Basics about signal isolators, barriers

Occasionally, analog signals cannot be fed directly from the sensor to the Beckhoff analog input terminal, in which case a special intermediary device must be added.

Reasons may include:

• The sensor may be installed in hazardous locations and protected according to the intrinsic safety ignition protection type (Ex i) while a Beckhoff ELX3xxx terminal is not yet available for the desired application.
• Separate electrical isolation between the sensor and the Beckhoff terminal is required.
• The sensor has an electrical output signal for which Beckhoff does not yet offer a suitable input terminal.

The type of intermediate device depends on the following criteria

• Electrical signal supplied by the sensor? voltage 10 V or µV, AC or DC, 20 mA or 1 A, resistance, …
• The sensor must be powered in some way, e.g.
• an IEPE sensor requires 2..8 mA constant current,
• a resistor requires a measuring current,
• an electronic sensor may need a 24V supply, or it may be fed via a 20 mA loop.
• What dynamic transmission quality for AC signals must the sensor provide via the intermediate device? Each intermediate device influences the analog signal, e.g. in terms of frequency-dependent attenuation, crosstalk, line resistance or bandwidth. This must be taken into account when an intermediate device is used in a metrological application.
• Is the device used for energy limitation in accordance with the intrinsic safety ignition protection type (Ex i)? In this case, a barrier with appropriate approval is required. Zener barriers are often used in such situations. They are made up of resistors, fuses and Zener diodes.
  
  As already mentioned, these can influence the analog signal quality with respect to the above features, e.g. by temperature-dependent change of the internal resistance.
  Terms: Zener barrier.
• Does it have to ensure electrical isolation of the analog signal?
  
  Does electrical isolation of the analog signal have to be provided? Devices that electrically isolate the transmitted signal reconfigure the signal, so that in this case special attention must be paid to the signal influence. In this case the analog properties of the isolator and the Beckhoff analog terminal are interlinked. The properties of the isolator are dominant, particularly when Beckhoff ELM measurement terminals or other high-quality analog terminals are used. On the output side, they typically supply standard signals, such as 10 V or 20 mA. Compared with the use of external devices for electrical isolation, the use of Beckhoff input terminals with channel-based electrical isolation is advantageous.
  Terms: signal isolator, signal converter, signal transducer, isolating amplifier, measuring amplifier, level transducer.
• Are both measures, i.e. explosion protection according to ignition protection type Ex i and electrical isolation necessary? In this case, so-called isolation barriers are used, which ensure energy limitation for intrinsic safety and also electrical isolation of the signal. Please refer to the notes on analog signal influence referred to above.

From a metrological point of view, signal-influencing intermediate devices should be avoided if possible.