Home > General > Which Digital Isolators for I2S? …or NOT

Which Digital Isolators for I2S? …or NOT

WHY USE DIGITAL ISOLATORS?

Use of digital isolators for I2S audio signals have been proposed by both industry people as well as diy people.

But why use a digital isolator? For one thing, it adds some jitter into the signals it is supposed to isolate. But I think one reason is to isolate the “dirty” digital side, from the “clean” side. In other words, isolate the “dirty” power (and signals) from the PC from the clean power in the DAC.

From the power supply point of view since there is no power shared between the two sides of the I2S interface, that leaves the Gnd connection that can be isolated between the PC-side and the DAC-side. From the signal point of view, noise on the I2S lines can also be isolated.

NVE has an application note describing the benefits of isolating I2S signals (and another paper on data converters in general):

This application note is about isolation electronics in high fidelity digital sound, and in particular, techniques that can be used to reduce the noise floor of any given system.

…the goal remains the same: elimination of known sources of noise and error in the forward path of the music signal. NVE’s unique GMR isolators can help achieve that goal.

…NVE manufactures the lowest distortion digital isolators in the world today. Precision edge placement with 50 ps of jitter allows the designer to transfer signal data from CD transport to audio DAC without noise induced phase error.

For those curious about how this technology work, here is a chip dissection of an NVE part: [link]

Texas Instruments has a paper describing the different technologies used in digital isolators. A design guide also provides good information for PCB designs with the use of isolators.

Spec ADUM 3440
NVE IL715
Si8440 ISO7240M
Speed 150 Mbps
110 Mbps 150 Mbps
150 Mbps
Propagation delay
20 min, 36 ns max
12 typ, 18 ns max.
3.0 min 9.5 ns max
12 min 24 ns max
Pulse width distortion
.5 typ, 2 ns max
2 typ, 3 ns max
1.5 typ, 2.5 ns max
1.0 typ, 2.0 ns max
Propagation delay skew
16 ns max
4 typ, 6 ns max
2.0 typ, 3.0 ns max
-
Channel to channel match
2 ns max
2 typ, 3 ns max
0.5 typ, 1.8 ns max
0 typ, 1 ns max
Pulse Jitter - 100 picosec typ.
250 picosec peak
1 ns peak to peak
Input-output capacitance
2.2 pf (1MHz test) 4 pf (1MHz test)
2 pf (1MHz test)
2pf (Input capacitance)
Transmit channels
4 4
4 4
Technology Magnetic coupling Giant Magnetoresistive
RF Capacitive coupling
Price $8.85 $10.20 $2.94 $6.63

Manufacturers are also comparing their offerings against competing offerings for example,

Analog Devices has a white paper comparing their ADUM devices with NVE’s isolators. The paper shows the following results:

Whether these issues are important for audio/I2S applications or not, probably depends on the actual conditions (noise levels, etc) in the signal lines.

DIY PROJECTS

There has been some discussion on using I2S isolators but no conclusive “wisdom”. Seems to be a good thing with no negative impact to the I2S signals.

I found these projects…

Project 1 [link] (IL715-based)

Project 2 [link] (IL715-based)

COMMERCIAL IMPLEMENTATIONS

Fidelix I2S over HDMI interface (based on PS Audio’s) [link] (IL715-based)

Exadevices EXAU21 [link]. Uses IL715.

Galvanic isolation of the I2S output from the FPGA core and the USB ground virtually eliminates jitter and common noise originated by the PC. Isolation is achieved by using Giant Magnetoresistive (GMR) Digital Isolators. The barrier capacitance between the board circuits and the I2S outputs is 17 pF.”

High Res photo from here [link]

Centrance DAC MINI [link]. Uses IL715.

There are two in the photo, (I think) one for the USB receiver chip and one for the SPDIF receiver chip

SDTrans uses TI ISO7241

XMOS Asynchronous USB-I2S [link]. Seems using IL715

NVE IL715

From the example projects above, we can see that the overwhelming favorite digital isolator is the NVE IL715.  It is an SOIC part and thus DIY-friendly.

