Circuit Note CN-0359 Devices Connected/Referenced 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 www.analog.com/CN0359. AD8253 10 MHz, 20 V/μs, G = 1, 10, 100, 1000, iCMOS Programmable Gain Instrumentation Amplifier ADuCM360 Low Power, Precision Analog Microcontroller with Dual SigmaDelta ADCs, ARM Cortex-M3 ADA4627-1 30 V, High Speed, Low Noise, Low Bias Current, JFET Operational Amplifier AD8542 CMOS Rail-to-Rail GeneralPurpose Amplifiers ADA4000-1 Low Cost, Precision JFET Input Operational Amplifiers ADP2300 1.2 A, 20 V, 700 kHz/1.4 MHz, Nonsynchronous Step-Down Regulator ADA4638-1 30 V, Zero-Drift, Rail-to-Rail Output Precision Amplifier ADP1613 650 kHz/1.3 MHz, Step-Up PWM DC-to-DC Switching Converters ADA4528-2 Precision, Ultralow Noise, RRIO, Dual, 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 2.1 Ω 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 3.3 V Slew Rate Limited, Half Duplex, RS-485/RS-422 Transceivers Fully Automatic High Performance Conductivity Measurement System EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0359 Circuit Evaluation Board (EVAL-CN0359-EB1Z) Design and Integration Files Schematics, Source Code, 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. A carefully selected combination of precision signal conditioning components yields an accuracy of better than 0.3% over a conductivity range of 0.1 µS to 10 S (10 MΩ to 0.1 Ω) with no calibration requirements. Automatic detection is provided for either 100 Ω or 1000 Ω 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. 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 Σ-Δ 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. 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 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2015 Analog Devices, Inc. All rights reserved. CN-0359 Circuit Note +3.3V J3 1 AIN6 RX AIN1 PWM1 +15V ADC2 AIN2 PWM2 −15V ADC3 AIN3 1kΩ +0.115V P0.0 U2WRI AIN6 P2.0 U4WRI P1.2 A0 A1 AIN9 R23 +VEXC –VEXC G1 = 1, 10, 100, 1000 PWM0 +15V AD8253 A0, A1, − U2WRI U15 430Ω + REF 2 V1P-P YX 5 3 ADA4000-1 P2.1 AGND EP ¼ ADG1211 ADG1419 1kΩ ADuCM360 AIN5/IEXC AIN8 36Ω 1 PWM0 AIN7/IEXC AIN9 J5 −15V U14 − CONDUCTIVITY CELL +1.65V −15V 470kΩ − 1nF IP-P U22B R47,1.0kΩ G2 × VOUT2 G1 × VOUT1 ADA4638-1 + C73 4.7µF R19 7.5kΩ U10 − +3.3V − +1.65V ½ ADA4528-2 + R29 ADA4638-1 7.5kΩ + − U13 − −15V 1k Ω 24Ω U12A +1.65V PWM1 − 10nF 820pF +15V C84 4.7µF ADA4638-1 − –15V 1k Ω +1.65V +15V ADC1 R52 1.2kΩ R48 10nF VOUT2 = G1 × 0.16 × V2P-P 820pF ADA4638-1 7.5kΩ − U21 − 24Ω ½ ADA4528-2 + 1nF + PWM2 +3.3V − U20B R42 430Ω C93 4.7µF R37 7.5kΩ U16 ¼ ADG1211 + U22A ADC2 ½ ADA4528-2 + R38 1.2kΩ + +15V +1.65V VOUT1 = G1 × 0.16 × V1P-P 820pF +15V ADC3 10nF R31 1.2kΩ 1nF R41 430Ω G2 = 1, 10, 100, 1000 × 1mS 24Ω U12B –15V 1nF −15V YX = 820pF +15V ¼ ADG1211 +15V AD8253 A0, A1, U19 U4WRI ADA4627-1 + V2P-P U18 + 430Ω −15V − REF − R34 430Ω ½ AD8542 1nF +VEXC R20 1.2kΩ R36 430Ω PWM2 POWER SUPPLIES +1.65V −VEXC ¼ ADG1211 + +3.3V 128 × 64 LCD MCCOG128064A6S-SPTLY 1k Ω + 4 J1 +4V TO +7V DAC C50 PWM1 4.7µF +3.3V +15V S1 ENCODER PEC11R-S AIN0 AIN5/IEXC 4 J4 J-LINK ADC1 +3.3V 3 J2 ADM3483 RS-485 ADC0 AIN7/IEXC AIN8 2 AVDD 24Ω ADC0 U20A –15V 1k Ω ½ AD8542 +1.65V ½ ADA4528-2 + 10nF 12970-001 100Ω OR 1000Ω Pt RTD R13 1.5kΩ 1nF Figure 1. High Performance Conductivity Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) CIRCUIT DESCRIPTION R14 110kΩ 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. 1nF R15 15kΩ +15V − DAC FROM ADuCM360 0V TO +1.2V R16 13kΩ G = 8.33 U9A + ½ ADA4077-2 0V TO +10V R27 27kΩ 1nF R24 27kΩ − R22 13kΩ −VEXC U9B + ½ ADA4077-2 −15V Figure 2. Excitation Voltage Sources Rev. A | Page 2 of 10 +VEXC 0V TO −10V 12970-002 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. 