Previous columns have stressed the need to eliminate ground loops that can cause digitized signals to include noise from nearby sources. Often, though, engineers may forget digital signals also can introduce ground loops that adversely affect analog measurements. So, when you examine an instrumentation system, look for possible problems with digital as well as analog signal-return paths and grounds. Isolation also provides another benefit: Equipment can operate at different voltage levels or “float” above zero volts.
Digital I/O ports provide a good place to look first. Unless you must sense or drive specific logic levels in nearby equipment, consider optically isolated I/O ports. Driving an input requires sufficient current to turn on an LED in an optocoupler. Isolated output ports usually provide low-side NPN phototransistors that act as an isolated electronic switch. Optical isolation helps eliminate ground loops and also helps protect equipment and PCs from transients and high-voltage short circuits in devices under test.
Serial ports that use RS-232, -422 or -485 levels require a common return path between receivers and transmitters. When this path connects with a common ground, it establishes a ground loop you should eliminate. Years ago, serial communications used 20-mA current loops designers could easily use to drive an optocoupler. Now, B&B Electronics, Photologic and other companies offer add-on isolation modules for various types of serial links. If you plan to design equipment, take a look at the isolated line driver/receiver ICs from Maxim Integrated Products. These devices provide 50-V isolation. You can find other optocouplers that offer several thousand volts of isolation.
USB ports might cause problems because they, too, require a return path for the data signals. You can buy individual USB cables that use a fiber-optic link to isolate equipment from computers. But because USB uses bidirectional signals, you usually cannot use optocouplers on the USB signals, themselves. In some cases, designers convert USB information to an intermediate bus such as SPI, isolate the SPI and timing signals and regenerate isolated USB signals. An optically isolated USB hub (SeaI/O-270U) from Sealevel Systems, for example, isolates seven USB ports that also deliver isolated power (500 mA at 5V) to each port. This hub also provides a locking USB connector.
You can use a power-over-Ethernet (PoE) connection to power a small remote instrument. But the IEEE 802.3af standard requires at least 1,500-V isolation between the power sourcing equipment (PSE) and your powered device (PD). So, if you buy off-the-shelf equipment that will consume PoE-delivered power, ensure manufacturers guarantee isolation between your application and the PSE. Or, use a remote power supply. Transformers at each transceiver isolate Ethernet signals, so an Ethernet communication link on its own will not produce a ground loop.