Signal conditioning—why and how

Modern integrated circuit (IC) technology and the advances made in microprocessor ICs have made it practical and cost-effective to distribute process control functionality throughout the plant. This architecture is implemented by partitioning process control functionality into sections, which can either be supervised by a central system computer or operate autonomously.

With this in mind, why should you use signal-conditioning modules (SCMs) in control and data acquisition systems?

To standardize all sensor output signals. Each sensor then delivers information at the same voltage (current) level. This simplifies wiring, standardizes all the receiving ADC [analog-to-digital converter] front-ends, and brings the transmitted sensor information signal above the injected EMI noise levels.

With standardized signal levels and scale factors, sensor-SCM combinations can be added or upgraded to facilitate system expansion or the use of new sensor technology with minimum impact on system control software.

To establish common scaling and linearization for sensor information directly at the sensor location.

The path from remote sensors and associated conditioning modules to the receiving device usually transverses many ground nodes and may experience undesirable ground loop induced voltages. SCMs, which provide three-way isolation (signal, field-side excitation, and computer-side power), eliminate these ground loop degradation problems.

To reduce system Mean Time to Repair as individual sensor signal-conditioning modules allow field failures to be repaired through SCM replacement.

Noise filtering is available on most ADC cards used as analog front-end input in microprocessor-based controllers. However, using individual SCMs with advanced high quality multi-pole filters provides better noise immunity and custom filtering.

SCMs with normalized isolated signal levels provide a simple, interface to intrinsic safety barriers required for hazardous environments.

SCMs that transmit analog field data using the 4-20 mA current loop provide system engineers with additional advantages. For example, an individual current-loop SCM can interface with field monitors, autonomous controllers, and the main controlcomputer all in one field loop.

Quality SCMs with input isolation and multi-pole filtering provide the following protections: normal-mode rejection exceeding 85dB at 60Hz; common-mode rejection exceeding 160dB at 50-60 Hz; continuous 1,500 Vrms common-mode voltage; 240 Vrms continuous differential input overload with reverse polarity; and ANSI/IEEE C37.90.1-1989 transient protection.

How to use them

Schematic of Dataforth’s SCM5B47 input protected three-way isolated signal-conditioning module.

SCMs have a multitude of standard package styles, offering the system engineer several ways to take advantage of the benefits listed above. SCMs come in standard 5B and 7B packages, each of which can be plugged into a field-mountable “motherboard” that can be interfaced with the controlling device via either direct multiplexed analog lines or an optically isolated serial RS-232 type link. Field sensor wiring is connected directly to the module motherboard with typical instrumentation field wiring terminals. Perhaps one of the most versatile SCM packaging methods is the DINrail system, which allows individual stand-alone module mounting, with removable field wiring connections for instant module replacement and a choice of output schemes using analog, serial, or high-speed bus techniques.

Bob Smith, vice president at Dataforth Corp., www.dataforth.com

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