CN-0359 (Rev. B) - Analog Devices

1 Circuit NoteCN-0359 Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve today s Analog , mixed-signal, and RF design challenges. For more information and/or support, visit Devices Connected/Referenced AD8253 10 MHz, 20 V/ s, G = 1, 10, 100, 1000, iCMOS Programmable Gain Instrumentation Amplifier ADuCM360 Low Power, Precision Analog Microcontroller with Dual Sigma-Delta ADCs, ARM Cortex-M3 ADA4627-1 30 V, High Speed, Low Noise, Low Bias Current, JFET Operational Amplifier AD8542 CMOS Rail-to-Rail General-Purpose Amplifiers ADA4000-1 Low Cost, Precision JFET Input Operational Amplifiers ADP2300 A, 20 V, 700 MHz, Nonsynchronous Step-Down Regulator ADA4638-1 30 V, Zero-Drift, Rail-to-Rail Output Precision Amplifier ADP1613 650 MHz, Step-Up PWM DC-to-DC Switching Converters ADA4528-2 Precision, Ultralow Noise, RRIO, Dual.

2 Zero-Drift Op Amp ADG1211 Low Capacitance, Low Charge Injection, 15 V/+12 V, iCMOS Quad SPST Switches ADA4077-2 4 MHz, 7 nV/ Hz, Low Offset and Drift, High Precision Amplifiers ADG1419 On Resistance, 15 V/+12 V/ 5 V, iCMOS SPDT Switch AD8592 CMOS, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers with Shutdown ADM3483 V Slew Rate Limited, Half Duplex, RS-485/RS-422 Transceivers Fully Automatic High Performance Conductivity Measurement System Rev. B 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.

3 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 be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, Box 9106, Norwood, MA 02062-9106, : Fax: 2015 2016 Analog Devices , Inc. All rights reserved. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0359 Circuit Evaluation Board (EVAL-CN0359-EB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a completely self-contained, microprocessor controlled, highly accurate conductivity measurement system ideal for measuring the ionic content of liquids, water quality analysis, industrial quality control, and chemical analysis.

4 A carefully selected combination of precision signal conditioning components yields an accuracy of better than over a conductivity range of S to 10 S (10 M to ) with no calibration requirements. Automatic detection is provided for either 100 or 1000 platinum (Pt) resistance temperature Devices (RTDs), allowing the conductivity measurement to be referenced to room temperature. The system accommodates 2- or 4-wire conductivity cells, and 2-, 3-, or 4-wire RTDs for added accuracy and flexibility. The circuit generates a precise ac excitation voltage with minimum dc offset to avoid a damaging polarization voltage on the conductivity electrodes. The amplitude and frequency of the ac excitation is user-programmable.

5 An innovative synchronous sampling technique converts the peak-to-peak amplitude of the excitation voltage and current to a dc value for accuracy and ease in processing using the dual, 24-bit - ADC contained within the precision Analog microcontroller. The intuitive user interface is an LCD display and an encoder push button. The circuit can communicate with a PC using an RS-485 interface if desired, and operates on a single 4 V to 7 V supply. CN-0359 Circuit Note Rev. B | Page 2 of 10 Figure 1. High Performance Conductivity Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) CIRCUIT DESCRIPTION The excitation square wave for the conductivity cell is generated by switching the ADG1419 between the +VEXC and VEXC voltages using the PWM output of the ADuCM360 microcontroller.

