When the voltages at the two ground points differ, the copper wires used for signal and ground form a circulating, closed current path. When this occurs, an additional and unpredictable amount of current is introduced into the loop, distorting the true measurement. This current path, known as a ground loop, is a common source of signal inaccuracy.
We use isolation to prevent signal inaccuracy. This isolation is called Signal Isolation. Signal isolation prevents direct current and unwanted alternating current from being transferred between two parts of a system while allowing signal and power transfer between those two parts.
Optical isolation and electromagnetic isolation are the most commonly used methods for signal isolation. However, other methods of isolation exist, such as the use of a capacitor, magnetoresistance, and a hall effect sensor.
Transformer isolation is also known as electromagnetic isolation. It uses a transformer to electromagnetically couple the desired signal across an air gap or non-conductive isolation gap.
A transformer’s primary and secondary windings are insulated from one another. They are not electrically connected, so there is no metal-to-metal contact. They use magnetic field flux, generated by wire coils overlapping a ferromagnetic material; signals are inductively coupled to/from the ferromagnetic material using a varying magnetic field.
The principle of optical isolation is to transfer electrical signals between two isolated circuits by using varying levels of light intensity. Opto-couplers and opto-isolators are typically self-contained in small compact modules that are mounted on a circuit board. The Opto Isolator consists of an LED that serves as a light source that converts an electrical signal to light, a closed electrical channel, and a photosensor that converts light directly to electric current or modulates electric current flowing from an external power source. This sensor can be a photoresistor, a photodiode, a phototransistor, a silicon-controlled rectifier (SCR), or a triac.
Optical isolation has better common-mode noise rejection, is usually seen in digital circuits, is not frequency sensitive, is smaller, and can sometimes provide higher levels of isolation than transformer isolation.
The other three methods of electrical isolations are not normally used to convert, transfer and isolate low level ac/dc signals.
The capacitor allows AC current to flow and block DC currents. They efficiently couple AC signals between circuits, at different DC voltages, via a varying electric field. Many modern devices will even use isolation-rated capacitors to connect between grounds on each side of an isolation barrier. This provides a conduction path for transient signals; perhaps to earth ground (also helpful in quelling radiated emissions). Capacitive isolation is faster than optical isolation.
Capacitors have their disadvantages; they are prone to failure at voltages above voltage ratings. This may result in situations of short circuits. Also, they are not very good when used to isolate digital signals.
A Hall effect sensor detects the presence and magnitude of a magnetic field using the Hall effect. A Hall sensor’s output voltage is proportional to the strength of the field.
Magnetocouplers use Giant Magneto Resistance (GMR) to couple alternating current to direct current. GMR is an isolation scheme that uses a material’s property to change the value of its electrical resistance when an external magnetic field is applied to it.
It’s important to remember that GMR works like a transformer, employing the variable magnetic field of an alternating current coil. It does this, however, to change the DC resistance of a physically isolated sensing element linearly.
Signal isolation is used in all types of industrial measurements and controls, where electrical separation is required to prevent loss of signal accuracy and stability and to ensure safety of equipments from the field sensors.
Some of the field applications and benefits of low level signal isolation are as follows: