Circuit Note CN-0289 Devices Connected/Referenced Circuits from the Lab™ reference circuits 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/CN0289. AD8226 Wide Supply Range, Rail-to-Rail Output Instrumentation Amplifier ADR02 Ultracompact, Precision 5.0 V Voltage Reference ADA4091-2 Precision Micropower, OVP, RRIO Dual Op Amp Flexible, 4 mA-to-20 mA, Loop-Powered Pressure Sensor Transmitter with Voltage or Current Drive The design is optimized for a wide variety of bridge based voltage or current driven pressure sensors, utilizes only four active devices, and has a total unadjusted error of less than 1%. The loop supply voltage can range from 12 V to 36 V. EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN0289 Evaluation Board (EVAL-CN0289-EB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The input of the circuit is protected for ESD and voltages beyond the supply rail, making it ideal for industrial applications. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a robust and flexible looppowered current transmitter that converts the differential voltage output from a pressure sensor to a 4 mA-to-20 mA current output. +5V +VLOOP +5V VOUT U2A 1/2 ICIRCUIT VIN J2-1 ILOOP ADR02 ADA4091-2 39.60µA I12 G = 50.00 +VLOOP VDRIVE J1-3 RBRIDGE R5 10kΩ U3 RBRIDGE R6 10kΩ 1nF RBRIDGE R1 J1-2 4.02kΩ 10nF R2 J1-4 4.02kΩ RBRIDGE R3 1.008kΩ U1 AD8226 U2B 1/2 ADA4091-2 R9 31.56kΩ I9 0µA, 158.42µA I10 VIN: 0mV, 100mV 39.60µA, 198.02µA VLOOP_SUPPLY ILOOP – ICIRCUIT 12V TO 36V Q1 ZXT13N50DE6TA 30kΩ ICIRCUIT REF 0V +VLOOP +VLOOP VOUT 0V, 5.00V 1nF J1-1 R12 126.25kΩ R10 1kΩ J2-2 3.960mA, 19.802mA PCB GROUND –39.60mV, –198.02mV R8 10Ω I10 I8 LOOP LOAD – R7 250Ω + ILOOP ILOOP: 4.000mA, 20.000mA LOOP GROUND 10947-001 NOTES 1. R8, R10 ARE STANDARD 0.1% VALUES. R5, R6 ARE STANDARD 1% VALUES. R3, R9, R12 ARE CALCULATED VALUES (SEE TEXT). 2. VOLTAGES MEASURED WITH RESPECT TO PCB GROUND. Figure 1. Robust Loop Powered Pressure Sensor Signal Conditioning Circuit with 4 mA-to-20 mA Output (Shown in Sensor Voltage Drive Mode) 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 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, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2013 Analog Devices, Inc. All rights reserved. CN-0289 Circuit Note CIRCUIT DESCRIPTION Sensor Excitation Drive The design provides a complete solution for the 4 mA-to-20 mA transmitter for pressure sensor measurements where the supply for the entire circuit is provided by the loop. The circuit has three critical stages: the sensor excitation drive, sensor output amplifier, and the voltage to current converter. Depending on the pressure sensor selected, either a voltage or current drive is required. The circuit uses one half of the ADA4091-2 (U2A) with different configurations chosen by switching S1 to support either option. The switch S1 provides the selection for either drive. The total current required by the circuit is 1.82 mA (maximum) as shown in Table 1. Pressure sensors requiring bridge drive currents of up to 2 mA can therefore be used without exceeding the maximum available loop current of 4 mA. Excitation: Voltage Drive Configuration Figure 2 shows the configuration for the voltage drive with S1 in the position labeled VOLTAGE DRIVE on the PCB (see complete circuit layout and schematics in the CN0289 design support package: http://www.analog.com/CN0289DesignSupport). Table 1. Maximum Circuit Currents @ 25°C Current (mA) 0.80 0.50 0.43 0.05 0.04 1.82 +VLOOP 5V IDRIVE = 2 mA FOR VDRIVE = 10V, RBRIDGE = 5kΩ 1/2 ADA4091-2 VDRIVE IDRIVE U2A J1-3 PCB GROUND R5 10kΩ R6 10kΩ PCB GROUND RBRIDGE RBRIDGE RBRIDGE RBRIDGE