How to Use a High-Voltage, High-Current Driver Op Amp in a 4–20mA Current Loop

How to Use a High-Voltage, High-Current Driver Op Amp in a 4–20mA Current Loop

This article discusses how to use high-voltage, high-current drive op amps to convert voltage signals into ±20mA or 4–20mA current signals in process control industrial applications. Using the MAX9943 op amp as an example, experimental instructions and test results are given.

introduction

Current loops have a long history of use in process control industrial systems. Current loops can be used to convey information from remote sensors to central processing units, or from these central processing units to remote excitation sources. 4–20mA current loops are very common, but some systems use ±20mA current loops. For low-resistance loads, using high-voltage op amps to provide high current drive can eliminate external power FETs and simplify circuit design.

This article discusses how to use high-voltage, high-current drive op amps in 4–20mA current loops. The op amp converts the voltage signal from the DAC to a ±20mA or 4–20mA current output. The MAX9943 op amp was used in the experiment, and the test data is given in the article.

Current Loop Basics

A current loop typically includes a sensor, a transmitter, a 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 output into a proportional 4mA to 20mA current signal; the receiver converts the 4–20mA current into a voltage signal, and the ADC or microcontroller converts the receiver's voltage output into a digital signal.


Figure 1. Major components of a simple current loopIn

a current loop, information is transmitted via a current-modulated signal. For a 4–20mA system, 4mA typically represents zero output from the sensor, and 20mA represents full-scale output. It is easy to distinguish between an open loop (0mA, a fault condition) and zero output from the sensor (4mA).

Compared to voltage-modulated signals, current loops are inherently more immune to interference, making them ideal for noisy industrial environments. Signals can be transmitted over long distances, and information can be sent to or received from a remote location. Typically, the sensor is located far from the control center where the system microcontroller is located.

More complex systems include another current loop from the microcontroller or DSP to the excitation source ( Figure 2 ). The DAC converts the digital information into an analog voltage signal. The current loop transmitter converts the DAC output voltage into a 4–20mA or ±20mA current signal that drives the excitation source. Similar applications exist in power grid monitoring systems, which use sophisticated algorithms to determine the current state of the system, predict the direction of system change, and dynamically adjust the system through a control loop.


Figure 2. A complex system that uses another current loop to control the excitation source

Using operational amplifiers to achieve VI conversion and provide high current drive

The circuit shown in Figure 3 uses two op amps and a few external resistors to create a simple VI (voltage-to-current) converter. When powered by ±15V, the op amp (in this case, the MAX9943) can provide more than ±20mA of output current into a low-impedance load.

The MAX9943 is a 36V op amp with high-current output drive capability. It is stable when driving load capacitance up to 1nF. This device is ideal for industrial applications that need to convert the voltage signal of a DAC output into a proportional 4–20mA or ±20mA current signal.


Figure 3. Using a VI converter to convert the DAC output to a load current, this circuit uses two MAX9943 op amps.

The relationship between the input voltage, V _IN, and the load current is shown in Equation 1:

Generate ±20mA Current Drive from ±2.5V

The circuit shown in Figure 3 can also be used to generate a ±20mA current drive. When V _REF  = 0V, an input range of -2.5V to +2.5V produces a nominal ±20mA current output, as shown in Figure 4. The relationship between

the input voltage (V _IN ) and the “forward” op amp output voltage (V1) is as follows:

Generates 4–20mA current drive from 0 to 2.5V input range

From Equation 5 above, when V _REF  = -0.25V, the input range is from 0V to +2.5V and the current output can be 2mA to 22mA ( Figure 6 ). Often in a 4–20mA current loop, designers want some additional "headroom" in the dynamic range (for example, 2mA to 22mA) to allow for software calibration. If higher current is required, the MAX9943 can be powered from a ±18V dual supply, as described above.


Figure 6. Generating 4–20mA output current from a 0V to 2.5V input voltage range. The blue curve is the ideal gain curve; the red curve is measured data. V _CC  = +15V; V _EE  = -15V.

in conclusion

Current loops are widely used in industrial applications that need to transmit information from remote sensors to central processing units, or from central units to remote excitation sources.

Experiments have shown that the MAX9943 op amp is well suited for control loop applications that convert voltage signals from sensors or DAC outputs into 4–20mA or ±20mA currents. The MAX9943 has high-current drive capability over the entire temperature range. It can maintain stable operation when driving capacitive loads up to 1nF, which are often encountered in long-distance transmission.