Circuit Note CN-0314 Devices Connected/Referenced Wide Supply Range, Rail-to-Rail AD8420 Circuits from the Lab reference designs are engineered and Output Instrumentation Amplifier tested for quick and easy system integration to help solve todays Ultracompact, Precision 5.0 V Voltage analog, mixed-signal, and RF design challenges. For more ADR02 Reference information and/or support, visit www.analog.com/CN0314. Configurable 4-20 mA Loop Powered Transmitter/Receiver using a Micropower Instrumentation Amplifier EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 is a configurable 4 mA-to-20 mA CN0314 Evaluation Board (EVAL-CN0314-EB1Z) loop-powered transmitter based on an industry-leading Design and Integration Files micropower instrumentation amplifier. Total unadjusted error Schematics, Layout Files, Bill of Materials is less than 1%. It can be configured with a single switch as either a transmitter (Figure 1) that converts a differential input voltage into a current output, or as a receiver (Figure 5) that converts a 4 mA-to-20 mA current input to a voltage output. P5-1 I CIRCUIT I + I I CIRCUIT AMP LOOP 5V 4mA TO 20mA ADR02 195mV 100A TO 200A TO I Q1 R4 R2 990mV 22.6k 20.5k I AMP R3 330pF 0V TO 10V 11.0k P2-1 20k +VIN +IN +V S R1 P3 + 0V TO 5V 5.05k LOOP OR SUPPLY 0V TO 10V 0V TO 5V Q1 12V TO 36V AD8420 V ZXT13N50DE6 OUT R5 3.3nF 1k FB REF 20k IN V I P2-2 S Q1 VIN 250 330pF I 0V CIRCUIT R LOOP + 100A 3.9mA R9 TO TO V NOTE: R1, R2, R3, R4 AND R5 ARE 0.1% R9 50 19.8mA 200A 0.1% P5-2 I AMP I LOOP V : 195mV TO 990mV R9 LOOP GROUND Figure 1. Robust Loop Powered Configurable Transmitter Circuit with 4 mA-to-20 mA Output Rev. 0 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 2014 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) 11284-001CN-0314 Circuit Note The design is optimized for precision, low noise and low power I = I + I LOOP AMP R9 industrial process control applications. The circuit can accept In order for the circuit to operate properly, the total circuit 0 V to 5V or 0 V to 10 V input range as a transmitter. As a supply voltage must be greater than 7 V to provide sufficient receiver it can provide 0.2 V to 2.3 V or 0.2 V to 4.8 V output headroom for the ADR02 voltage reference. range compatible with ADCs using 2.5 V or 5 V references. The V > 7 V + R I LOOP SUPPLY Load Loop(max) supply voltage can range from 12 V to 36 V as a transmitter and 7 V to 36 V as a receiver. For RLOOP of 250 , Since the circuit is configurable, a single hardware design can be VLOOP SUPPLY > 7 V + (250 )(20 mA) used as a backup for both transmitter and receiver at the same Therefore, time, minimizing customer inventory requirements. V > 12 V LOOP SUPPLY CIRCUIT DESCRIPTION The loop supply voltage is also limited to 36 V maximum. The circuit features the AD8420, an instrumentation amplifier An advantage of AD8420 is its high impedance reference pin based on an indirect current feedback architecture. Because of that eliminates the need for an additional op-amp to drive it, this architecture, the AD8420 has excellent input and output thus, saving power, cost and space in the transmitter circuit. characteristics. Unlike conventional instrumentation amplifiers, the AD8420 can easily amplify signals at or even slightly below It is important for proper loop operation that the circuit board ground without requiring dual supplies. The AD8420 has rail- ground and the loop ground are not connected except for the to-rail output voltage swing that is completely independent of R9, 50 sense resistor. the input common-mode voltage. This exempts the AD8420 Selection of Scaling Resistors from the restrictions caused by interaction between the The differential input voltage range of AD8420 is limited to common-mode input and output voltage associated with most maximum of 1V. Hence, to be able to accept higher industrial conventional instrumentation amplifier architectures. The input voltage range, the circuit makes use of a scaling resistor flexible input and output characteristics, together with network to translate the 0 V to 5 V or 0 V to 10 V input into micropower consumption (80 A maximum for 0 V input) and 0.195 V to 0.990 V. The following equations use nodal analysis wide supply range, make the AD8420 ideal for flexible low to obtain the values for R1, R2, R3, and R4 in the circuit: power industrial applications. VIN 0.195 V V 0.195 V 0.195 V MIN REF (1) Transmitter Configuration + = R1 R2 R5 Figure 1 shows the simplified schematic for the 4mA-to-20mA VIN 0.990 V V 0.990 V 0.990 V transmitter configuration. The circuit only consumes around MAX REF (2) + = 1 mA of current, which makes it suitable for loop-powered R1 R2 R5 applications. The transmitter has input ranges of 0 V to 5 V and where: 0 V to 10 V, which can be selected using Jumper P3. The input V = 5 V REF voltage range is then scaled down to 0.195 V to 0.990 V range R5 = 1 k since the differential input voltage of AD8420 is limited to For 0 V to 5 V input range, Equation 1 and Equation 2 lead to maximum of 1 V. Equation 3 and Equation 4, respectively. The input to the AD8420 has a differential mode noise filter 0 V 0.195 V 5 V 0.195 V 0.195 V (3) (40 k/3.3 nF) with a 1.2 kHz bandwidth and a common -mode + = R1 R2 1 k noise filter (20 k/330 pF) with a 24 kHz bandwidth. 5 V 0.990 V 5 V 0.990 V 0.990 V The indirect current feedback architecture of AD8420 forces the (4) + = differential input voltage of the amplifier to appear between its R1 R2 1 k FB and REF pins. The transistor Q1 then translates the 0.195 V Solving for R1 and R2 using Equation 3 and Equation 4: to 0.990 V range into 3.9 mA to 19.8 mA flowing through the R9 50 sense resistor . R1 = 5044 The current through the R9 sense resistor includes the circuit R2 = 20564 current and the Q1 current, but not the AD8420 current, IAMP. For 0 V to 10 V input range, Equation 1 and Equation 2 results The unique architecture of the AD8420 makes its supply to Equation 5 and Equation 6, respectively. current predictable from 100 A to 200 A for a 0.195 V to 0 V 0.195 V 5 V 0.195 V 0.195 V (5) + = 0.990 V input applied between +IN and IN. This supply R3 R4 1 k current will add to the current through R9, bringing the total 10 V 0.990 V 5 V 0.990 V 0.990 V output loop current to 4 mA-to-20 mA. Therefore, the total (6) + = current in the loop is given by the equation: R3 R4 1 k Rev. 0 Page 2 of 8