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.