10 January 2012

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Sensors transmitters are commonly used in process industries to help control parameters such as temperature, pressure and flow. Temperature transmitters (TT), for example, can be found in food and beverage, pharmaceutical and environmental control processes. In the chemical industry, the TT will be used to ensure the solution in a chemical reactor stays at a constant temperature.

Fundamentally a sensor transmitter performs three functions: it gathers data from the sensor; isolates the sensor from the central controller; and sends a current representing the measured quantity down a 4 to 20mA loop. Engineers developing sensor transmitter applications are faced with time consuming designs that require configuration and features specific to each type of sensor.

Temperature transmitter basics
Figure 1 shows a typical temperature transmitter block diagram. It reads a temperature sensitive element, such as a thermocouple and transmits the information over a long cable.

Various temperature sensors can be connected to the transmitter’s analogue front end (AFE), which handles the acquisition and conditioning of the sensor’s signal. The information is transferred to an mcu for signal processing and finally sent as current down the loop through isolation. The temperature transmitter is powered by the loop itself; therefore, it is critical to minimise power consumption while achieving the highest possible accuracy.

Temperature sensing applications
Various sensors are used to monitor temperature; examples are Resistance Temperature Detectors (RTDs) and thermocouples.

The resistance of a RTD changes with temperature, the most common material used for these sensors is platinum. RTDs are available in a range of impedances; three common industry standards are Pt100 (100O), Pt500 (500O) and Pt1000 (1000O) and RTDs are usually used in applications between -200 and 800°C.

Thermocouples are made from two dissimilar metals tied together. A voltage is produced at the metal junction relative to the temperature. The junction voltage is compared to a reference voltage called the cold junction. All thermocouples require a cold junction reference to be maintained at a known temperature. A sensor at the cold junction monitors the cold junction temperature and this can be implemented with an analogue temperature sensor such as the LM94022. The temperature at the thermocouple junction is calculated from the difference in the thermocouple voltage and cold junction voltage. Thermocouples are found in similar applications to RTDs, but are not as accurate. However, they are rugged and address temperatures ranging from -270 to 2300°C.

In Fig 2, the LMP90080 provides the complete signal path between the sensor and the microcontroller. It features a flexible input multiplexer, allowing any input pin to be connected to any a/d converter input, making it possible to interface with a wide range of sensors, including: thermocouples; two, three and four wire RTDs; and thermistors. The Sensor AFE provides two programmable match current sources, adjustable in 100µA steps to a maximum of 1mA. This support resistive sensors, such as RTDs. The gain is adjustable in binary factor from 1 to 128 and designers can select the output date rate with single cycle settling (eight values are offered between 1.6775 and 214.65sample/s), enabling it to interface with a variety of sensors.

The LMP90080 is part of a family of pin to pin compatible products. This allows the designer to use the same layout and to optimise the design by selecting different features, such as resolution and differential channels, as well as integrated current sources. Other features include continuous background calibration and diagnostics.

Continuous background calibration compensates for gain and offset errors in the a/d converter, virtually eliminating any drift with time and temperature. Calibration is performed in the background without any conversion interruption. This improves performance over the life span of the end product and helps reduce the down time associated with field calibration.

Sensor diagnostics are also performed in the background, without interfering with signal path performances, allowing the detection of sensor shorts, opens and out of range signals. The LMP90080 also provides a robust serial peripheral interface (spi) with cyclic redundancy check to ensure data transfer integrity.

These products are supported by the Webench Sensor AFE Designer Tool that allows for the design, the performances’ estimation and the optimisation of a sensor analogue signal path solution. It permits the AFE configuration to be exported and used for the microcontroller code. In addition an evaluation system allows the designer to attach sensors, modify a design, load configuration data to the Sensor AFE IC and evaluate the complete solution.

The 4 to 20mA current loop is an industrial standard for analogue measurement transmission over long distances, in which the two wire transducers use current for representing the measured value. Current loop transmission is commonly used in industry because it is robust to interference and well suited for long distance transmission. It is also low cost and can be used in potentially explosive areas.

The DAC161P997 (see fig 3) eases the design of current loops by integrating all precision elements on chip. Only a few external components are needed to realise a low power, high precision industrial 4 to 20mA transmitter.

The DAC161P997 features a single wire interface, a robust solution for transmitting digital data which allows galvanic isolation to be implemented easily, compared to solutions with spi interfaces. The digital data format supports transmission without loss of data, an issue that can affect pulse width modulation schemes.

The output current sourced by the OUT pin of the device is expressed by:

Iloop = DACCODE/2^16) / 24mA

The valid DACCODE range is the full 16bit code space (0x0000 to 0xFFFF).

As shown in fig 3, the DAC161P997 cannot interface directly to a typical 4 to 20 mA loop due to the excessive loop supply voltage. The loop supply has to be stepped down to 3.3V and this can be done using either an ultra low quiescent current linear regulator, such as the LM2936, or a low power switching regulator, like the LM2840. The designer should keep in mind that the regulator’s quiescent current will have a direct effect on the minimum achievable loop current.

As the temperature transmitter is powered by the loop itself, the current consumption of the various components should be as low as possible to respect the system’s 3.5mA power budget. The DAC161P997’s consumption is less than 190µA, with an output current temperature coefficient of 29ppm/°C and a long term output current drift of 90ppm full scale.

Integrated solutions, such as the LMP90xxx and DAC161P997, ease the challenge of bringing together sensors and transmitters, while reducing component count and board space. The LMP90xxx can interface with various sensors, including temperature, pressure and other voltage output detectors. The device is supported by the Sensor AFE Designer Tool, which enables engineers to design and evaluate their solutions online.

Carine Alberti is a product marketing engineer with National Semiconductor Europe.

Carine Alberti

Supporting Information


National Semiconductor UK Ltd

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