As I develop circuits that connect to the USB port on my computer I have been thinking that a short circuit could damage my computer costing a lot to repair. The USB port on a host computer has limited protection. A polyfuse protects the VBUS supply rail while a few ESD diodes prevent static from destroying the interface. What happens if you short a +12V stepper motor circuit to the D+/D- signals? Magic smoke! Once you let the magic smoke out of a chip its never the same. The solution is to isolate the USB port on the computer from the USB device your testing.
The isolation is provided by a small circuit that isolates the D+/D- signals as well as the VBUS supply. Analog devices has two devices that make this possible. First the ADMU4160 isolates the USB signals that connect the host to the device. The device provides 5kV of isolation which means your device can be jolted with an alarming 5000 volts and the host PC will not be impacted. Well there is your device under test and with 5kV applied to it you probably let the smoke out, but at least you didn’t kill your computer in the process. The second device the ADMU5000 provides an isolated power supply that drives VBUS to your device under test; again with 5kV of isolation.
The USB isolator has options that allow it to be configured for low or full speed however I only allow full speed in this design. I did this because all of the USB chips I use can operate at full speed and this gives better performance than their low speed counterparts. To use the isolator connect a short 1 meter cable (A to mini B) from the computer to the isolation circuit and then use a 1 meter A to mini B cable to connect to your USB device. The short cables are important to help maintain the USB signal integrity.
Status: The ADMU4160 costs $12 and the ADMU5000 cost $8 so the protection is quite inexpensive. I have built and tested the board and it now serves as an isolator for my LPC1347 and Arduino boards.
Eagle files: USB_Isolator