Signal isolators provide electrical (galvanic) isolation between the input and output circuits. They couple the signal to the output through a transformer or optical isolator. Signal isolators also break the direct electrical (galvanic) path between two or more loop points. They are used to protect against dangerous measured-variable voltages and provide increased protection from surges and spikes.
Selecting Signal Isolators
Selecting signal isolators requires an analysis of product specifications. There are three basic types of products: four-wire isolators, loop-powered isolators, and two-wire isolator transmitters. Each type of device should provide signal isolation between the input, output, and power source. A typical signal isolator is capable of withstanding 1000 V, or more common-mode signals at the input.
Signal ground isolators are used to remove the DC circuit path from ground circuit. Generally, ground signals are present wherever multiple grounding is involved. Signal sources can be categorized as continuous wave (CW), which generates a single frequency; fixed sine wave; swept, which sweeps over a range of frequencies and may be phase-continuous; or signal generator, which adds modulation and produces a “real world” signal. An input signal is a control signal or sensor and can be isolated, filtered, amplified, scaled and/or liberalized by a micro-processor. Input signals are converted to a binary, weighted, digital word and transmitted serially (one bit at a time).
Signal isolators are used in industrial, medical and other environments in which electrical isolation is essential for safety. Signal isolators can also be used to amplify signals, enabling instruments to be added to an overburdened loop, or stepping down dangerous, high-voltage signals to safer levels. Signal isolators are useful in breaking ground loops (circuits where extraneous currents flow in the signal path and add to the signal).