DIY High Speed (480Mbps) USB Isolator for audio using ISOUSB211

[drksy]

Hey guys,

I had been suffering with common mode noise and ground loops in my home studio set up for as long as I remember. Whenever I moved a window on the PC for example, I would hear this faint buzzing noise in my studio monitors. Tried PCIe interfaces, USB, different outlets for speakers, mains filter, nothing seemed to help. It seemed I had to find a way to isolate the analog audio ground from my PC's power supply ground somehow, while maintaining the audio fidelity of my sound card. A transformer in the analog signal path wasn't an option because of increased distortion. So I decided it's best to isolate the USB interface to the PC.

Long story short, after trying different USB isolators on eBay based on ADUM3160, none of them seemed to work. My USB audio interface (MOTU M4) refused to startup if the USB port was not capable of running at 480Mbps. Looking at the market, there aren't many buy options with a reasonable price to get high speed USB isolation. But after a googling this a few months ago, I came across an ASIC from Texas called ISOUSB211. This hasn't even been released yet as of today and is still in preview, but could be ordered directly from TI. There is of course a EVAL board for it for about 50 bucks which is a very good price, but I decided I could even beat that price by designing a 4 layer PCB since PCB's are so cheap these days, and I had all the passives and linear regulators, etc. Also I wanted a dedicated, low noise, linear power supply on the peripheral side.

So I did, and I wanted to share my design here. I designed the circuit in EasyEDA and manufactured it with JLCPCB. After assembling it, I've run it for about two months now, and it really works amazingly. All ground noise has been totally removed from my set up. Please note that I'm not an EE by trade, I only do this as more or less a hobby. So if you find things that are odd or can be improved, please feel free to let me know. Here are my design files so far:







What it looks like after building it:



To protect my audio interface from the power supply misbehaving, I added a voltage supervisor and protection IC (LTC4361). It cuts off power in case of over-voltage, under-voltage, and over-current in the matter of nanoseconds.

The PCB was manufactured with JLCPCB's 7628 stackup to get controlled impedance on the high speed traces. These were calculated to be 90 Ohm differential impedance using JLCPCB's calculator (https://cart.jlcpcb.com/impedanceCalculation). So if you want to order this PCB, make sure to use this exact stackup, called JLC7628.

Since I'm by no means a pro hardware designer, I have shared this here for the community to perhaps improve it or use it in other ways as you see fit. The design json files are attached at the end of this post. You can drag these to any EasyEDA projects. Comments, suggestions, welcome.

[baconsteak]

Nice Job! Did you by chance measure the round trip latency with and without on your M4? I'm wondering if this setup would be good for use with a guitar.

[moffy]

Thanks for sharing. Could be very usefull.

[trobbins]

Very useful also for people who use their soundcard for audio circuit assessment using software like RE, where isolating as many items from mains powered equipment is a bonus.

[jonpaul]

no need for this.

Use TOS-link optical digital audio, complet isolation.

format SPDIF.

Most high quality external DAC, ADC use TOS-link, SPDIF or AES EBU

Jon

[trobbins]

.... but not all.

[Nominal Animal]

no need for this.

Use TOS-link optical digital audio, complet isolation.

But not sufficient bandwidth for multichannel audio.

I do believe four channels is the maximum that existing S/PDIF optical audio stuff supports, i.e. 4 channels at 24 bits per sample at 48 kHz sampling rate.

On the other hand, even microcontrollers like Teensy 4.0 and 4.1 can do over 25 Mbytes/sec (I've tested this over plain USB Serial; you can do much better with USB bulk transfers), which corresponds to over 150 channels at 24 bits per sample at 48 kHz sampling rate, or over 75 at 96 kHz.

This particular use case –– Teensies –– is exactly why I too am waiting for ISOUSB211 to become available.  Mouser has the EVMs available for about 65€, but the chips are still not stocked (and only priced for a full reel of 2000 chips, at 12.62€ per chip, as of this moment).  Digikey claims it has the chips in stock for 14.25€ apiece in singles, which would be okay price for me, but being Digikey, their "in stock" is suspect, in my opinion... 

