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Sensors Mag

Designing Intelligent 4–20 mA Transducers

August 1, 2006 By: Brendan Cronin Sensors

Intelligent transmitters are only as good as their components. Here's how ADCs, and other low-power devices, come into play.


The next step up is an even smarter circuit called an intelligent transmitter (Figure 5). In an intelligent transmitter, the functions of the microcontroller are shared among deriving the primary measurement signal, storing information regarding the transmitter itself, and managing a communications system that can superimpose two-way communications on the same circuit used to carry the measurement signal. A smart transmitter incorporating the commonly used HART protocol is an example of an intelligent transmitter.

Figure 5. Block diagram of an intelligent 4–20 mA transmitter
Figure 5. Block diagram of an intelligent 4–20 mA transmitter
 

Once the data have been appropriately conditioned and digitized, they can be transmitted through the 4–20 mA current loop using a DAC. Figure 6 shows how to create a digitally controlled 4–20 mA current loop by combining a resistor and a MOSFET with a DAC, such as the Analog Devices AD5660. The DACs used to control 4–20 mA current loops must be high precision, low power, low voltage, and ideally have internal amplifiers able to control the gate voltage of the external MOSFET, thus simplifying the design task. The SPI-, QSPI-, I2 C-, and Microwire-compatible interfaces on the DAC simplify interfacing, allowing their use with most of the processors and controllers found today.

Figure 6. Digitally controlled 4–20 mA current loop using the AD5660 DAC
Figure 6. Digitally controlled 4–20 mA current loop using the AD5660 DAC
 

The only drawback to this configuration is the need to drive the external MOSFET, which requires a much higher supply voltage. Most industrial control applications provide low and high voltages to support both 3 V and 5 V logic control and the sensors, which can demand voltages as high as 36 V (24 V nominal). In the example in Figure 6, the external MOSFET drives the current loop with a current given by Equation 1 and determined by the 50 α sense resistor.



 

The DAC's resolution is N and the decimal equivalent of its input code is D, which represents the sensor's digitized output after processing. Resolution for the serial-input AD5660 DAC is 16 bits, yielding a full-scale range of 2N = 216 = 65536 codes. For a 1.25 V reference, 50 α sense resistor, and full-scale DAC output of D = 65536, the output current is IOUT = VRef/RSense = 1.25 V/50 Ω = 25 mA. (In most applications, the current loop must be capable of delivering a 10% or greater over-range.)

For transducers in remote locations with no local power source available (such as in flowmeter applications), the electronics must be powered from the 4–20 mA loop. In this case, the application needs a voltage regulator, which converts the loop voltage to a fixed 3 V/5 V for operating the various sensor electronics. The voltage regulator can be any low-cost device with a quiescent current sufficiently lower than the 4 mA budget.

 

Moving Ahead

 

More and more industrial automation applications are requiring low-power converters both to accurately measure and control various processes and to transmit data afterward. These end applications demand increased performance, robustness, and feature sets, while simultaneously reducing costs and board area. Intelligent 4–20 mA transducers are one such application. Component manufacturers are addressing these challenges and offering a number of converter and isolation products to address the needs of system designers for current and future designs.

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