R1 J1-2 4.02kΩ +V AD8226 R2 J1-4 4.02kΩ INPUT VCM = 5V –V J1-1 PCB GROUND Figure 2. Sensor Voltage Drive Configuration for RBRIDGE = 5 Ω, VDRIVE = 10 V Rev. 0 | Page 2 of 7 10947-002 Component ADR02 ADA4091-2 AD8226 R5, R6 @ 10 V R12 @ 5 V TOTAL Circuit Note CN-0289 The voltage drive circuit is normally configured for a bridge drive voltage of 10 V. In this mode, the minimum allowable bridge resistance is: R BRIDGE ≥ 2 V REF = 2 mA 10 V where V DRIVE = 5 kΩ Note that the loop voltage, VLOOP, should be at least 0.2 V greater than the bridge drive voltage to allow sufficient headroom for U2A. 2 mA For lower than 5 kΩ bridge resistance, the drive voltage can be decreased down to 5 V using a buffer configuration by removing R6. V LOOP ≥ V DRIVE + 0.2 V Excitation: Current Drive Configuration Other values of drive voltage can be obtained by properly selecting R6 using the equation: The circuit can be switched to the current drive configuration shown in Figure 3 by moving S1 to the position labeled CURRENT DRIVE on the PCB. R5 V DRIVE = 5 V 1 + R6 5 V × R5 V DRIVE – 5 V +VLOOP 5V 1/2 ADA4091-2 U2A VDRIVE = 5V + IDRIVE × RBRIDGE = 11V, FOR RBRIDGE = 3kΩ VDRIVE J1-3 PCB GROUND RBRIDGE RBRIDGE RBRIDGE RBRIDGE R1 J1-2 4.02kΩ +V AD8226 R2 J1-4 4.02kΩ INPUT VCM = 8V –V J1-1 IDRIVE × R4 = 5V IDRIVE = 2mA R4 2.49kΩ PCB GROUND Figure 3. Sensor Current Drive Configuration for RBRIDGE = 3 kΩ Rev. 0 | Page 3 of 7 10947-003 R6 = ≥ 2 mA R BRIDGE CN-0289 Circuit Note In the current drive mode, the 2 mA maximum allowable bridge drive current must be observed. The circuit is configured for R4 = 2.49 kΩ, and IDRIVE = 2 mA. Lower values of IDRIVE can be obtained by using the following equation to select the value of R4: 5V R4 = I DRIVE The current through R12 is given by: The resulting drive voltage VDRIVE is calculated from: The current through R9 is given by: I 10 = I LH = 101 I 12 = V REF 20 mA = 198.02 μA 101 = 5V = 39.60 μA 126.25 kΩ R12 I9 = I10 − I12 = 198.02 μA − 39.60 μA = 158.42 μA V DRIVE = 5 V + I DRIVE × R BRIDGE A headroom of 0.2 V is required for the U2A supply, therefore: The value of R9 can now be calculated from: V LOOP ≥ V DRIVE + 0.2 V R9 = For the values shown in Figure 3, RBRIDGE = 3 kΩ, IDRIVE = 2 mA, VDRIVE = 11 V, and VLOOP ≥ 11.2 V. V OUT I9 = 5V 158.42 μA = 31.56 kΩ The ADA4091-2 op amp is chosen for the circuit because of its low current consumption (250 µA/amplifier), low offset voltage (250 µV), and rail-to-rail inputs and outputs. In practice, the calculated values for R3, R12, and R9 will not be available as standard values, so there will be fixed errors introduced by the actual values used in the circuit. These errors can be calculated as follows. Bridge Output Instrumentation Amplifier and Selection of Gain and Offset Resistors Gain, offset, and total error, measured as a %FSR (where FSR = 16 mA) due to resistors R3, R9, and R12: Gain Error (%FSR) = The output of the bridge is filtered by a common-mode filter (4.02 kΩ, 1 nF) with a bandwidth of 39.6 kHz and a differentialmode filter (8.04 kΩ, 10 nF) with a bandwidth of 2 kHz. 1008 Ω – R3 31.56 kΩ – R9 + 1008 Ω 31.56 kΩ The AD8226 is an ideal choice for the in-amp because of its low gain error (0.1%, B-grade), low offset (58 µV @ G = 50, B-grade; 112 µV @ G = 50, A-grade), excellent gain nonlinearity (75 ppm = 0.0075%), and rail-to-rail output. The AD8226 instrumentation amplifier amplifies the 100 mV FS signal by a factor of 50 V to 5 V using a gain setting resistor R3 = 1.008 kΩ. The relationship between the gain, G, and R3 is given by R3 = 49.4 kΩ For the output zero-value loop current, ILO = 4 mA: I LO = I 8 + I 10 