One of the most complex problems in measuring a signal from a sensor or an electronic product is getting the signal to the final instrument without electrical interference. Often the only path for the wiring is in a cable trough or in conduit which also carries AC power leads, relay contact leads which switch inductive loads such as relay coils or motors or other potential sources of interference.

The most common signals measured are AC voltage, AC current, DC voltage, DC current, RTD, Thermocouple, Frequency, Potentiometer, and Strain Gage (Bridge). Strain gauge sensors and thermocouple have millivolt signal levels while AC voltages can be millivolts or hundreds of volts.

Some sensors are commonly inherently isolated. RTD's, bridges, and potentiometers normally have no connections except to the signal conditioner so they do not need an isolated signal conditioner.

Others can be grounded or isolated, depending on requirements of the system. Thermocouples may not be connected to anything but the signal conditioner, but they often are welded to the probe which houses the thermocouple. This probe is usually grounded to the device which it is mounted to. To prevent a ground loop, an isolated signal conditioner is required.

A decision has to be made to select an isolated or non-isolated signal conditioner. A careful analysis of the system wiring must be done to prevent interference from "noise" which can create errors in the intended measurement.

The desired system is one in which the signal leads have no ground loops and do not pick up electrical interference because of the path the conductors follow.

The isolated signal conditioner breaks ground loops and also provides a high voltage common mode capability. The isolated signal conditioner and the proper use of twisted, shielded wiring can prevent most interference to the intended signal.

The non-isolated signal conditioner cannot prevent ground loops and has very limited common mode rejection. It is best used where signal leads are short and are easily kept away from interference from other wiring.

THE VALUE AND USE OF DC INPUT ISOLATED SIGNAL CONDITIONERS

Why 4mA to 20mA Signals?

OUTPUT
This standard output has a powerful characteristic which gives it great value.  The output is a constant current proportional to the INPUT signal.  Since the current is constant,  the resistance of the wire has no effect on the signal.  This gives one the ability to run the output wire leads enormous distances without an error.  It also gives one the ability to put more resistors in series with the signal loop without introducing an error.  Each resistor could be the input of another signal conditioner.  These signal conditioners can be chosen to create any output needed for a given use.

The 4mA to 20mA current is enough to power a Loop Powered Display which can be put in the loop at any needed location. 

The current signal can also be used to power a Loop Powered Isolator which creates another isolated 4mA to 20mA signal without any other power.

LIMITATIONS

INPUT
A popular instrument for isolation and long signal leads is a 2 Wire Transmitter.  This instrument receives its power and provides its output over the same pair of wires.  The input signal leads can be extremely short due to the transmitter being able to be mounted at the signal sensor physical location.

The long output leads of the isolated signal conditioner makes it difficult to not pick up interference from all types of electrical noise generators.  Noise is defined as any electrical signal one does not want on one's signal leads.

Noise can be picked up by the input or output wire leads of an isolated signal conditioner.  Proper routing and placement of signal wiring can reduce noise to a none interference level.  The signal conditioner can be no better than the installation made for noise reduction in the wiring.  The only advantage of the isolated signal conditioner is that it breaks the galvanic connection between the input and output of the signal conditioner.  This prevents creating a ground loop in the wiring.

A well designed system requires careful selection of electronic products and routing of wiring to achieve the results expected.

For more information on wiring issues, Click Here to go to a wiring article on our Blog page.

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