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. Circuit Note CN-0359 The ADG1419 is a 2.1 Ω, on-resistance SPDT analog switch with an on-resistance flatness of 50 mΩ 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 mΩ ÷1 kΩ = 50 ppm. Resistor R23 limits the maximum current through the sensor to 10 V/1 kΩ = 10 mA. 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/µs, programmable gain (G = 1, 10, 100, 1000) instrumentation amplifiers with gain error of less than 0.04%. The AD8253 has a slew rate of 20 V/µs and a settling time of 1.8 µs to 0.001% for G = 1000. 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 µV (typical, A grade), a bias current of 1 pA (typical), a slew rate of 40 V/µs, and a 550 ns settling time to 0.01%. The low bias current and offset voltage make it ideal for this stage. The symmetry error produced by the 120 µV offset error is only 120 µV/10 V = 12 ppm. The U22A and U22B (AD8542) buffers supply the 1.65 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). The operation of the current channel (U17C, U17D, U16, U21, U20A, and U20B) is identical. The ADuCM360 generates the PWM0 square wave switching signal for the ADG1419 switch as well as PWM1 and PWM2 synchronizing signals for the synchronous sampling stages. The cell voltage and the three timing waveforms are shown in Figure 3. +VEXC CELL VOLTAGE 0 –VEXC PWM0 PWM1 TRACK −V TRACK −V HOLD –V 20µs 500ns PWM2 TRACK +V HOLD +V 20µs HOLD +V 500ns 12970-003 The ADA4077-2 has a typical offset voltage of 15 µV (A grade), a 0.4 nA bias current, a 0.1 nA offset current, and an output current of up to ±10 mA, with a dropout voltage of less than 1.2 V. The U9A op amp has a closed-loop gain of 8.33 and converts the ADuCM360 internal DAC output (0 V to 1.2 V) to the +VEXC voltage of 0 V to 10 V. 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 µV offset voltage of U9A is approximately (2 × 15 µV) ÷ 10 V = 3 ppm. The primary error introduced by the inverting stage is therefore the error in the resistor matching between R24 and R27. Figure 3. Cell Voltage and Track-and-Hold Timing Signals The output of the AD8253 in amp (U15) drives two parallel track-and-hold circuits composed of ADG1211 switches (U17A/U17B), series resistors (R34/R36), hold capacitors (C50/C73), and unity-gain buffers (U10/U13). The ADG1211 is a low charge injection, quad SPST analog switch, operating on a ±15 V power supply with up to ±10 V input signals. The maximum charge injection due to switching is 4 pC, which produces a voltage error of only 4 pC ÷ 4.7 µF = 0.9 µV. The PWM1 signal causes the U10 track-and-hold buffer to track the negative cycle of the sensor voltage and then hold it until the next track cycle. The output of the U10 track-and-hold buffer is therefore a dc level corresponding to the negative amplitude of the sensor voltage square wave. Similarly, the PWM2 signal causes the U13 track-and-hold buffer to track the positive cycle of the sensor voltage and then hold it until the next track cycle. The output of the U13 trackand-hold buffer is therefore a dc level corresponding to the positive amplitude of the sensor voltage square wave. The bias current of the track-and-hold buffers (ADA4638-1) is 45 pA typical, and the leakage current of the ADG1211 switch is 20 pA typical. Therefore, the worst-case leakage current on the 4.7 µF hold capacitors is 65 pA. For a 100 Hz excitation frequency, the period is 10 ms. The drop voltage