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Instrumentation Moves from the Lab to the Factory Floor
Analog circuitry eases the transition
Randy Frank, Contributing Editor -- Design News, November 3, 2008
Portable. Battery-powered. Wireless. While these terms usually describe the latest consumer products, they are the enablers for taking laboratory-grade instrumentation into the factory.
“People want to be in better control of their processes,” says Art Eck, senior product marketing manager, Microchip Technology. With instrumentation becoming increasingly portable with easy connectivity, factory engineers can afford to measure things they didn't measure before. Eck sees customers making more temperature, pressure, acceleration, motion control and even more chemical measurements. “Once you do that, it opens up extreme process control, your yields go up and it's a great investment,” he says.
“A lot of instrumentation companies are making their instruments much more available to general purpose, industrial-type applications going hand in hand with a lot of the connectivity,” says Trent Butcher, product marketing manager, Microchip Technology.
This transition is driving the development of several new analog integrated circuit products. Microchip's MCP6V0X auto-zero operational amplifiers with rail-to-rail input/output were specifically designed to ease the transition from the laboratory to the factory.
“We see that auto-zero as being a strong play into instrumentation and industrial markets,” says Eck. Part of the reason is the increased use of technology by personnel that may tend to give it more abuse than it experienced in a laboratory environment.
“Expensive probes aren't good in the hands of less technical folks,” says Eck. “You need a more durable probe.” This puts more demand on the input circuitry to the instrument for trimming out errors to avoid putting the amplifier in the probe. Amplification in the probe makes it more expensive and replacing a broken probe becomes quite costly. And as Eck says, “That becomes a big issue as this equipment gets more ubiquitous.”
Lower cost probes require higher-resolution analog to digital converters (ADCs) and lower-offset amplifiers. In addition to the MCP6V0X solution for the amplifier portion, Microchip has its MCP3551, a low-power, Single-Channel 22-Bit delta-sigma ADC to avoid amplifying at the probe.
“If you go with a less expensive probe you are going to have more noise, more trouble getting out the signal that you want and therefore having less offset becomes much more critical,” says Eck. As a result, improved common-mode rejection at the input side of the measurement unit becomes much more critical, a situation the MPCV0X addresses. “A signal where you don't need quite as much resolution, you can now get away with putting an auto-zero on the front end and eliminating a lot of the noise that way,” he says. This avoids introducing large offset errors.
Connectivity is another aspect of instrumentation in the factory. “Sometimes from the industrial side we are seeing a lot of interesting bus architectures like the CANbus being used,” says Eck. In fact, the use of USB and CAN networks with RF wireless nodes is increasing and creating additional problems for analog circuits to solve. With many distributed network locations and circuitry powered by 5.5V or less, dynamic range is a major issue. “You are limited on what your input voltages can be, so offset voltage is important,” says Butcher. This provides greater need for a high-performance amplifier. Eck adds, “You need an operational amplifier that is rail-to-rail because the dynamic range is so much smaller.”
Even though the probes are being extended to new applications and becoming ubiquitous, that does not mean that the performance has been compromised. “It's actually improving,” says Eck. The new probes are more rugged and lower-end but the overall system constraints are actually higher.
The improved analog circuits also provide something for laboratory instruments. Applications with expensive probes still exist, since the experienced lab personnel continue to push the envelope in terms of accuracy in their measurements. Using higher performance amplifiers in the probe can take instrumentation to the next level.
Wide Temperature Operation
With instruments coming out of the controlled laboratory environment, temperature is another area that requires special attention for analog products. Products such as the Linear Technology LT6107, a high-side current sense amplifier addresses this problem with a Military Plastic grade (MP-grade) package and temperature operation from -55 to 150C.
“The LT6107 offers a simple high-side current sense solution for extremely challenging environments,” says Mike Kultgen, design manager for Linear Technology.
Operating with common mode voltages up to 36V, the amplifier has an offset voltage guaranteed to be less than 400 µV over temperature and can accept a full-scale differential input of 500 mV, resulting in a minimum dynamic range of 1,250:1. The performance and operating temperature range make the current sense amplifier well-suited for low power and battery-operated industrial applications.
Fault Protection
In USB and CAN architectures, transmitting data and power along the same cable requires special design considerations to avoid problems. To implement this increased connectivity, Maxim Integrated Products' MAX13448E provides a full duplex transceiver that integrates fault protection on the RS-485 interface pins allowing it to recover from shorts up to ±80V. The integrated protection eliminates the need for external fault-protection circuitry such as positive temperature coefficient limiters and Zener diodes. The internal protective circuitry prevents data transmission during a fault condition and allows normal operation after the fault condition has been removed.
To avoid problems that can result from high levels of radiated noise, slew-rate limiting circuitry reduces electromagnetic interference (EMI) and reflections on the data bus. With a 5V supply this can mean up to 500 kbps of error-free data transmission or up to 250 kbps with a 3.3V supply.
Specified over the -40 to 85C temperature range, the transceiver solves not only industrial issues but those in HVAC and motor-driver control systems, as well.
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