That said, I do have used TOSLINK for other microcontroller uses; they're definitely nice when they suffice.

[jonpaul]

Ignore the last

Instead, see AES67 STD, 128 ch 24 bit 192 kHz audio over IP
https://en.m.wikipedia.org/wiki/AES67
Industries standard
The Ethernet PHY uses transformers so no ground or noise issue

Jon

[dmills]

Only issue with AES67 is that you need to lock to PTP at least on the transmitter, and really need PTP capable switches.

It can be done and it does work, but it is a bit of a pain.

[Nominal Animal]

Instead, see AES67 STD, 128 ch 24 bit 192 kHz audio over IP
https://en.m.wikipedia.org/wiki/AES67

Good suggestion, if one can find and afford the hardware

The ISOUSB211 EVM off Digikey/Mouser is under 60€, and the chips themselves on the order of 12-15€ in singles, and the EVM uses only three layers, so it is likely a two-layer design with a shelf price of around 25-30€ (compared to the ADuM3160-based ones) should be possible.  It's just that the chipageddon has delayed the chips from becoming easily available.

The Motu M4 OP has is quite a capable USB device, with 4 channels in and 4 channels out at 24 bits per sample, 96 kHz sample rate, and a round-trip USB latency of only 2.5ms, with off-the-shelf price of about 250-275€ currently.  In the same class, typically labeled as "USB C Audio", there is a wide selection available right now at various prices, many with S/PDIF optical inputs and outputs and even BT Audio support, in addition to the main USB interface.

Looking at AES67 and "Dante" devices, the cheapest AES67 device I can find is a plain 24 bit, 48 kHz, stereo receiver by HasseB on eBay.  Anything more capable seems to be quite a bit more expensive.

So, I don't think it is fair to tell people just ditch their existing equipment and start anew, because a 25-60€ isolator happens to be needed/useful with the USB stuff.  But, it is good to point out the alternatives, because especially non-music-enthusiasts might not know they exist.

[drksy]

Nice Job! Did you by chance measure the round trip latency with and without on your M4? I'm wondering if this setup would be good for use with a guitar.

I was not able to measure any difference in round-trip latency with or without the USB isolator whatsoever. With 48kHz and 256 samples buffer, I'm getting precisely 12.771 ms delay with and without the isolator. This was measured with a loopback connecting analog input to the analog output.

I think if anything, the delay might be in the order of nanoseconds in input to output skew, nothing in the ballpark of milliseconds that you could measure as audio latency.

no need for this.

Use TOS-link optical digital audio, complet isolation.

format SPDIF.

Most high quality external DAC, ADC use TOS-link, SPDIF or AES EBU

Jon

The problem with Toslink or AES EBU is that then you have two interfaces to control. One interface connected to PC, and another interface connected to the speakers. It's a real pain. Even simple things like recording a track and monitoring becomes a nightmare because you have to manually route the digital output back and fourth through two interfaces for every channel - assuming Toslink even supports many channel outputs. It's much nicer to just have one interface.

no need for this.

Use TOS-link optical digital audio, complet isolation.

On the other hand, even microcontrollers like Teensy 4.0 and 4.1 can do over 25 Mbytes/sec (I've tested this over plain USB Serial; you can do much better with USB bulk transfers), which corresponds to over 150 channels at 24 bits per sample at 48 kHz sampling rate, or over 75 at 96 kHz.

This particular use case –– Teensies –– is exactly why I too am waiting for ISOUSB211 to become available.

It's finally available! 

[jonpaul]

Alternative solutions, beside TOS-LINK, industry standard AES-67 for Audio over IP, via Ethernet, up to 128 channels 24 bit, with précis time stamp.

For wired  2 ch AES/EBU, AEQ3id, SP/DIF, use our high CMRR Shielded transformers.

jon

[drksy]

industry standard AES-67 for Audio over IP, via Ethernet, up to 128 channels 24 bit, with précis time stamp.

Yes, this is a good suggestion of course, but maybe suited for bigger studios. I don't know if I can afford such equipment for a small home studio!