over one-half the period (5 ms) due to the 65 pA leakage current is (65 pA × 5 ms) ÷ 4.7 µF = 0.07 µV. The offset voltage of the ADA4638-1 zero-drift amplifier is only 0.5 µV typical and contributes negligible error. The final stages in the signal chain before the ADC are the ADA4528-2 inverting attenuators (U12A and U12B) that have a gain of −0.16 and a common-mode output voltage of +1.65 V. The ADA4528-2 has an offset voltage of 0.3 µV typical and therefore contributes negligible error. Rev. A | Page 3 of 10 CN-0359 Circuit Note The attenuator stage reduces the ±10 V maximum signal to ±1.6 V with a common-mode voltage of +1.65 V. This range is compatible with the input range of the ADuCM360 ADC input, which is 0 V to 3.3 V (1.65 V ± 1.65 V) for an AVDD supply of 3.3 V. The attenuator stages also provide noise filtering and have a −3 dB frequency of approximately 198 kHz. The differential output of the voltage channel, VOUT1, is applied to the AIN2 and AIN3 inputs of the ADuCM360. The differential output of the current channel, VOUT2, is applied to the AIN0 and AIN1 inputs of the ADuCM360. The equations for the two outputs are given by VOUT1 = G1 × 0.16 × V1P-P (1) VOUT2 = G2 × 0.16 × V2P-P (2) The cell current is given by IP-P = V1P-P × YX (3) System Accuracy Measurements The following four resistors affect the accuracy in the VOUT1 voltage channel: R19, R20, R29, and R31. The following five resistors affect the accuracy in the VOUT2 current channel: R47, R37, R38, R48, and R52. Assuming that all nine resistors are 0.1% tolerance, and including the 0.04% gain error of the AD8253, a worst-case error analysis yields approximately 0.6%. The analysis is included in the CN-0359 Design Support Package. In practice, the resistors are more likely to combine in an RSS manner, and the RSS error due to the resistor tolerances in the positive or negative signal chain is √5 × 0.1% = 0.22%. Accuracy measurements were taken using precision resistors from 1 Ω to 1 MΩ (1 S to 1 µS) to simulate the conductivity cell. Figure 4 shows the results, and the maximum error is less than 0.1%. 0.04 The V2P-P voltage is given by V2P-P = IP-P × R47 0.02 (4) Solving Equation 4 for IP-P and substituting into Equation 3 yields the following for YX: V2P − P YX = V1P − P × R 47 (5) ERROR (%) 0 Solving Equation 1 and Equation 2 for V1P-P and V2P-P and substituting into Equation 5 yields the following: YX = –0.06 –0.08 G2 × VOUT2 (6) G1 × VOUT1 × R 47 G2 × VOUT2 G1 × VOUT1 × 1 mS (7) The AD8253 gain error (G1 and G2) is 0.04% maximum, and R47 is chosen to be a 0.1% tolerance resistor. From this point, the resistors in the VOUT1 and VOUT2 signal chain determine the overall system accuracy. The software sets the gain of each AD8253 as follows: • –0.10 1µ Equation 7 shows that the conductivity measurement depends on G1, G2, and R47, and the ratio of VOUT2 to VOUT1. Therefore, a precision reference is not required for the ADCs within the ADuCM360. • –0.04 If the ADC code is over 93.2% of full scale, the gain of the AD8253 is reduced by a factor of 10 on the next sample. If the ADC code is less than 9.13% of full scale, the gain of the AD8253 is increased by a factor of 10 on the next sample. 