New ASIC Technology Enables RTD Signal Conditioning Advances
By Donald Lupo, Marketing and Sales, Acromag, Inc.
Recent advances in Resistance Temperature Detector (RTD) sensor measurement and transmitter
technology enable a combination of analog accuracy with digital convenience and stability. Amplifiers
are now available in flexible, small, easy-to-implement packages and at a low cost. With the availability
of Application-Specific Integrated Circuits (ASICs), end users and OEMs can take advantage of innovative,
low-cost, commercial-off-the-shelf (COTS) temperature measurement solutions that deliver advanced
features and benefits. New ASIC technology delivers higher performance with more reliable RTD
temperature measurements. And when combined with USB technology, users get compatibility with
modern PCs for more convenient transmitter configuration and calibration.
A Brief History of RTD Signal Conditioning
Analog RTD transmitter technology became popular in the 1960s with offerings from various
manufacturers. With assistance from magnetic amplifiers early on and later from solid state transistor
technology, RTD signal conditioners improved rapidly. Through the use of jumpers and zero/span
potentiometers, which are still used today, users are able to calibrate and set ranges on the transmitter
to measure temperatures to within a fraction of a degree and output a common analog signal such as 420mA. Over the years, manufacturers of analog-based RTD signal conditioning instruments offered
many innovative designs to combat performance issues commonly associated with RTD measurements.
These performance challenges include:
1. Providing higher accuracy and more repeatable measurements while minimizing the long-term
effects of drift due to some analog components and potentiometers.
2. Improving lead-wire compensation circuitry for 3- and 4 wire measurements.
3. Improving linearization for non-linear RTD temperature sensors.
4. Adding over/under-range capabilities for failsafe systems that can detect the difference
between a sensor fault or “runaway” process.
5. Developing faster and easier methods for field ranging, configuration, calibration, and general
instrument maintenance.
Seeking to answer these challenges, the industry began adopting digital technology in the early 1980s.
Technology evolved in the 1990s to include A/Ds, D/As, and microcontrollers. Digital designs commonly
used today solve most of the RTD measurement challenges. They provide excellent stability and high
accuracy with regard to linearization, lead-wire compensation, and fault detection. In addition, digital
transmitters offer users easier configuration and faster calibration. Although the costs of digital-based
RTD transmitters are somewhat higher, their advantages typically offset the extra cost for most
requirements. But for critical or safety-type applications, analog RTD transmitters still provide an
advantage over their digital counterparts. Analog RTD signal conditioners are preferred for use in critical
systems because they offer better reliability. For example, it is believed they are more immune to
severe electrical surges or electromagnetic impulses that may cause a digital “upset” within a system.
Along Comes ASIC
New, innovative ASIC technology for RTD signal conditioners now offers the performance advantages of
digital technology with the reliability advantage of analog technology. Additionally, ASIC technology is
available at a much a lower cost and with a smaller footprint. Today’s ASICs allow users to digitally
control and calibrate an analog amplifier. They increase performance and reliability by consolidating
several functions into a single integrated circuit and use digital control of calibration, linearization, and
sensor fault parameters. No potentiometers, jumpers, dip-switches or pushbuttons are needed for
adjustment.
RTD ASICs do not run an instruction set like a microprocessor, nor do they use an A/D on the front end.
Essentially, they use an analog programmable gain amplifier (PGA) internally which is digitally
controlled. For example, digitally controlled D/A converters precisely set the zero offset, sensor
excitation and linearization for the analog signal path. This scheme delivers the equivalent of 12-bit
performance without digitizing the signal. It’s simply an analog transmitter with digital control and
calibration. When this RTD ASIC is combined with non-volatile memory and a USB interface, users gain
many advantages and flexibility. As shown in the block diagram, EEPROM is pre-loaded with information
via a USB connection to a PC that runs easy-to-use configuration software. The RTD ASIC uploads this
content at power-up to define its operation. It supports precision 2-wire, 3-wire, or 4-wire RTD
measurement and offers a standard 4-20mA loop powered output.
ASICs offer all the features you would expect from a digital microprocessor-based RTD transmitter and
more: excellent accuracy (<0.1%), increased stability (<100ppm/C), programmable 2, 3, or 4-wire
measurements, improved lead-wire compensation, dynamic sensor linearization, programmable
over/under range values, and selectable sensor fault detection.
Acromag’s new ASIC-based RTD transmitter is a miniature, DIN Form B, field-mount instrument. Users
benefit from the flexibility and stability of a digitally controlled device with the accuracy and reliability of
a fully analog signal path. This technology, combined with a USB interface, offers easy configuration and
fast calibration using a modern PC. For more information on these technology advancements and their
implementation, go to www.acromag.com and search for the Model ST131.
Acromag
Wixom, MI.
(248) (248) 295-0880.
[http://www.acromag.com].