Summarizing the data from the data sheet we have:

•  +5 V / +3.3 V CMOS/TTL Compatible
•  High Speed: 110 Mbps
•  High Temperature: −40°C to +125°C (IL715T/IL716T/IL717T)
•  2500 V RMS  Isolation (1 min.)
•  2 ns Typical Pulse Width Distortion
•  100 ps Typical Pulse Jitter
•  4 ns Typical Propagation Delay Skew
•  10 ns Typical Propagation Delay
•  30 kV/µs Typical Common Mode Rejection
•  Low EMC Footprint
•  2 ns Channel-to-Channel Skew
•  0.3″ and 0.15″ 16-pin SOIC Packages
•  UL1577 and IEC 61010-2001 Approved

NVE manufactures the lowest distortion digital isolators in the world today. Precision edge placement with  50 ps of jitter allows the designer to transfer signal data from CD transport to audio DAC without noise induced phase error.

  1. Nary
    2011/11/24 at 19:46

    Hi,
    Now I use NVE IL715. I think it is DIY-friendly, too.
    For playing Hi-Res on my dac, it is the one of key parts, I think.

  2. BlogGeanDo
    2011/11/25 at 09:02

    Please describe your project some more. What is your source and what is your DAC. Thanks…

    • Nary
      2011/11/25 at 19:05

      OK,
      My project is using XMOS USB Audio 2.0 L1 board and AckoDAC.
      The source is mac mini using Puremusic(all time using upsample function for 192kHz).
      Now I isolate I2S and I2C line, it is very good effect for sound for me.

  3. 2011/11/25 at 10:58

    Thanks for flagging up the Si Labs devices, I hadn’t come across those before. On paper they look very good and the price is remarkably cheap so I’ll definitely look into them some more. How about including the input-output capacitance in your table of interesting specs, to me that’s the no.1 consideration as I find common-mode noise the main enemy which isolation helps me to defeat :p

    • BlogGeanDo
      2011/11/25 at 21:35

      Hi Richard, I’ve added the input-output capacitance values. They are all within the 2pf ballpark. Tell me more about that parameter.

  4. BobS
    2011/12/08 at 09:43

    It would be interesting to compare the IL715 solution to the one Twisted Pear chose (LVDS) for I2S isolation.

  5. 2012/02/22 at 15:02

    Ah wordpress didn’t send me an update that you’d asked me a question – so better late than never! The isolation capacitance parameter tells us something useful about how much isolation we’re going to get at RF (where it matters most). Low frequency isolation is not the main reason for using these parts for digital audio, rather we went to prevent RF common-mode noise coming in from (very) nasty sources like PCs and laptop SMPSUs.

    A value of 2pF means the impedance is about 8kohms at 10MHz and will fall to 800ohms at 100MHz. This will give good rejection of SMPSU noise which tends to be relatively low freq. A common-mode choke (I use Murata DLW31SN222) can boost this impedance beyond 100MHz. Another way to raise the common-mode impedance is ferrite beads. Is the capacitance value you quoted per channel or per package?

    • BlogGeanDo
      2012/02/24 at 01:41

      The capacitance value is per device

  6. regal
    2012/12/23 at 15:00

    These NVE’s certainly make more sense than the Adums which just pass thru any noise. But the NVE jitter performance isn’t better than a spdif receiver.

    The trick is to keep the masterclock on the correct side (dac side) and realign after the NVE again on the DAC side, then the jitter is in the 2-3ps range. One day someone will get it right. Until then one is best off just going sans galvanic isolation and rely on good PCB layout.

    • BlgGear
      2012/12/24 at 10:02

      Ian’s Fifo reclocker does exactly that: clocking after the isolator…

  7. Suprted1
    2013/04/11 at 22:50

    Is IL715 1 way communication only ?
    If yes, would it work to just connect two of these optocoupler gates in paralell, but in opposite direction ? Of course two different circuits, to keep inside/outside voltage separated.

    • BlgGear
      2013/04/12 at 17:40

      Yes. You can get chips with some pins going one direction and other pins going the other direction…

      • Suprted1
        2013/04/24 at 20:31

        Maybe you got me wrong. I wonder how you may isolate a wire with two way communication. Is it possible to connect 2 IL715 gates in paralell with opposite directions ?

      • BlgGear
        2013/04/24 at 20:59

        Maybe it is possible, I have not tried. I know there are chips that support two way communication, for example I2C isolators.

  1. 2012/02/21 at 18:14
  2. 2012/03/14 at 16:21
  3. 2012/04/01 at 23:24
  4. 2012/10/18 at 15:55
  5. 2012/10/19 at 14:15

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