Analog Core Vision | Designing a 4-20mA loop-powered transmitter

introduction

In process control systems, sensor transmitters collect data such as pressure, temperature, flow, and level and send this information to a programmable logic controller (PLC) or distributed control system.

These transmitters rely on a 4-20mA signal to send data to the controller. Although standards such as IO-Link and Profibus are emerging, the 4-20mA signal offers long-distance recovery, reliability, noise immunity, and universal compatibility with every PLC system.

This article will provide an overview of 4-20mA transmitter construction and its operating principles, as well as design alternatives for implementing such transmitters using common semiconductor products.

4-20mA Transmitter Basics

4-20mA transmitters are divided into four-wire, three-wire and two-wire types according to power and number of wires. This article will focus on the two-wire type.

The two-wire field transmitter in Figure 1 connects to a field power supply and an analog input module, which forms a current loop. The first subsystem in a field transmitter is the sensing subsystem, which connects to the physical sensor, conditions its output, and converts the signal into a digital code for processing (including linearization and calibration). The second subsystem is the transmitting subsystem, which powers the transmitter by extracting power from the loop, transmits processed data by converting digital signals back to analog signals, and controls the loop current. The transmitter transmits the signal by regulating the current in the loop as a voltage-controlled current source.

Figure 1: Universal two-wire 4-20mA sensor transmitter

4-20mA Transmitter Design Considerations

4-20mA transmitter design considerations include:

• Low power operation.

• Small package size.

• High accuracy and low noise are provided over the entire industrial temperature range.

• Supports HART protocol.

• low cost.

Design performance indicators

There are several transmitter performance metrics to evaluate:

Loop Compliance Voltage This is the loop voltage range for normal operation of the transmitter, which is primarily determined by the LDO limit and is affected by the series components in the loop (including protection devices). The typical loop compliance voltage range is 12V to 36V.

Resolution is the number of different current output values ​​that the transmitter can generate and is directly related to the native resolution of the DAC. Commercial 4-20mA transmitters have a resolution between 12 bits and 16 bits.

Linearity error Mainly determined by the integral nonlinearity of the DAC, it is the maximum error (in least significant bits [LSBs]) over the entire output range.

Noise Measured as the root mean square (RMS) of the output noise current. This noise can make some output levels indistinguishable, reducing the effective resolution. In this case, effective resolution is a measure of noise performance. For a 16-bit resolution system, the effective resolution is expected to be between 13 and 15 bits, depending on the signal bandwidth.

Accuracy A measure of the deviation of the current output from the ideal current value. This includes the RMS sum of offset error, gain error, and nonlinearity error, as well as the temperature drift of these values. Total unadjusted error indicates the degree of inaccuracy.

Dynamic performance Includes signal bandwidth and transmitter stability. Bandwidth is the maximum current signal bandwidth that can be transmitted through the loop. This bandwidth is determined by the DAC settling time and the amplifier circuit bandwidth and the transconductance of the bypass transistor. The use of a degeneration resistor can eliminate the effects of changes in the transistor transconductance (gm). The amplifier circuit is usually also externally compensated. Stability is a function of the loop bandwidth and the compensation capacitor value. Reducing the capacitance at critical nodes in the loop will improve stability. For HART-enabled transmitters, using external components to reduce the bandwidth can help prevent interference with the HART signal.

Circuit protection features Protect the transmitter from abnormal conditions such as reverse loop polarity and surge events. Diodes block reverse polarity. If the transmitter operates with reverse polarity, use a rectifier bridge as shown in Figure 3. Surge protection requires the use of transient voltage suppressor diodes (such as TVS3301) and passive components to limit current during high voltage events. These protection components require some margin during operation and will increase the minimum compliance voltage value.

Figure 3: Typical protection section of a two-wire transmitter

Transmitter circuit implementation

The difference between the block diagram implementations in Figure 2 is the integration approach. The bypass transistor is always a discrete component for better thermal management. All of the following implementations can support the HART protocol by adding a HART modem such as the DAC8740H.

Dedicated loop converter

One approach is to use a DAC, such as the DAC161S997, which has an integrated voltage reference and output amplifier. This solution consists of a DAC, a wide input voltage LDO, and an NPN transistor, as shown in Figure 4. This implementation consumes 130µA, has excellent accuracy, and requires no calibration.

