Designing Intelligent 4–20 mA TransducersAugust 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.
Modern industrial automation applications—such as PLCs, factory process control, and intelligent transmitters—all demand high-performance, low-power components. Current-mode data transmission is no exception. Intelligent 4–20 mA current-mode data transmission is the preferred technique in many industrial automation applications and is a well-established standard for communications between the host computer and smart/intelligent transmitters in harsh factory environments. This article describes intelligent transmitters and explains their need for high-resolution, low-power ADCs, DACs, and isolation devices.
Why Use 4–20 mA Loops for Data Transmission?
When transmitting low-amplitude, low-frequency data signals over several hundred meters in a noisy industrial control environment, current-loop transmission is well established and is preferred over voltage-mode transmission for the following reasons:
- 1. Insensitivity to IR drops makes current loops suitable over long distances. In contrast, voltage at any point depends on line resistance and capacitance and therefore varies with cable length.
- 2. Current transmission allows a single 2-wire cable to carry both power and signals at the same time, an important factor when powering electronic components in remote locations.
- 3. Current loops don't require a precise or stable supply voltage.
- 4. Inexpensive 2-wire twisted-pair cables offer good noise immunity and lower EMI sensitivity.
- 5. It's easy to detect offline sensors, broken transmission lines, and other fault mechanisms.
Intelligent 4–20 mA Transducer Design
In a 2-wire, 4–20 mA current loop, the supply current for the sensor electronics must not exceed 4 mA (the remaining 16 mA carries the signal information), so the components that make up the transmitter must be low power. As shown in Figure 1, smart transmitter systems use five building blocks: an ADC, a microcontroller or DSP, memory (RAM), a DAC with an optional integrated amplifier and reference, and a sensor or transducer.
Figure 1. Block diagram of a smart 4–20 mA transmitter
The microprocessor or microcontroller performs linearization and other functions on the sensor data and communicates them back to the host system. The transducer voltage—usually ranging from a few millivolts to a few volts, depending on the type of sensor (Figure 2)—must be digitized by a high-precision ADC before the signal reaches the processor. Choosing a high-resolution ADC with an onchip low-noise programmable-gain amplifier (PGA) and good offset, full-scale, and drift specifications ensures precise conversion of the sensor inputs with minimum noise and drift due to external temperature variations.
Figure 2. Frequently used industrial automation sensors and their input spans Note: *A/D input spans from resistive sensors depend on the magnitude of the excitation current sources
For high-end applications, the rms noise requirements must be <100 nV at high gain settings (e.g., 64 or 128), with offset and gain drift of 10 nV/°C and 1 ppm/°C, respectively. Operating current consumption ideally should be <400 µA. The ADCs used in 4–20 mA transducer designs often include simultaneous 50/60 Hz rejection filters, onchip matched current sources for cold junction compensation and resistance temperature detector (RTD) biasing, and a precision reference. If integrated, these features simplify the design task significantly by eliminating some of the board and layout challenges presented by discrete components while simultaneously reducing cost.
Interfacing to RTDs
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