Because of the 100:1 ratio of R10 to R8: I 8 = 100 × I 10 However, the total error at full-scale output (20 mA) is equal to: • R9 = 30.9 kΩ + 655 Ω = 31.555k Ω (calculated = 31.56 kΩ) ILO = 101 × I10 • R12 = 124 kΩ + 2.26 kΩ = 126.26 Ω (calculated = 126.25 Ω). The AD8226 output is 0 V for ILO = 4 mA, and the offset resistor R12 can now be calculated from: = The total error at zero output (4 mA) is not affected by the gain error. • R3 = 1 kΩ + 8.06 Ω = 1008.06 Ω (calculated = 1008 Ω) Combining the last two equations: I 10 126.25 kΩ – R12 × 100% 0.25 126.25 kΩ In the practical circuit, the nearest EIA standard 0.1% resistor values must be chosen, thereby resulting in fixed gain and offset errors as described in the previous equations. It is possible to use combinations of two 0.1% values to come closer to the calculated values. For instance, the following series combinations of 0.1% resistors come very close to the calculated values: where G = 50, and R3 = 1008 Ω. R12 = Offset Error (%FSR) = Full Scale Error = Gain Error + Offset Error G –1 V REF × 100% 101 × V REF I LO = 101 × 5 V • Offset Error = −0.008% FSR • Gain Error = +0.010% FSR = 126.25 kΩ 4 mA For VOUT = 5.00V, the output loop current is ILH = 20 mA, and hence: I LH = I 8 + I 10 = 100 × I 10 + I 10 = 101 × I 10 The error calculated with these combinations is as follows: • Full Scale Error = +0.002% FSR In some cases, however, even standard 0.1% resistor values may not be obtainable from resistor suppliers, and substitutions are required. Rev. 0 | Page 4 of 7 Circuit Note CN-0289 For example, the resistors supplied with the EVAL-CN0289EB1Z board are as follows: In order for the circuit to operate properly, the supply voltage, VLOOP, must be greater than 7 V in order to provide sufficient headroom for the ADR02 voltage reference. • R3 = 1000 Ω (calculated = 1008 Ω) Therefore, • R9 = 31.6 kΩ (calculated = 31.56 kΩ) VLOOP_SUPPLY > 7 V + R7 × ILOOP • R12 = 124 kΩ (calculated = 126.25 kΩ) With the values supplied with the board, the errors due to the resistor values are: VLOOP_SUPPLY > 7 V + 250 Ω × 20 mA = 12 V The minimum loop supply voltage is also dependent on the configuration of the drive circuit for the bridge. In the voltage drive mode with VDRIVE = 10 V, the supply voltage VLOOP must be greater than 10.2 V in order to maintain sufficient headroom for U2A (see Figure 2). • Offset Error = +0.45% FSR • Gain Error = +0.66% FSR • Full Scale Error = +1.11% FSR Voltage Reference The ADR02 5 V reference is used to set the drive voltage or current to the bridge and to set the 4 mA zero offset. It has an initial accuracy of 0.1% (A-grade), 0.06% (B-grade), and 10 µV p-p voltage noise. In addition it will operate on supply voltages up to 36 V and consumes less 1 mA maximum, making it an ideal choice for loop-powered applications. Voltage-to-Current Conversion The 4 mA-to-20 mA output is produced by forcing a current through R10 that is the sum of the signal component (I9) and the offset component (I12). The current I10 = I9 + I12 develops a voltage across R10 that is applied to the sense resistor R10 through U2B and Q1. The current through R8 is 100× the current in R10. The loop current, ILOOP, is therefore : I LOOP = I 8 + I 10 = 100 × I 10 + I 10 = 101 × I 10 The values of R8 (10 Ω) and R10 (1 kΩ) were chosen to be values that are easily obtainable in 0.1% tolerances. In order for the circuit to operate properly, the circuit current, ICIRCUIT, must always be less than the minimum loop current of 4 mA. In addition, the ground of the PCB must not be connected to the loop ground in any manner, and the PCB ground and the sensor must be