The DAC161S997 has diagnostic features that detect current loop errors at low supply voltages or high current loads and signal a low error current of less than 4mA.

The design is simple and requires only a few external components to ensure loop stability and limit inrush current. This approach has a maximum operating temperature of 105°C.

Figure 4: Two-wire 4-20mA transmitter using DAC161S997

Loop Transmitter Devices

Another implementation is to use a low-power DAC such as the DAC8551, followed by a dedicated loop transmitter such as the XTR115 with an integrated LDO, voltage reference, and output amplifier. This approach minimizes noise and has a gain error of less than 1%.

There are a few constraints here: The XTR115 is limited to 85°C operating temperature, and the maximum input voltage of the integrated LDO is 36V. As an alternative, the XTR117 has a smaller package, consumes lower quiescent current, and can operate up to 125°C. The XTR117’s integrated LDO can operate at voltages up to 40V. The XTR117 does not integrate a voltage reference, so it relies on an external reference, resulting in a three-device solution: an LDO, a DAC, and a voltage reference, as shown in Figure 5.

Figure 5: Two-wire 4-20mA transmitter using XTR117

DAC with integrated MCU

Cost-sensitive applications use MCUs with analog resources. The MSPM0G MCU enables a transmitter-level implementation that includes an integrated 12-bit DAC, internal reference, and output amplifier. The only external device required is an LDO, as shown in Figure 6. Because the analog functions are implemented in the digital processing of the MCU, their power consumption is relatively high compared to their dedicated analog counterparts. This approach is very attractive for applications that require 11-bit effective resolution at a very low cost. Using the VREF– pin as the internal reference negative pin instead of ground improves performance. Isolating the VREF– pin isolates digital noise from the analog reference.

Figure 6: Two-wire 4-20mA transmitter using MSPM0G

PWM-based DAC

A more common approach using an MCU (without an integrated DAC) relies on pulse width modulation (PWM) to generate the DAC output. Simple PWM DACs have a resolution of 10 to 12 bits. However, using more advanced techniques such as dual-path PWM and active ripple suppression, DACs with 16-bit resolution can also be achieved.

To achieve high effective resolution, the PWM signal is buffered using logic gates powered by a voltage reference; the MCU requires proper bypassing to avoid digital noise injection into the loop current. The implementation shown in Figure 7 has low power consumption, is stable over temperature, and achieves more than 13 bits of effective resolution at very low cost.

Figure 7: Two-wire 4-20mA transmitter using PWM DAC

Standalone low power DAC

Excellent resolution and linearity performance can be achieved when implementing a 4-20mA transmitter using a low-power, stand-alone DAC such as the AFE88101 in Figure 8. To further reduce power consumption, a low-power voltage reference such as the REF35125 can reduce the current to 180µA. In addition, the AFE88101 has extensive diagnostic features and includes a 12-bit ADC and defined fail-safe modes.

The AFE881H1 is pin-to-pin compatible with the AFE88101 and features an integrated HART modem for compact HART-enabled transmitters. The AFE881H1 has low current consumption when HART is enabled. The HART modem typically consumes 10µA during operation, making it an ideal device for low-power HART-enabled transmitters. Another feature of the AFE88101 is compatibility with 1.8V logic levels, enabling low-voltage digital operation, further reducing power consumption on the MCU input/output side, and reducing electromagnetic emissions.

Figure 8: Two-wire 4-20mA transmitter using AFE88101

A low-cost variant using the DAC8311 DAC, LDO, and external low-power reference runs at 130µA and still achieves adequate performance.

Comparison of implementation plans

Table 1 and Table 2 show each implementation with its suggested bill of materials (BOM) and expected performance. Performance data is obtained through limited measurements.

Table 1: 4-20mA transmitter design, recommended BOM and performance (MSPM0 DAC12, PWM using M0, XTR117)

Conclusion

When designing a 4-20mA transmitter, this selection process can help you determine the correct implementation:

• If you are building a safety system and require ultra-high accuracy and ultra-low noise performance, or are looking for a sub-200µA HART-enabled transmitter, the AFE88101 and AFE881H1 should be your first choice.

• If performance is the priority over power consumption, the DAC161S997 implementation has the lowest power consumption and space usage, followed by the DAC8311 implementation, and then the XTR117 implementation.

• If the lowest cost is required, choose the MSPM0G implementation. If its performance is not good enough, the next low-cost solution is the PWM solution.