10µ 0.1m 1m 10m CONDUCTIVITY (S) 0.1 1 12970-004 YX = –0.02 Figure 4. System Error (%) vs. Conductivity of 1 µS to 1 S RTD Measurement Conductivity measuring system accuracy is only as good as its temperature compensation. Because common solution temperature coefficients vary in the order of 1%/°C to 3%/°C or more, measuring instruments with adjustable temperature compensation must be used. Solution temperature coefficients are somewhat nonlinear and usually vary with the actual conductivity, as well. Therefore, calibration at the actual measuring temperature yields the best accuracy. The ADuCM360 contains two matched, software configurable, excitation current sources. They are individually configurable to provide a current output of 10 µA to 1 mA, and matching is better than 0.5%. The current sources allow the ADuCM360 to easily perform 2-wire, 3-wire, or 4-wire measurements for either Pt100 or Pt1000 RTDs. The software also automatically detects if the RTD is Pt100 or Pt1000 during the setup procedure. The following discussion shows simplified schematics of how the different RTD configurations operate. All mode switching is accomplished in the software, and there is no need to change the external jumper settings. Rev. A | Page 4 of 10 Circuit Note CN-0359 The 3-wire connection is another popular RTD configuration that eliminates lead resistance errors, as shown in Figure 6. Figure 5 shows the configuration for 4-wire RTDs. IEXC V7 – V8 TO ADC – VP + RP AIN8 RP 2 AIN6 RX RP 2 RX +3.3V V6 – V5 V7 – V8 +0.115V – VP + RP AIN5/ IEXC 3 1kΩ 36Ω × 1.5kΩ RP RX = Figure 5. Configuration for 4 Wire RTD Connection The parasitic resistance in each of the leads to the remote RTD is shown as RP. The excitation current (IEXC) passes through a precision 1.5 kΩ resistor and the RTD. The on-chip ADC measures the voltage across the resistor (V7 − V8). It is important that the R13 resistor and the IEXC excitation current value be chosen such that the ADuCM360 maximum input voltage at AIN7 does not exceed AVDD − 1.1 V; otherwise, the IEXC current source does not function properly. The RTD voltage is accurately measured using the two sense leads that connect to AIN6 and AIN5. The input impedance is approximately 2 MΩ (unbuffered mode, PGA gain = 1), and the current flowing through the sense lead resistance produces minimum error. The ADC then measures the RTD voltage (V6 − V5). V8 – V5 V7 – V8 4 V6 − V5 V7 − V8 × 1.5 kΩ +0.115V 36Ω Figure 6. Configuration for 3-Wire RTD Connection The second matched IEXC current source (AIN5/IEXC) develops a voltage across the lead resistance in series with Terminal 3 that cancels the voltage dropped across the lead resistance in series with Terminal 1. The measured V8 − V5 voltage is therefore free of lead resistance error. Figure 7 shows the 2-wire RTD configuration where there is no compensation for lead resistance. RP 100Ω OR 1000Ω Pt RTD 1 R13 1.5kΩ 0.1% J3 ADuCM360 AIN7/ IEXC IEXC V7 – V8 TO ADC AIN8 2 AIN6 3 AIN5/ IEXC RX V8 – V9 TO ADC +3.3V (8) 1kΩ RP The measurement is ratiometric and does not depend on an accurate external reference voltage, only the tolerance of the 1.5 kΩ resistor. In addition, the 4-wire configuration eliminates the error associated with the lead resistances. The ADuCM360 has a buffered or unbuffered input option. If the internal buffer is activated, the input voltage must be greater than 100 mV. The 1 kΩ/36 Ω resistor divider provides a 115 mV bias voltage to the RTD that allows buffered operation. In the unbuffered mode, Terminal 4 of J3 can be grounded and connected to a grounded shield for noise reduction. AIN9 × 1.5kΩ The RTD resistance is then calculated as RX = IEXC +3.3V AIN9 12970-005 RX = 4 V7 – V8 TO ADC AIN6 1kΩ RP IEXC V8 – V5 TO ADC AIN5/ IEXC 3 ADuCM360 AIN7/ IEXC AIN8 100Ω OR 1000Ω Pt RTD V6 – V5 TO ADC 1 R13 1.5kΩ 0.1% J3 12970-006 1 ADuCM360 AIN7/ IEXC RX = V8 – V9 V7 – V8 4 +0.115V AIN9 36Ω × 1.5kΩ – 2R P 12970-007 RP 100Ω OR 1000Ω Pt RTD R13 1.5kΩ 0.1% J3 Figure 