6 It is important that the square wave has a precise 50% duty cycle and a very low dc offset. Even small dc offsets can damage the cell over a period of time. The +VEXC and VEXC voltages are generated by the ADA4077-2 op amps (U9A and U9B), and their amplitudes are controlled by the DAC output of the ADuCM360, as shown in Figure 2. Figure 2. Excitation Voltage Sources REFREF+ 64 LCDMCCOG128064A6S-SPTLY+ + 15V1k 15V1k 15V1k 15V 15V+15V470k 15V+15V 15V+15 VAD8253 15V+15 VADA4000-1 +1nF+ 1nF + +AD8253 ADA4627-11k PWM0+VEXC + 36 + + , A1,U2 WRIADA4638-1+ 1nF+15VU10 ADA4638-1+ 1nF+15VU13430 R34430 R36430 R41430 R42430 F ADG1211 ADG1211 ADG1211 ADG1211 PWM1 PWM2 ADA4638-1+ 1nF+15VU16 ADA4638-1+ 1nF+ FPWM1 PWM2 + 820pF 24 10nF +820pF ADA4528-2U12A24 10nF+ + 820pF ADA4528-2U20B24 + + +820pF ADA4528-2U20A24 100 OR1000 Pt RTDCONDUCTIVITYCELLAIN0 AIN1 AIN2 AIN3 AIN5/IEXCAIN6 AIN7 + +15V 15V+ + , A1.

7 U4 WRI430 ADuCM360 POWERSUPPLIES+VEXC VEXCADM3483RS-485J3J5123412345J1+4V TO +7 VENCODERPEC11R-SJ2S1+ + + + AD8542 AD8542U22AJ-LINKJ4 DACVOUT1 =G1 V1P-PV1P-PV2P-PR47, G1 = 1, 10, 100, 1000G2 = 1, 10, 100, 1000IP-PYX =G2 VOUT2G1 VOUT1 FVOUT2 =G1 V2P-P12970-001+15V +R14110k 1nF ADA4077-2U9AR1515k R1613k R2213k DACFROMADuCM3600V TO + +1nF ADA4077-2U9BR2427k R2727k 15V+VEXC VEXCG = TO +10V0V TO 10V12970-002 Circuit Note CN-0359 Rev. B | Page 3 of 10 The ADA4077-2 has a typical offset voltage of 15 V (A grade), a nA bias current, a nA offset current, and an output current of up to 10 mA, with a dropout voltage of less than V. The U9A op amp has a closed-loop gain of and converts the ADuCM360 internal DAC output (0 V to V) to the +VEXC voltage of 0 V to 10 V.

8 The U9B op amp inverts the +VEXC and generates the VEXC voltage. R22 is chosen such that R22 = R24||R27 to achieve first-order bias current cancellation. The error due to the 15 V offset voltage of U9A is approximately (2 15 V) 10 V = 3 ppm. The primary error introduced by the inverting stage is therefore the error in the resistor matching between R24 and R27. The ADG1419 is a , on-resistance SPDT Analog switch with an on-resistance flatness of 50 m over a 10 V range, making it ideal for generating a symmetrical square wave from the VEXC voltages. The symmetry error introduced by the ADG1419 is typically 50 m 1 k = 50 ppm. Resistor R23 limits the maximum current through the sensor to 10 V/1 k = 10 mA.

9 The voltage applied to the cell, V1, is measured with the AD8253 instrumentation amplifier (U15). The positive input to U15 is buffered by the ADA4000-1 (U14). The ADA4000-1 is chosen because of its low bias current of 5 pA to minimize the error in measuring low currents associated with low conductivities. The negative input of the AD8253 does not require buffering. The offset voltages of U14 and U15 are removed by the synchronous sampling stage and do not affect the measurement accuracy. U15 and U18 are AD8253 10 MHz, 20 V/ s, programmable gain (G = 1, 10, 100, 1000) instrumentation amplifiers with gain error of less than The AD8253 has a slew rate of 20 V/ s and a settling time of s to for G = 1000.

10 Its common-mode rejection is typically 120 dB. The U19 (ADA4627-1) stage is a precision current to voltage converter that converts the current through the sensor to a voltage. The ADA4627-1 has an offset voltage of 120 V (typical, A grade), a bias current of 1 pA (typical), a slew rate of 40 V/ s, and a 550 ns settling time to The low bias current and offset voltage make it ideal for this stage. The symmetry error produced by the 120 V offset error is only 120 V/10 V = 12 ppm. The U22A and U22B (AD8542) buffers supply the V reference to the U18 and U15 instrumentation amplifiers, respectively. The following is a description of the remainder of the signal path in the voltage channel (U17A, U17B, U10, U13, U12A, and U12B).