Top 10 Advances in RTD Signal Conditioning
Recent innovations in signal conditioning technology for RTD temperature measurements are made possible by employing application-specific integrated circuits (ASICs) in commercial-off-the-shelf (COTS) instruments.
Here’s a Top 10 list of advanced features and benefits of available RTD transmitters:
1. Easy Configuration and Calibration Using a PC’s USB Interface.
Simple USB digital configuration and calibration is made possible by new RTD transmitters that incorporate an ASIC. This design eliminates the need for zero/span pots, jumpers, DIP switches, or pushbuttons. The PC interface is easy, calibration is faster, and transmitter performance is improved. Users select functional parameters from Windows-based configuration software, and the settings download via USB into non-volatile EEPROM memory to control the transmitter’s operation. Setup is reduced to clicking the input type input wiring, sensor alpha coefficient, and sensor fault detection settings then entering the input/output range values on an intuitive configuration screen.
2. Analog Amplifier Enhanced with Digital Calibration
ASIC-based RTD transmitters have an input signal that is not digitized. There are no microcontrollers in the I/O signal path and no embedded firmware requirements for signal processing. Instead, an analog signal path is maintained for the highest accuracy and performance. The first benefit is that measurements are continuous and have no steps or update time delays in the output signal, unlike digital designs. Second, ASIC-based transmitters deliver the reliability of an analog design with the flexibility, convenience and stability of a digital design. The fully analog ASICs are also more immune to high electromagnetic interference for increased reliability in critical applications.
3. Improved Temperature Measurement Through Simultaneous Differential Sampling
Simultaneous differential sampling eliminates small measurement errors from digital multiplexer sampling of sensor wires at different points in time. An ASIC samples all sensor wires at the same time and measures the analog input signal continually across the entire signal to produce an accurate differential measurement. Advanced auto-zero circuit techniques disregard anomalous data points and accuracy improves.
4. Superior Lead-wire Compensation
A third wire is often used to compensate for resistance differences in the sensor’s lead wires. However, traditional RTD measurement circuits typically require all three sensor wires to be the same length, gauge, and ohmic impedance for best results. Any impedance differences in a 3-wire measurement produce a small, “static” error unless calibrated out. ASICs, on the other hand, can measure and reject the third wire’s resistance so their length and gauge are not factors. This design adds flexibility when installing a transmitter with a 3-wire input configuration. The RTD measurement accuracy improves and reduces recalibration requirements for systems with long lead wires.
5. Reduced Sensitivity to Ambient Temperature Changes
The ambient (surrounding) operating temperature range for industrial-grade RTD transmitters is typically –40 to 80oC. ASIC technology offers higher performance across a wider ambient temperature range making ASIC RTD transmitters well-suited to harsh environments. An ASIC that has a programmable gain amplifier (PGA) with an auto-zeroing internal frequency to offset temperature drift can achieve drift values as low as 5 ppm/oC. By comparison, digital RTD transmitters drift around 50 ppm/oC for ambient temperature changes. So for applications that must endure variable ambient temperatures, there are now commercially available solutions selling for under $100 that approach military-grade drift specifications.
6. User-Programmable Over/Under Range Values
Typical analog 2-wire transmitters have a 4-20mA output signal which is difficult to control beyond the endpoints. But, an ASIC can support configuration of output ranges with specific over/under range levels and different sensor fault/alarm limit levels. This feature enables identification of differences between a “runaway” process temperature and a sensor fault condition. For example, the output signal could be set to allow a level up to three percent above/below the expected range. However, a broken lead wire would send the output signal further beyond these over/under-range levels. Controller devices could then react to these different circumstances appropriately.
7. User-Configurable Failsafe Modes
In the event of a measurement fault, the configured failsafe mode drives the output signal either “upscale” or “downscale”. The ASIC can produce an output level that is 1mA above the normal over-range value (upscale) or 0.4mA below the normal under-range value (downscale). The control system can then set an alarm condition trip point based on these signal levels. Such programmability helps ensure safe, predictable execution of alarm or shutdown operations in critical applications. ASIC technology is also Namur-compliant, meeting the specification’s linear output range and failure limit requirements.
8. Better Linearization
New RTD measurement techniques employ a unique, low noise, voltage‐to‐current conversion scheme that rivals 12‐bit performance. ASIC technology goes even further to digitize the signal by using an integrated digital‐to‐analog converter. In so doing, zero offset is adjusted, excitation currents are controlled, and a linearization correction of 40:1 is applied to the input. This innovation along with a programmable gain amplifier yields even more accurate temperature measurements. Because the temperature measurement technology is analog, users can calibrate the amplifier for unique narrow ranges without sacrificing resolution. Exceptional accuracy (±0.05%) is achieved over ranges within 250°C. In contrast, digital RTD transmitters require expensive circuitry to handle multiple, pre-set ranges so accuracy does not degrade on narrow ranges.
9. Faster Response Times for Tighter Control
RTD sensors are seldom used when a fast response is required because of the “thermal lag” time to heat up and show a resistance change. RTD transmitters can also have a slower response time due to added filtering for noise immunity. However, ASIC‐based RTD transmitters have response times of less than one millisecond compared to 20 to 1200 milliseconds for traditional RTD transmitters. In applications where thermocouples were once the only solution, quick‐response ASIC RTD transmitters can now work with faster, low thermal mass RTD sensors. Faster measurements enable more detailed temperature profiles, tighter temperature control, and reduce the time to shut down a “runaway” process.
10. Higher Reliability
ASIC RTD transmitters have fewer parts than conventional analog or digital transmitters. Fewer parts translate to a lower failure probability and higher reliability. ASIC‐based RTD transmitters can have as few as three major parts—the ASIC IC, EEPROM memory, and a USB‐to‐serial converter.
So there they are—the Top 10 advances in RTD signal conditioners. Acromag’s ST131 & TT231 ASIC‐based transmitters feature many of these advances. They have a USB interface for easy PC configuration and the compact, DIN Form B connection head is well suited for thermo well or rail mount applications.
Acromag has designed and manufactured measurement and control products for more than 50 years. They are an AS9100 and ISO 9001-certified international corporation with a world headquarters near Detroit, Michigan and a global network of sales representatives and distributors. Acromag offers a complete line of industrial I/O products including a variety of process instruments, signal conditioners, and distributed fieldbus I/O modules that are available with a 2-year warranty. Industries served include chemical processing, manufacturing, defense, energy, and water services.
For more information about Acromag products, call the Inside Sales Department at (248) 295-0880, FAX (248) 624-9234. E-mail firstname.lastname@example.org or write Acromag at 30765 South Wixom Road, Wixom, MI 48393 USA. The web site is www.acromag.com.