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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 4394
Keywords: op amp, high-voltage op amp, operational amplifier, high voltage operational amplifier, VI
converter, current-loop system, 4-20mA
APPLICATION NOTE 4394
How to Use High-Voltage and High-Current-Drive
Op Amps in ±20mA or 4–20mA Current-Loop
Systems
By: Maurizio Gavardoni, Product Definer
Feb 16, 2010
Abstract: This application note shows how to use a high-voltage, high-current-drive operational amplifier
to convert a voltage signal into a ±20mA or 4–20mA current signal for use in process-control industrial
applications. The MAX9943 op amp serves as the example device. Experiments are described and test
results presented.
This application note was also featured in Maxim's Engineering Journal, vol. 68 (PDF, 2.72MB).
Introduction
Current loops have long been used in process-control industrial applications. Current loops enable the
transfer of information from remote sensors to central processing units, or from those central units to
remote actuators. Although 4–20mA current loops are quite common for the application, ±20mA current
loops can be used as well. For low-impedance loads, the use of a high-voltage op amp with a high-
current-drive output simplifies the circuit design by eliminating the need for external power FETs.
This application note shows how to use a high-voltage, high-current-drive op amp in a ±20mA or 4–
20mA current-loop application. The op amp converts a voltage signal from a DAC into a ±20mA or 4–
20mA current output. Application experiments feature the MAX9943 op amp. Test data are presented.
Basics of a Current Loop
A current loop typically includes a sensor, transmitter, receiver, and an ADC or microcontroller (Figure
1). The sensor measures a physical parameter (such as pressure or temperature) and provides a
corresponding output voltage. The transmitter converts the sensor's output into a proportional 4mA-to-
20mA current signal. The receiver then converts the 4–20mA current into a voltage. An ADC or a
microcontroller digitizes the receiver's voltage output.
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