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[FAQ] How to Select Amplifiers for 4-20mA Field Transmitter Applications

Field transmitter is a general term that encompasses the common industrial process control systems: pressure, temperature, and flow. While pressure, temperature, and flow transmitters each pose distinct design challenges and considerations, some signal conditioning similarities exist between the three.

A sensor will convert the physical quantity (i.e. pressure, temperature, flow, etc.) into an electrical signal. More often than not, the sensors used to detect the aforementioned physical quantities produce a small differential electrical signal that needs to be amplified (i.e. bridge sensors, thermocouples, etc.). An instrumentation amplifier is used to amplify the signal and perform the differential to a single ended conversion.

Figure 1 - Simplified field transmitter implementation

The amplified signal is then used to create the renowned 4-20mA current output to transport the sensor data to a remote processer. There are several ways to create this 4-20mA output. For example, a common implementation is to take the analog signal from the instrumentation amplifier and convert it to the digital domain using an analog-to-digital converter (ADC). The digital information is processed by a microprocessor that then controls a digital-to-analog converter (DAC). The DAC output is then used to create the 4-20mA current output.

The voltage to current conversion can be done discretely using an op-amp and some external components, but the simplest way is to use a 4-20mA transmitter (XTR) that integrates the op-amp and offers some additional functionality (i.e. excitation currents, reference voltages, error detection flags, etc.). The XTR amplifier can be easily interfaced with a DAC or directly with the instrumentation amplifier as shown in Figure 1.

Table 1 shows some recommended instrumentation amplifiers for interfacing with many pressure, temperature, and flow sensors. The requirements here are high common-mode rejection (CMR), low offset and offset drift, low noise, and low power.

Table 1 - Instrumentation amplifiers for field transmitters

Device

CMRR (min, G=1000)

Input offset voltage (max)

Input offset drift (max)

Noise density

Quiescent current (typ)

INA823

100 dB

100 µV

2 µV/°C

21 nV/√Hz

250 µA

INA819

140 dB

35 µV

0.4 µV/°C

8 nV/√Hz

350 µA

INA821

140 dB

35 µV

0.4 µV/°C

7 nV/√Hz

650 µA

INA317

100 dB

75 µV

0.3 µV/°C

50 nV/√Hz

50 µA

 

Table 2 shows some recommended XTR devices for 2 and 3 wire applications. XTRs offer an easy, ready to use solution for creating a 4-20mA transmitter. Also visit [FAQ] How do I choose a 4-20mA transmitter? for more details.

Table 2- XTR devices for field transmitters

Device

Functionality

Supply voltage range

Features

XTR115

2 wire

7.5V – 36V

5 V regulator and 2.5 V reference

XTR111

3 wire

8V – 40V

Configurable regulator 3V -5 V

XTR305

3 wire

10V – 40V

User selectable current/voltage output, output error flags, thermal protection

 

Table 3 shows some recommended op-amps for a discrete implementation of the 4-20mA function. The requirements here are low power, low offset, and offset drift.

Table 3- Op-amps for field transmitters

Device

Quiescent current (typ)

Bandwidth

Input offset voltage (max)

Offset voltage drift (typ)

Supply voltage range

OPA391

24 µA

1 MHz

45 µV

1.00 µV/°C

1.7V – 5.5 V

OPA187

100 µA

550 kHz

10 µV

0.001 µV/°C

4.5V – 36 V

OPA333

17 µA

350 kHz

10 µV

0.02 µV/°C

1.8 V – 5.5 V

 

For useful resources, please make sure to check out the following content.

How to Select Amplifiers for Pressure Transmitter Applications

[FAQ] Designing with 4-20mA current loop transmitters (XTRs): FAQ links

[FAQ] How do Instrumentation Amplifiers (INAs) fit into my design?

TI Precision Labs – Instrumentation Amplifier Video Curriculum

TI Precision Labs - Amplifiers: Introduction to 4-20mA current loop transmitters

Why precision matters with fully differential amplifiers