We have talked about common isolation topologies for instruments and where the isolation can be applied to the signal within the instrument, but we have not talked about the isolation barrier itself or how the signal crosses the isolation barrier. In this section we will quickly cover the isolation barrier and then we will move into three common isolation types, which use different techniques to transmit the signal data across the isolation barrier.
Physical isolation is the most basic form of isolation, meaning that there is a physical barrier between two electrical systems. This can be in the form of insulation, an air gap, or any nonconductive path between two electrical systems. With pure physical isolation, you can imply that no signal transfer exists between electrical systems. When dealing with isolated measurement systems, the signal of interest needs to cross the isolation barrier with the benefits of removing ground loops. Therefore, you must have a transfer, or coupling, of the signal’s energy across the isolation barrier. Three common techniques of transferring the signal across the isolation are discussed below.
Capacitive isolation, as seen in Figure 6, uses an electrical field as the form of energy to transfer the signal across the isolation barrier. The electric field changes the level of charge on the capacitor. This charge is detected across the isolation barrier and the charge detected is proportional to the level of the measured signal.
Figure 6: Capacitive isolation uses an electrical field as the form of energy to transfer the signal across the isolation barrier.
Inductive isolation uses a transformer, shown in Figure 7, to transfer a signal across an isolation barrier. The transformer generates an electromagnetic field, proportional to the measured signal, as the form of energy to cross the isolation barrier.
Figure 7: Inductive isolation uses a transformer, notated with the above symbol, to transfer a signal across an isolation barrier.
As in capacitive coupling, inductive isolation can provide relatively high-speed data transmission rates. In addition to high-speed transmission, inductive coupling uses low power for the data transmission. However, inductive coupling is susceptible to interference from surrounding magnetic fields because it uses electromagnetic fields as the method to cross the isolation barrier. If external magnetic fields do interfere with the electromagnetic field produced by the transformer, this could affect the accuracy of the measurement.
Optical isolation uses an LED and a photodetector to transmit the signal information across the isolation barrier. The isolation barrier in optical isolation is typically an air gap and the signal is transmitted using light. The light intensity produced by the LED is proportional to the measured signal.
Figure 8: Optical isolation uses an LED and a photodetector to transmit the signal information across the isolation barrier.
Because optical isolation uses light as the energy to transfer the measured signal across the isolation barrier, it gains the advantage of immunity from electrical- and magnetic-field interference. This can make optical isolation an effective technique in industrial areas where strong electric or magnetic fields could be present. The advantages gained by using light are balanced by some disadvantages. Optical isolation typically has slower data transfer rates, which are limited to the LED switching speed. It also has relatively high power dissipation when compared to capacitive and inductive isolation.