free to float with respect to the loop ground. The bipolar NPN transistor that is controlled by the output of U2B and generates the loop current should have a gain of at least 300 to minimize the linearity error. It should also have a breakdown voltage of at least 50 V. The output transistor Q1 is a 50 V NPN power transistor capable of dissipating 1.1 W @ 25°C. The worst case power dissipation in the circuit is for an output current of 20 mA into a loop load resistance of 0 Ω with a VCC supply of 36 V. Under these conditions the power dissipation of Q1 is 0.68 W. The supply voltage, VLOOP, driving the circuit board is dependent on the loop supply VLOOP_SUPPLY, the loop load, R7, and the loop current, ILOOP. These values are related by the following equation: For a maximum loop current of 20 mA, and R7 = 250 Ω, In the current drive mode, the supply voltage VLOOP must be greater than 11.2 V in order to maintain sufficient headroom for U2A (see Figure 3). The loop supply voltage is limited to 36 V maximum. Error Analysis for Active Components The maximum and rss errors due to the active components in the system for A- and B-grade levels of the AD8226 and ADR02 are shown in Table 2 and Table 3. Note that the ADA4091-2 op amp is only available in one grade level. Table 2. Errors Due to Active Components (A-Grade) Error Component AD8226-A ADR02-A ADA4091-2 AD8226-A RSS Offset RSS Gain RSS FS Error Max Offset Max Gain Max FS Error Error Offset Offset Offset Gain Error Value 112µV 0.10% 250µV 0.15% Error %FSR 0.11% 0.02% 0.16% 0.15% 0.20% 0.15% 0.35% 0.29% 0.15% 0.44% Table 3. Errors Due to Active Components (B-Grade) Error Component AD8226-B ADR02-B ADA4091-2 AD8226-B RSS Gain RSS Offset RSS FS Error Max Offset Max Gain Max FS Error VLOOP = VLOOP_SUPPLY – R7 × ILOOP Rev. 0 | Page 5 of 7 Error Offset Offset Offset Gain Error Value 58µV 0.06% 250µV 0.10% Error %FSR 0.06% 0.01% 0.16% 0.10% 0.10% 0.17% 0.27% 0.23% 0.10% 0.33% CN-0289 Circuit Note Total Circuit Accuracy 0.9 A good approximation to the total error contributed by the resistor tolerances is to assume that each of the critical resistors contribute equally to the total error. The five critical resistors are R3, R8, R9, R10, and R12. Worst-case tolerance build up of 0.1% resistors yields a total resistor error of 0.5% maximum. If rss errors are assumed, then the total rss error is 0.1√5 = 0.224%. 0.8 TOTAL ERROR (%FS) 0.7 Adding the worst case resistor tolerance error of 0.5% to the previous worst-case errors due to the active components (Agrade) yields: 0.6 AD8226 OUTPUT ZERO ERROR = +0.35% 0.5 0.4 0.3 0.2 GAIN ERROR = –0.47% FULL SCALE ERROR = –0.02% 0.1 –0.1 • Gain Error = 0.15% + 0.5% = 0.65% 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 BRIDGE OUTPUT (mV) • Full Scale Error = 0.44% + 0.5% = 0.94% 10947-004 0 • Offset Error = 0.29% +0.5% = 0.79% Figure 4. Total Error in Output Current (%FSR) vs. Bridge Output for 3 kΩ Bridge, 24 V Loop Supply These errors assume ideal resistors are selected and that the only errors are due to their tolerances. Although the circuit is capable of 1% or less total error, if better accuracy is required offset and gain adjustment capability should be added to the circuit. Offset can be calibrated by adjusting R12 for 4 mA output with zero input, and full-scale can then be adjusted by varying R9 for a full-scale 100 mV input. The two adjustments are independent provided the offset is calibrated first. Actual error data from the circuit is shown in Figure 4. The total output error (%FSR) is calculated by taking the difference between the ideal output current and the measured output current, dividing