7. Configuration for 2-Wire RTD Connection The 2-wire configuration is the lowest cost circuit and is suitable for less critical applications, short RTD connections, and higher resistance RTDs such as Pt1000. Rev. A | Page 5 of 10 CN-0359 Circuit Note Conductivity Theory the temperature, a second temperature sensor and compensation network must be used. The resistivity, ρ, of a material or liquid is defined as the resistance of a cube of the material with perfectly conductive contacts on opposite faces. The resistance, R, for other shapes can be calculated by R = ρL/A (9) where: L is the distance between the contacts. A is the area of the contacts. Resistivity is measured in units of Ω cm. A 1 Ω cm material has a resistance of 1 Ω when contacted on opposite faces of a 1 cm × 1 cm × 1 cm cube. Conductance is the reciprocal of resistance, and conductivity is the reciprocal of resistivity. The unit of measurement of conductance is Siemens (S), and the unit of measurement of conductivity is S/cm, mS/cm, or µS/cm. All aqueous solutions conduct electricity to some degree. Adding electrolytes such as salts, acids, or bases to pure water increases the conductivity (and decreases resistivity). For the purposes of this circuit note, Y is the general symbol for conductivity measured in S/cm, mS/cm, or µS/cm. However, in many cases, the distance term is dropped for convenience, and the conductivity is simply expressed as S, mS, or µS. A conductivity system measures conductivity by means of electronics connected to a sensor called a conductivity cell immersed in a solution, as shown in Figure 8. The contacting-type sensor typically consists of two electrodes that are insulated from one another. The electrodes, typically Type 316 stainless steel, titanium palladium alloy, or graphite, are specifically sized and spaced to provide a known cell constant. Theoretically, a cell constant of 1.0/cm describes two electrodes, each sized 1 cm2 in area, and spaced 1 cm apart. Cell constants must be matched to the measurement system for a given range of operation. For instance, if a sensor with a cell constant of 1.0/cm is used in pure water with a conductivity of 1 µS/cm, the cell has a resistance of 1 MΩ. Conversely, the same cell in seawater has a resistance of 30 Ω. Because the resistance ratio is so large, it is difficult for ordinary instruments to accurately measure such extremes with only one cell constant. When measuring the 1 µS/cm solution, the cell is configured with large area electrodes spaced a small distance apart. For example, a cell with a cell constant of 0.01/cm results in a measured cell resistance of approximately 10,000 Ω rather than 1 MΩ. It is easier to accurately measure 10,000 Ω than 1 MΩ; therefore, the measuring instrument can operate over the same range of cell resistance for both ultrapure water and high conductivity seawater by using cells with different cell constants. The cell constant, K, is defined as the ratio of the distance between the electrodes, L, to the area of the electrodes, A: K = L/A (10) The instrumentation then measures the cell conductance, Y: Y = I/V (11) The conductivity of the liquid, YX, is then calculated: YX = K × Y There are two types of conductivity cells: those with two electrodes, and those with four electrodes, as shown in Figure 9. The electrodes are often referred to as poles. TO I/V CONVERTER V 2-POLE EVAL-CN0359-EB1Z I A L 4-POLE A = AREA OF ELECTRODE SURFACE (cm2) L = DISTANCE BETWEEN ELECTRODES (cm) V = EXCITATION VOLTAGE I = CELL CURRENT K = L/A = CELL CONSTANT (cm–1) Y = MEASURED CONDUCTANCE = I/V (S) YX = WATER CONDUCTIVITY = K × Y (S/cm) 12970-008 A (12) The electronic circuitry impresses an alternating voltage on the sensor and measures the size of the resulting current, which is related to the conductivity. Because conductivity has a large temperature coefficient (up to 4%/°C), an integral temperature sensor is incorporated into the circuitry to adjust the reading to a standard temperature, usually 25°C (77°F). When measuring solutions, the temperature coefficient of the conductivity of the water itself must be considered. To compensate accurately for 12970-009 Figure 8. Interface Between Conductivity Cell and EVAL-CN0359-EB1Z Figure 9. 2-Pole and 4-Pole Conductivity Cells The 2-pole sensor is more suitable for low conductivity measurements, such as purified water, and various biological and pharmaceutical liquids. The 4-pole sensor is more suitable the high conductivity measurements, such as waste water and seawater analysis. Rev. A | Page 6 of 10 Circuit Note CN-0359 Power Supply Circuits The cell constants for 2-pole cells range from approximately 0.1/cm to 1/cm, and the cell constants for 4-pole cells range from 1/cm to 10/cm. To simplify system requirements, all the required voltages (±15 V and +3.3 V) are generated from a single 4 V to 7 V supply, as shown in Figure 10. The 4-pole cell eliminates the errors introduced by polarization of the electrodes and field effects that can interfere with the measurement. The ADP2300 buck regulator generates the 3.3 V supply for the board. The design is based on the downloadable ADP230x Buck Regulator Design Tool. The actual configuration of the electrodes can be that of parallel rings, coaxial conductors, or others, rather than the simple parallel plates shown in Figure 8. The ADP1613 boost regulator generates a regulated +15 V supply and an unregulated −15 V supply. The −15 V supply is generated with a charge pump. The design is based on the ADP161x Boost Regulator Design Tool. Regardless of the type of cell, it is important not to apply a dc voltage to any electrode, because ions in the liquid will accumulate on the electrode surface, thereby causing polarization, measurement errors, and damage to the electrode. Details regarding the selection and design of power supplies are available at www.analog.com/ADIsimPower. Take special care with sensors that have shields, as in the case of coaxial sensors. The shield must be connected to the same potential as the metal container holding the liquid. If the container is grounded, the shield must be connected to Pin 5 of J5, the circuit board ground. Use proper layout and grounding techniques to prevent the switching regulator noise from coupling into the analog circuits. See the Linear Circuit Design Handbook, the Data Conversion Handbook, the MT-031 Tutorial, and the MT-101 Tutorial for further details. The final precaution is not to exceed the rated excitation voltage or current for the cell. The CN-0359 circuit allows programmable excitation voltages from 100 mV to 10 V, and the R23 (1 kΩ) series resistor limits the maximum cell current to 10 mA. 100nF +4V TO +7V BST VIN 22µH +3.3V +3.3V EMI FILTER SW EN B120 ADP2300 10kΩ 100nF 10µF 22µF 100nF FB +0.8V GND 7.5kΩ +1.65V 10kΩ 2.4kΩ −15V UNREGULATED ½ BAT54S 22µF −15V EMI FILTER 100nF 22µH 10µF 47µF SW VIN 100nF ½ BAT54S 1N4148 +15V REGULATED EMI FILTER +15V EN 200kΩ ADP1613 FREQ FB 33nF SS COMP GND +1.235V 18kΩ EMI FILTERS: BNX025H01 56pF 3.3nF 47kΩ Figure 10. Power Supply Circuits Rev. A | Page 7 of 10 12970-010 100nF CN-0359 Circuit Note Figure 11 shows the LCD backlight driver circuit. After connecting, the conductivity cell and the RTD the board are powered up. The LCD screen appears as shown in Figure 12. +3.3V Push the encoder knob to enter the setting menu, and then input the EXC Voltage, EXC Frequency, TEMP Coefficient, and Cell Constant, as shown in Figure 13. 