Need Isolation? Capacitive Solutions Outperform Opto, Magnetic Options (.PDF Download)

If you’re designing circuits and equipment that require electrical/electronic isolation, it may be time to consider electronic isolation via capacitance. Of the methods available, capacitive isolation provides outstanding advantages over magnetic isolation by transformer or optoisolation with an LED and photodetector.

Such isolation is a common requirement in most industrial and medical applications. That’s where capacitive-isolation ICs, which have been developed and refined for implementation into these critical designs, step in.

Isolation Defined

Isolation is the process of blocking some signals and electrical connections while allowing others to occur. Known as galvanic isolation, this process prevents direct electrical contact between input and output, but allows for the transfer of signals. For instance, a typical isolator prevents dc or ac supply voltages from being passed on, yet at the same time permits data signals to pass.

A major function of isolators is to separate the common grounds of input-signal devices and the equipment receiving the signals. Using a single common ground almost always introduces ground loops and the attendant unwanted offset voltages.

Keeping high voltages as great as 10 kV from industrial equipment away from computers, sensitive equipment, and human operators is another function of isolators. In addition, isolators protect sensitive equipment from electrostatic discharge (ESD), electrical fast transients (EFTs), and other variations from electrical surges that are common in an industrial setting.

Such protection gives isolated equipment good electromagnetic compatibility (EMC) as required to meet selected certification standards. Capacitive-isolation ICs meet all of these requirements while supporting high-speed data rates and lower power consumption over other methods.

Common Isolation Methods

Figure 1 shows the three common isolation methods. The transformer in Fig. 1a is the most obvious—it uses two electrically isolated windings on a common magnetic core. Signals are passed by magnetic induction from primary winding to secondary winding. The isolation is excellent, but transformers have some downsides. They’re typically larger, heavier, and more expensive than other options. Though they do a good job of blocking dc, their frequency response can limit data rate unless special high-speed transformers (e.g., Ethernet) are used.