by the FSR (16 mA), and multiplying the result by 100. Note that the error between 0 mV and 1 mV input is due to the saturation voltage of the AD8226 output stage that can range from 20 mV to 100 mV under the loading conditions in the circuit. All rail-to-rail output stages are limited in their ability to approach the supply rails by either the saturation voltage (bipolar outputs) or on-resistance (CMOS outputs). If the error caused by the output saturation voltage is troublesome, the input signal from the bridge can be offset by connecting an appropriate resistor from the +5 V reference to one side of the bridge output. COMMON VARIATIONS The circuit is proven to work with good stability and accuracy with component values shown. Other voltage references, precision op-amps and in-amp can be used in this configuration to develop 4 mA-to-20 mA analog current output and for other various applications for this circuit. The ADA4091-4 is a quad version and can be used as a substitute for the ADA4091-2, dual channel, if additional precision op amps are needed. The AD8426, a dual-channel, low cost, and wide supply range instrumentation amplifier can also be used for multiple input channel applications. The ADR4550, high precision, low power, low noise voltage references can be used to replace ADR02 for a low voltage supply applications. CIRCUIT EVALUATION AND TEST Equipment Required • EVAL-CN0289-EB1Z evaluation board • Agilent E36311A dual dc power supply or equivalent • Agilent 3458A multimeter or equivalent Current Output Measurement The current output of the evaluation board was measured with a setup seen in Figure 5. The test conditions were as follows: • Loop supply: 24 V • Loop load: 250 Ω • RBRIDGE = 3 kΩ • VDRIVE = 5 V • VCM = 2.5 V The bridge resistors are connected to both terminal inputs of the in-amp to simulate sensor output. Rev. 0 | Page 6 of 7 Circuit Note CN-0289 Test Setup Configuration and Tests LEARN MORE The circuit was tested using the test setup in Figure 5. CN-0289 Design Support Package: http://www.analog.com/CN0289-DesignSupport. The Agilent E36311A dual power supply was used to generate a common-mode voltage of 2.5 V and a 0 mV to 100 mV differential input voltage. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. The Agilent 3458A was used to measure the actual loop current output of the evaluation board. MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. EVAL-CN0289-EB1Z DUAL POWER SUPPLY J1-2 RBRIDGE ÷ 2 24V J1-4 COM VDIFF J1-3 VOUT1 CHANNEL 1 COM1 MT-087 Tutorial, Voltage References. Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. CURRENT METER RBRIDGE VCM MT-066 Tutorial, In-Amp Bridge Circuit Error Budget Analysis. Analog Devices. VOUT J2-1 COM J1-1 PCB GND J2-2 Voltage Reference Wizard Design Tool. I FOR TESTS, VCM = 2.5V, RBRIDGE = 3kΩ, VDRIVE = 5V Figure 5. Functional Block Diagram of Test Setup 10947-005 VOUT2 CHANNEL 2 COM2 MT-065 Tutorial, In-Amp Noise. Analog Devices. POWER SUPPLY RBRIDGE ÷ 2 Data Sheets and Evaluation Boards CN-0289 Circuit Evaluation Board (EVAL-CN0289-EB1Z) AD8226 Data Sheet ADA4091-2 Data Sheet ADR02 Data Sheet REVISION HISTORY 5/13—Revision 0: Initial Version (Continued from first page) Circuits from the Lab circuits 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 circuits 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 circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits 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 circuits at any time without notice but is under no obligation to do so. ©2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN10947-0-5/13(0) Rev. 0 | Page 7 of 7