100nF 36kΩ +3.3V 6.8Ω 400mV + U6B − K1 K2 + U6A − ½ AD8592 ½ AD8592 A1 60mA +3.3V 6.8Ω +3.3V 400mV 12970-011 +3.3V A2 LCD DISPLAY 60mA SETTING SCREEN Figure 11. LCD Backlight Drivers Each half of the AD8592 op amp acts as a 60 mA current source to supply the LCD backlight currents. The AD8592 can source and sink up to 250 mA, and the 100 nF capacitor ensures a soft startup. Software Operation and User Interface The EVAL-CN0359-EB1Z comes preloaded with the code required to make the conductivity measurements. The code can be found in the CN-0359 Design Support Package, in the CN0359-SourceCode.zip file. The CN-0359 user interface is intuitive and easy to use. All user inputs are from a dual function push button/rotary encoder knob. The encoder knob can be turned clockwise or counterclockwise (no mechanical stop), and can also be used as a push button. Figure 12 is a photo of the EVAL-CN0359-EB1Z board that shows the LCD display and the position of the encoder knob. SETTING EXCITATION VOLTAGE Figure 13. LCD Display Screens Rotating the knob moves the cursor up and down through the various parameters. Set the cursor to EXC Voltage and push the knob until it clicks. Position the cursor over the first digit in the number to be set by rotating the knob. Push the button, and the cursor blinks. Change the number by rotating the knob, and push the knob when the desired number is reached. After setting all the digits in the number, position the cursor on Save and push the button to save the setting. Continue the process and set the EXC Frequency, TEMP Coefficient, and Cell Constant. After setting all the constants, select RETURN TO HOME and push the knob. The system is now ready to make measurements. If numbers are entered that are outside the allowable range, the buzzer sounds. If the conductivity cell is incorrectly connected, the screen displays Sensor Incorrect. If the RTD is incorrectly connected, the screen displays RTD Incorrect use 25°C. The system can still make measurements without the RTD connected, but uses 25°C as the compensation temperature. Ensure that the RTD is connected before power is applied so that the system can detect the resistance value of the RTD (Pt100 or Pt1000) and the configuration (2-wire, 3-wire, or 4-wire). 12970-012 COMMON VARIATIONS Figure 12. Photo of EVAL-CN0359-EB1Z Board Showing Home Screen in Measurement Mode 12970-013 5.1kΩ +2.9V The system shown in the CN-0359 uses the ADuCM360 precision analog microcontroller for a highly integrated conductivity measurement. If the user desires a discrete ADC, the AD7794 24-bit, Σ-Δ ADC is a good choice. Rev. A | Page 8 of 10 Circuit Note CN-0359 CIRCUIT EVALUATION AND TEST 3. This circuit uses the EVAL-CN0359-EB1Z circuit board, an external power supply, conductivity cell, and an RTD. 4. Equipment Needed The following equipment is needed: The EVAL-CN0359-EB1Z circuit board A 6 V power supply or wall wart (EVAL-CFTL-6V-PWRZ) A conductivity cell Pt100 or Pt1000 2-wire, 3-wire, or 4-wire RTD (if the RTD is not connected, the conductivity measurement is referenced to 25°C) Setup Connectivity for Prototype Development Take the following steps to set up the circuit for evaluation: 1. 2. 