USB Isolator
[ Update ] I now have bare PCBs and parts kits of this design for sale in store, as well as assembled and tested boards.
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.
Oleg.
Related posts:
[...] over at Circuits@Home has made a USB isolator for his hacking needs. This isolator separates the signal, ground, and power lines of a USB host [...]
This is a great reference design, but it does not provide mains isolation. The PCB layout clearance is only 1.63 mm, which means it can only isolate 150 volts DC at less than 1 watt of current.
The board should be redone with at least a 5.5mm between the primary and secondary side of the isolation to provide the isolation specified in the datasheet.
Here is a rule of thumb: If you are working with a component that provides isolation, do not create copper traces that are closer together than the footprints for the part.
Thanks for catching that. I’ll get the layout changed in the next batch.
On the other hand, I did not claim this board as providing mains isolation in the first place.
Mains isolation not required for my purposes! I don’t even need 100V for my app. Nice work, now I don’t have to do it.
:-D
Interesting device, could this be used to isolate the ground hum you get on a USB sound card? For example, on my laptop I get a hum which is quite noticeable when amplified however when the laptop is run on batteries or has the earth pin disconnected it disappears, would this provide a suitable alternative?
Thanks.
Could very well be. If earth ground is a source of hum this device would disconnect it. Is your soundcard bus-powered or self-powered?
It’s bus powered so sounds like it would work, thanks!
Nick,
it WOULD likely remove the hum you hear if the card is powered by the USB bus. Obviously if it’s powered by a separate power supply then separating the USB power supply will not do you any good (unless that bus power is bleeding in to your other source). Most of the time that hum comes from the switching power supplies either in the brick or on the main board itself. Find a clean power supply away from those and you can get much higher quality audio.
I’m not nitpicking but pointing this out. Generally a “Hum” is 50/60hz. Considering that it is not likely to be caused by a switched mode supply (which operate in the KHZ/MHZ range).
[...] over at Circuits@Home has made a USB isolator for his hacking needs. This isolator separates the signal, ground, and power lines of a USB host [...]
Personally I’d make it entirely USB powered. The MAX256 is a switch mode isolation power converter. That’d save for the need for a barrel jack. But otherwise VERY neat.
I ordered and one of these & hopefully will receive it shortly. While I’m waiting I have a question – I see on the schematic a jumper, JP6 which seems to connect USB GND to AGND – what’s this for as it negates the purpose of the chip. I don’t, however see this jumper on the pics of the board – maybe not implemented?
I will be supplying an external 5V DC to Vin, where does my Gnd-in connect to?
I’m sure some of these questions would be resolved with the board in front of me but it isn’t so …….
This jumper is mislabeled on the schematic – thank you for catching that! It is labeled correctly on the board though – it’s just spare ground connection on the host side.
External power for the peripheral side can be applied several ways. Straight answer to your question is “connection marked AGND” on the right side of the board next to “SHDN”. It also can be soldered to either of two groung moles of the barrel connector if left unpopulated. My original idea was to solder barrel connector, then run a wire from VIN to VBUS, then supply 5V through the barrel connector – I have so many 5V wall warts that this seems logical.
I shipped your kit two weeks ago, on Oct.1 – you should have received it by now. If you won’t get it this week, let me know, I’ll send you another one.
Sorry, I see that I should be connecting my external regulated 5V DC supply to VBus & AGND
Thanks for the info.
I’m having the ground loop problem with my USB audio headset. For that reason, I ordered this USB Isolator (prebuilt). As the USB Isolator is traveling towards me, I was wondering what kind of power adapter I should buy for it to get the best audio quality (i.e. eliminate the possibility of USB Isolators power supply generating the 50/60Hz hum). Anyone?
The power supply for the peripheral side has to be 7-25V 500ma center-positive barrel connector type. It is further stabilized on the isolator board with 500khz DC-DC type converter; you won’t hear any hum from it, the frequency is too high for the human ear.
To connect it to the computer, you will need standard A to B USB cable, not the extension cable.
Also, I suppose I will need a small USB cable to connect the computer and the isolator together, right? If so, what kind? I have some regular USB extension cords like this one: http://en.wikipedia.org/wiki/File:Usb_extension_cable.jpg
Will one of those do?
Hi Oleg,
I would like to ask you whether both sides of IC ADuM4160 can be powered (i.e. also wired) in the same way. Each part powered by USB bus (+5V). One part is the +5V power supply of PC (USB port) the other one is the +5V power supply from the pheripheral device (USB port). Thanks for the info. Ivan
Yes this is possible. The host side is powered from the host connector in the standard configuration. The peripheral side can’t be powered from the connector since peripherals don’t provide 5V on the VBus pin but if a peripheral is self-powered, you can use this power (3.3-5V) to power peripheral side of ADuM4160.