USB Isolator - ADuM4160 - diyAudio
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Old 5th February 2010, 06:56 PM   #1
Ekkeri is offline Ekkeri  Finland
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Default USB Isolator - ADuM4160

Hi!

I was thinking to build an USB isolator for my soundcard, but I'm not so experienced with these kind of things..

So I have a few questions, chip I will try is Analog Device ADuM4160 . Since my card is M-Audio Transit which is quite old, the speed should be enough for it.. Manual says:
"Note: Due to USB 1.1 bandwidth limitations and hardware constraints,sampling rates above 48000 Hz do not support full duplex (i.e.,simultaneous input and output) mode."

USB1.1 is 12 Mbit/s (Full-Speed) so ADuM4160 should be enough?

According to Charles in here Transit can use external clock, so I think I will like to use full duplex and only (max.) 48kHz, but I think it's enough for me. Please inform me if you find a failure in my "logic".

But to the biggest problem with this.. USB line termination

ADuM4160 datasheet says that it needs 24Ω resistors in the output. And TAS1020A datasheet has 27.4Ω in the datalines. And Transit uses 24.3Ω. USB Faq says:

What characteristic impedances should I design for in laying out my USB device's traces?

A: 30 ohms to ground, 90 ohms differential between the USB data lines. Be aware that many differential impedance formulas don't take into account the presence of a plane next to the data lines and tend to overestimate the reduction in single-ended impedance due to coupling between the two lines. For typical buried microstrips on spacings FR4 stackups the USB traces should be somewhere around 11 or 12mils wide with roughly a 33mil edge to edge spacing (45mils center to center). This is far enough apart that there's almost no coupling between the lines---the single-ended impedance is around 1.5O lower than what a single trace's impedance would be.

Can anyone give me idea what resistors to use, and what kind of wire between them? Will 24Ω in both ends with any short wire work? I want to use as short wire as I can in there and no connectors between ADuM and TAS1020A. And I'm not very familiar with these impedance things.. But wires should be equal lenght, and maybe twisted if space allows that? Or is it pointless in short distance?

There are some discussion about this chip in few topics, but I think seperate topic for this could be nice. Circuits@Home have ready-made PCBs and parts, but I'm not 100% happy with that design.. But kudos to Oleg for offering that kind of thing.
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Old 13th February 2010, 03:21 AM   #2
felis is offline felis  United States
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You don't need to worry about resistors if you are not planning to transmit into 5m cable. Most people solder isolator directly to the peripheral input anyway, in this case, anything from 0 to 100 ohms will work just fine.
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Old 2nd March 2010, 08:17 PM   #3
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Hi,

please look at the folowing website:
USB Isolator. « Circuits@Home
It works perfectly.
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