While developing USB products I sometime have rather complex USB setups. My device under test is USB connected to one PC. Power comes from an external power supply and JTAG connection for debugging is connected to my development PC. Especially in the lab with different mains circuits for lab equipment, IT and IT2go,I sometimes run into issues with ground loops. Switching mode PSU like the one on my laptop are weird things…
So I created a small PCB that isolates USB data and power. This avoids ground loops and references and prevents dangerous voltages spreading from one side to the other damaging all equipment down the line.
The build is rather easy with an AnalogDevices USB iCoupler ADUM4160 and Texas Instruments isolated DC/DC converter DCR010505. The datacoupler requires one to set full speed or high speed USB2.0 based on the connected device – jumpers let you make that selection. To draw more than 1W power, there are 3 DC/DC converters in parallel. According to the datasheet this should work, but I have not tested it yet.
All in all, it is now
- safe(r) to develop on mains (I look at you E-Meter developers with non isolated debuggers hanging of your board)
- create ±5V from one USB line (for those of you still weird devices like split rail OpAmps – can’t you just use an Arduino for that?)
- easy to use a single ended USB meter to a differential ground-free meter (I just love my cheap regression test setup)
The image above shows 2 MSP-EXP430F5529 LaunchPads. One is connected via the USB Isolation Card, the other is directly connected to the black hub. GND from the LaunchPad on the top is connected to +5V on the bottom (via the Ampmeter on the left) and used as new zero volt. From 5V top to GND bottom we now have 10V (Voltmeter on the right), or ±5V when measuring from the center. I used LaunchPad because they come with handy pins to connect meters, also this way I can create ±3.3V.