6. Connect the conductivity cell as follows: a. 4-wire cell: connect an outside current electrode to J5 Pin 1 and the closest inner voltage electrode to J5 Pin 2. Connect the second outside current electrode to J5 Pin 4 and the closest inner voltage electrode to Pin 3. b. 2-wire cell: connect one electrode to J5 Pin 1 and Pin 2 and connect the second electrode to J5 Pin 3 and Pin 4. c. If the conductivity cell has a shield, connect it to J5 Pin 5. Connect the RTD as follows (if used): a. 4-wire RTD (see Figure 5): connect the positive current excitation wire to J3 Pin 1 and the positive voltage sense wire to J3 Pin 2. Connect the negative current excitation wire to J3 Pin 4 and the negative voltage sense wire to J3 Pin 3. b. 3-wire RTD (see Figure 6): connect the positive current excitation wire to J3 Pin 1. Connect the negative current excitation wire to J3 Pin 4. Connect the negative voltage sense wire to J3 Pin 3. c. 2-wire RTD (see Figure 7): connect one RTD wire to J3 Pin 1 and the other wire to J3 Pin 4. d. If the RTD wires are shielded, connect the shield to J5 Pin 5. The EVAL-CN0359-EB1Z is designed to be powered with the EVAL-CFTL-6V-PWRZ 6 V power supply. The EVAL-CN0359EB1Z requires only the power supply and the external conductivity cell and RTD for operation. The EVAL-CN0359-EB1Z also has an RS-485 connector, J2, that allows an external PC to interface with the board. Connector J4 is a JTAG interface for programming and debugging the ADuCM360. Figure 14 is a typical PC connection diagram showing an RS-485 to RS-232 adapter. Rev. A | Page 9 of 10 6V SUPPLY PC J1 CONDUCTIVITY CELL J5 J2 RS-485 TO RS-232 ADAPTER EVAL-CN0359-EB1Z RTD J3 J4 JTAG Figure 14. Test Setup Functional Diagram 12970-014 • • • • 5. Connect the 6 V power supply (EVAL-CFTL-6V-PWRZ) to J1 of the EVAL-CN0359-EB1Z circuit board. Turn on the power by connecting the EVAL-CFTL-6VPWRZ, and then push the button on the EVAL-CN0359EB1Z circuit board. Follow the procedure previously described in the Software Operation and User Interface section and enter the following parameters: EXC Voltage, EXC Frequency, TEMP Coefficient, and Cell Constant. Return to the main screen and wait for the ADuCM360 to flush the buffers and display the conductivity and the temperature. If the screen shows an error and the buzzer beeps more than 20 times, check the sensor connections. CN-0359 Circuit Note LEARN MORE Data Sheets and Evaluation Boards CN-0359 Design Support Package: www.analog.com/CN0359-DesignSupport AD8253 Data Sheet ADIsimPower Design Tool. Analog Devices ADA4000-1 Data Sheet Linear Circuit Design Handbook. Analog Devices/Elsevier. ADA4638-1 Data Sheet Op Amp Applications Handbook. Analog Devices/Elsevier. ADA4528-2 Data Sheet The Data Conversion Handbook. Analog Devices/Elsevier. ADA4077-2 Data Sheet A Designer's Guide To Instrumentation Amplifiers, 3rd Edition. Analog Devices. AD8592 Data Sheet MT-031 Tutorial. Grounding Data Converters and Solving the Mystery of “AGND” and “DGND”. Analog Devices. ADuCM360 Data Sheet ADA4627-1 Data Sheet MT-101 Tutorial. Decoupling Techniques. Analog Devices. "Section 7: Temperature Sensors" in Sensor Signal Conditioning. Analog Devices. AD8542 Data Sheet ADP2300 Data Sheet ADP1613 Data Sheet ADG1211 Data Sheet ADG1419 Data Sheet ADM3483 Data Sheet REVISION HISTORY 11/15—Rev. 0 to Rev. A Change to Setup Section ...................................................................9 1/15—Revision 0: Initial Version (Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab reference designs at any time without notice but is under no obligation to do so. ©2015 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN12970-0-11/15(A) Rev. A | Page 10 of 10