Building USB Isolator.

ADuM4160 USB Isolator PCB panel

[ Update ] I now have ICs and complete parts kits of this design for sale in the store, as well as assembled and tested boards.

USB Isolator PCBs has arrived. In this article I will be talking about building and configuring one. The PCB was designed with hobbyist-friendly large size SMD packages and 10mil clearances and I hope it won’t be difficult to made one at home. The layout files are available from downloads section. If you never made PCBs before, buying it from me would probably be more economical.

Let’s talk about parts procurement. This is a BOM at Mouser sans ADuM4160, LT1376-5, and PCB. This is BOM at Digi-Key, which includes LT1376-5. Last time I checked, Digi-Key was way more expensive. As far as ADuM4160, since no one sells it in single quantities right now (check this using FindChips), the easiest way to get it is to ask Analog Devices for a sample, they are generous folks. I am planning on selling ADuM4160 too as soon as I get them in my hands.

Now, let’s discuss building the circuit. First of all, you need to understand one thing. This circuit is an isolator and consequently has two power rails and two grounds, marked GND and AGND. Don’t connect these two grounds together. Uplink side of ADuM4160 is powered from host USB bus, downlink power has to be provided form separate supply, usually wall wart. If you have 5V wall-wart capable of providing enough power to the USB device connected to the downlink port plus 10ma for isolator itself, you don’t need to build a regulator. Simply connect pad marked “Vin” to a pad marked “Vbus”. If your device is self-powered and have 3.3-5V supply, it can also be used to power downlink side of ADuM4160 by connecting it to “Vbus” pad.

If you decide to build a power supply it is very important to use wall wart which is center-positive(has “plus” contact in the center). If you reverse polarity, LT1376 will burn. Pads for pins 2 and 3 will be damaged as well so any subsequent repair will be difficult and will look ugly. Don’t ask me how I know. Also, if you have several wall warts, using one in 9-12V range would help the circuit to stay cool. Maximum input voltage is 25V, do not exceed it.

How to substitute parts. I used CDRH5D18 inductor from Sumida. Values from 6.2 to 12uH work well. You can use another inductor if it of “shielded” variety, fits into the pads and its current rating is 1A or more. Another important components are input and output capacitors. They have to be low-ESR tantalum, similar to AVX TPS, which I use. 24 ohm resistors on D+ and D- must be 1% accurate. D1 must be 30V 2A Shottky; the one I use (MBRS130) works very well and is cheap. Other parts are not critical.

The USB speed of the isolator has to be set manually using two jumpers, separate for uplink and downlink. They must be set to the same speed. Full speed will work for almost anything, excluding maybe USB keyboards and mice.

After the board is built, the easiest way to test is to connect an USB flash drive to the isolator and isolator to PC, make sure it gets detected, copy large archive to flash drive and try to unpack or run integrity check on this archive. If you end up with no errors, your USB isolator is ready to use.

If you have difficulties with this circuit, drop me a line and I will try to help.

Oleg.

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3. USB Isolator.
4. Arduino USB Host Shield build log. Part 1.