liquids, water quality analysis, and chemical analysis. U2B +3.3V AD8606 +3.3V U6 +3.3V FXO-HC536R-1 R12 +3.3V E/D 10k U4 VDD OUT U1 ADuM5000 ADG715 GND R13 U3 V V ISO DD1 V 10k RESET DD AD5934 NC RC R R SEL FB FB MCLK R9 100 R11 R10 S1 D1 V RC SEL OUT 20k 20k RFB AVDD R8 1k S2 D2 GND GND ISO 1 VIN J2 J1 R6 DNP +3.3V DVDD S3 D3 U5 IMOD CON 5-PIN OA ADuM1250 8-PIN 1 R3 100 S4 D4 V V +3.3V IN R1 DD2 DD1 SDA SDA VDD/2 DDS, SDA 10k RTD 2 U2A R4 1k S5 D5 DAC, SDA SDA SDA IN 2 1 AD8606 SCL SCL VOUT ADC, SCL R7 10k S6 D6 Pt100 DSP 3 SCL SCL SCL IN 2 1 C1 R R CAL CAL 470nF AGND DGND GND GND GND IN 4 2 1 S7 D7 R5 Y 10k CELL 5 R2 100 S8 D8 GND IN Y /RTD X CONDUCTIVITY CELL SDA SCL A0 A1 GND V SS Y X Y = CELL 1 R2 Y X SDA SCL Figure 1. Fully Isolated Data Acquisition System for Conductivity Measurements (Simplified Schematic: all Pins, Connections, and Decoupling not Shown) Rev. A 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 20142017 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) 11908-001CN-0349 Circuit Note All aqueous solutions conduct electricity to some degree. The CIRCUIT DESCRIPTION measure of the ability of a solution to conduct electricity is The circuit shown in Figure 1 incorporates the AD5934 12-bit called conductance, and it is the reciprocal of resistance. The impedance converter, the ADG715 octal single-pole, single-throw unit of measurement of conductance is Siemens (S). Adding (SPST) switch, the AD8606 rail-to-rail op amp, the ADuM1250 electrolytes such as salts, acids, or bases to pure water increases 2 dual I C isolator, and the ADuM5000 isolated dc-to-dc converter to the conductance (and decreases resistance). Resistivity is create a complete data acquisition system for conductivity expressed in cm, and conductivity is expressed in S/cm, measurements. The circuit has an 8-pin IMOD connector on mS/cm, or S/cm. board that can be used for connection to a customer For the purposes of this circuit note, Y is used as the general microprocessor or a field programmable gate array (FPGA). symbol for conductivity measured in S/cm, mS/cm, or S/cm. The AD5934 is a high precision impedance converter system However, in many cases, the distance term is dropped for solution that combines an on-chip, programmable direct digital convenience, and the conductivity is simply expressed as S, synthesizer (DDS) with a 12-bit, 250 kSPS analog-to-digital mS, or S. converter (ADC). The tunable frequency generator allows an external complex impedance to be excited with a known frequency. A conductivity system measures conductance by means of The AD5933 is a similar device with a 1 MSPS ADC. The on- electronics connected to a sensor immersed in a solution. The analyzer circuitry impresses an alternating voltage on the sensor board ADC monitors the voltage and current of the unknown and measures the size of the resulting current, which is related impedance. An on-board digital signal processor (DSP) engine to the conductivity. Because conductivity has a large temperature calculates the discrete Fourier transform (DFT). The DFT coefficient (up to 4%/C), an integral temperature sensor is algorithm returns a real (R) and an imaginary (I) data-word at each output frequency. incorporated into the circuitry to adjust the reading to a standard temperature, usually 25C (77F). When measuring The AD8606 op amp was chosen because of its low offset solutions, the temperature coefficient of the conductivity of the voltage (65 V maximum), low bias current (1 pA maximum), water itself must be considered. To compensate accurately for and low noise (12 nV/Hz maximum). the temperature, a second temperature sensor and The ADG715 is a complementary metal oxide semiconductor compensation network must be used. (CMOS), octal SPST switch that is controlled via a 2-wire serial 2 Conductivity Sensors interface that is compatible with the I C interface standard. Low The contacting type sensor usually consists of two electrodes power consumption, a low operating supply range of 2.7 V to that are insulated from one another. The electrodes, typically 5.5 V, low on-resistance (typically 2.5 ), and a small 24-lead Type 316 stainless steel, titanium palladium alloy, or graphite, TSSOP package make this device ideal for many applications. are specifically sized and spaced to provide a known cell The ADuM5000 is an isolated dc-to-dc converter with a 3.3 V constant. Theoretically, a cell constant of 1.0/cm describes two or 5 V output, based on the Analog Devices isoPower technology, electrodes, each sized one square centimeter in area, and spaced and packaged in a 16-lead SOIC. one centimeter apart. Cell constants must be matched to the The ADuM1250 is a hot swappable, digital isolator with non- analyzer for a given range of operation. For instance, if a sensor latching, bidirectional communication channels that are compatible with a cell constant of 1.0/cm is used in pure water with a 2 with I C interfaces, based on the iCoupler chip scale transformer conductivity of 1 S/cm, the cell has a resistance of 1 M. technology from Analog Devices, Inc., and packaged in an Conversely, the same cell in seawater has a resistance of 30 . 8-lead SOIC. Because the resistance ratio is so large, it is difficult for ordinary instruments to accurately measure such extremes with only one Conductivity Theory cell constant. The resistivity, , of a material or liquid is defined as the resistance of a cube of the material with perfectly conductive When measuring the 1 S/cm solution, the cell is configured contacts on opposite faces. The resistance for other shapes can with large electrodes spaced a small distance apart. For be calculated by example, a cell with a cell constant of 0.01/cm results in a cell resistance of approximately 10,000 , which can be measured R = L/A quite accurately. Therefore, the measuring instrument can where: operate over the same range of cell resistance for both ultra- L is the distance between the contacts. pure water and high conductivity seawater, by using cells with A is the area of the contacts. different cell constants. Resistivity is measured in units of cm. A 1 cm material has Temperature Compensation a resistance of 1 when contacted on opposite faces of a 1 cm Conductivity measuring system accuracy is only as good as its 1 cm 1 cm cube. temperature compensation. Because common solution temperature Conductance is the reciprocal of resistance, and conductivity is coefficients vary in the order of 1%/C to 3%/C, or more, the reciprocal of resistivity. Rev. A Page 2 of 10