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

Daniel Terrazas

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