Analog I/O: connected but safely isolated

Today’s analog I/O device vendors are giving industrial I/O builders functionality that includes Ethernet communication, digital and capacitive isolation, built-in sensors and new levels of configurability.



By Wayne Labs, Contributing Editor

THE HIGH-TECH industries’ mantra since the debut of the microprocessor has been “smarter, faster, smaller and cheaper.” In response, I/O device (chip) manufacturers give industrial I/O designers tools to build-in more channels for less money with higher accuracy and throughput. However, the real news with industrial analog I/O systems concerns connectivity, isolation and configurability. Digital isolation allows more flexibility on the analog front end and increases overall performance with larger signal bandwidth, higher input impedance and lower costs..

High-end machine control requires increasingly faster sensing and response. “Current chip architectures, and especially communications technology, allow us to build analog I/O to achieve A/D conversion rates of 10 microseconds (µs) and sampling rates as fast as 50 µs for up to 100 16-bit analog points, resulting in input-to-output raction times of 150 µs,” reports Skip Hansen, Beckhoff Automation’s I/O systems product manager.

To get this level of performance, industrial I/O builders tap into the enhanced features of new and specialized chip technologies. For example, Texas Instrument’s TMS320F2801, -06, and -08 digital signal controllers allow for 150-picosecond (ps) edge placement on their on-chip PWM outputs. “This allows for more than 16 bits of resolution in lower frequency (100 kHz) applications, while still retaining more than 11 bits of resolution in 2.5 MHz applications,” says Chris Clearman, of TI’s C2000 applications division. These chips include a 12-bit analog-to-digital converter (ADC) and communication interfaces, and work in power conversion systems.

TI engineers note that RS-485 signaling is widespread in industrial networks using Modbus, Profibus, Interbus, and many other protocols, but Joe Benedetto, Schneider Electric’s senior product manager, says “the most popular communication protocol is Ethernet, and we have seen a big shift away from proprietary networks.” Ethernet is easy to implement, has a bevy of support devices, and is now industrially hardened.

Industrial I/O designers, however, demand several improvements in transceiver features for their latest products. TI responded by providing chips with higher ESD protection levels, lower unit loading, and a choice in driver speed by slew rate control, which reduces high-frequency effects and minimizes signal reflections at network discontinuities.

Ken Tang, business manager at chipmaker Maxim Integrated Products, notes that microcontrollers in chip-level ADCs now perform A/D conversion in real time, support complex displays, and provide communication interfaces. When asked what was hot, John Lehman, Dataforth’s engineering manager, replied, “Wireless technology has become readily available at low cost, which will benefit many industrial applications. Advances in the reduction of IC and discrete part package sizes allow Dataforth to reduce the size of isolated signal conditioning modules by 20%.”

“It appears that an increasing number of data acquisition manufacturers have moved away from analog isolation to digital isolation,” says Nipun Mathur, National Instruments’ industrial data acquisition product engineer. “Digital isolation allows more flexibility on the analog front end and increases overall performance with larger signal bandwidth, higher input impedance, and lower cost.” Mathur adds that newer digital isolation technologies, such as iCoupler from Analog Devices, use chip-scale transformers to offer several channels of isolation in a small form factor, cutting the need for slow and power-hungry optocouplers in I/O products.

Another isolation technology soon to be released is TI’s capacitive isolation, which is aimed specifically at industrial I/O. This technology can be applied to data couplers, isolated 485/CAN communications, data converters, and power supply modules, and is said to provide fast data rates while having immunity to electromagnetic interference (EMI).

Higher-voltage ADCs will reduce noise. According to Mike Britchfield, Analog’s precision signal processing products director, low and high-voltage (30-36 V) CMOS chips with small geometries (0.6 µm) intended for industrial I/O applications will improve the performance of both converters and amplifiers, and reduce noise figures while improving linearity and stability.

More interesting is the company’s iMEMS architecture, which allows accelerometers to be included on the chip; and its iSensor devices, which place intelligence at the sensor core, allowing designers to set up embedded solutions for motion control, temperature, vibration analysis and power systems.

Finally, configurable analog I/O chips hold a lot of promise for industrial applications. “Exciting new reconfigurable ‘on-the-fly’ analog technologies will allow future analog I/O modules to offer more features and allow customers to use generic analog I/O building blocks to create highly customized solutions for their specific applications,” believes Bill Black, GE Fanuc Automation’s controllers product manager. “Companies, such as Anadigm offer field-programmable analog arrays (FPAAs) that allow reconfiguring analog blocks on the fly, providing more customization options for end users.”