What Is a USB Isolator?

A USB isolator sits between a USB host, such as a computer, and a USB device, and removes every conductive path between them. Data still crosses, and the device is still powered, but the ground of the computer and the ground of the device are no longer joined. Most are small inline adapters with a USB plug on one side and a socket on the other; the same circuit is also built into some industrial hubs and instruments.

Inside there are two parts. A USB isolator chip carries the data across an insulating barrier, usually with on-chip transformers or capacitors. An isolated DC-DC converter takes power from the computer’s port and delivers it to the device side through a transformer, so the 5 V supply is isolated too. Without that second part, the ground would simply be rejoined through the power wiring.

The aim is the same as any form of galvanic isolation: stop current flowing between two circuits that sit at different earth potentials, while still letting them communicate.

An RS-485 or CAN port is easy to isolate because transmit and receive are separate signals, or the direction is controlled by a simple enable line. USB is different. Its two data lines, D+ and D−, carry traffic in both directions over the same pair, and the bus can be idle, sending, resetting or suspended. An isolator has to follow the USB protocol closely enough to know which way to drive the lines at every moment.

Speed adds another layer. Low Speed (1.5 Mbit/s) and Full Speed (12 Mbit/s) were the first to be isolated in a single chip, and Full Speed isolators are still the most common and cheapest. Hi-Speed USB 2.0 at 480 Mbit/s also needs the isolator to pass the “chirp” handshake that host and device use to agree on Hi-Speed, which is why Hi-Speed isolator chips, such as TI’s ISOUSB211, appeared much later. SuperSpeed USB at 5 Gbit/s and above uses separate high-speed pairs that are harder again to isolate, so isolated USB 3 links are uncommon.

The common reasons fall into four groups.

An isolator solves the ground problem but brings limits of its own. The built-in DC-DC converter often supplies less current than a full USB port, so a hard drive, camera or powered peripheral may need its own isolated supply on the device side. Some isolators have a terminal for exactly that.

Speed is the other common surprise. A Hi-Speed device plugged into a Full Speed isolator either runs at 12 Mbit/s, if it supports that mode, or does not enumerate at all. Webcams, storage and many audio interfaces fall into the second group. Check the isolator’s rating against the device, not just the USB version printed on its box.

The withstand rating, often 2.5 kV or 5 kV rms for one minute, is not the voltage the barrier can sit at all day. That is the working voltage, which is far lower and is the figure that matters for continuous separation.

On industrial computers, the USB port is not always the best place to isolate. If the device at the far end is really a CAN bus, an RS-485 network or a set of digital signals, it is usually simpler and faster to isolate that interface directly. Our guide to USB versions and industrial USB covers locking connectors, ESD and bandwidth as well as where isolation fits.

For vehicle and machine networks, our CAN bus I/O modules include USB to dual isolated CAN adapters, and our serial I/O modules include opto-isolated RS-232 and RS-485 cards, some isolated to 2500 V RMS per port. Isolating at the interface keeps the USB port free for ordinary peripherals and puts the barrier where the field wiring enters.

We specify isolated serial, CAN and digital I/O for the industrial computers we supply, and plan any USB isolation into the system as standard before it ships.