We all love USB. It is well supported across many platforms, easy to work with, and even able to provide a little power to the peripheral. However, the quirk of USB is that peripheral must share ground line with the host. The host is usually a PC and very often a desktop, which means it’s USB ground is electrically connected to earth ground in the wall outlet. With PC-based test instruments, like oscilloscopes, logic analyzers and such, It works fine most of the time, but not always.
There are situations when we prefer our ground separate. It happens when PC/earth ground is too “dirty” and we don’t want our circuit to pick up this dirt. Sometimes our device’s ground is not too good or even dangerous if connected to earth ground. Sometimes we are trying to overcome ground loops. Sometimes, we want our oscilloscope to behave like a multimeter, i.e. being able to show voltage drop between any two arbitrary points of the circuit. In any of this cases we want our USB data and ground isolated from the host.
Isolation improves common-mode voltage, enhances noise rejection, and permits two circuits to operate at different voltage levels. It also tends to save test equipment, as well as PC itself. It is also very useful in industrial setting, that probably why industrial USB isolator devices cost between $200 and $400. While looking for a solution for my lab, I found interesting USB isolator part, recently released by Analog Devices, and decided to give it a try.
ADuM4160 works at full/low speed and uses built-in transformers. The uplink (host) side of it is powered by USB bus, the downlink (peripheral) side uses external supply. The speed of the peripheral has to be manually set with jumpers on both sides of the isolator; if automatic speed detection is desired, a hub can be connected to the peripheral side and isolator configured for full speed.
The PCB also contains a 5V switch mode regulator with wide input range. It supplies downlink side of AdUM4160 and bus-powered device connected to it, if necessary. It is built around Linear Technology’s LT1376 buck converter IC. Input range 6-25V, output 5V 600ma. The power connector is usual center-positive barrel type.
I tested this circuit with my Bitscope DSO and Logicport logical analyzer; both work flawlessly. I also made an extensive data integrity test by moving and zipping/unzipping large file archives to and from USB flash drive, connected to a PC via this isolator; I haven’t seen a single error over more than 100Gb transferred.
Project files, including Eagle schematic and board layout, as well as Gerbers, are available from downloads section. Additionally, PDF of the schematic is available in case you don’t have Eagle installed. The board can be easily made at home. It was designed using SOIC IC packages and 1206 capacitors and resistors. It is routed almost entirely over top side with two ground planes at the bottom side.