Some engineers argue, that a gavlanic isolation is not a solution to our problems. Here is an interesting reading:
Originally Posted by JohnSwenson
Due to the large number of questions I'm not going to quote each one here, I hope I cover them all.
The isolation between computer and DAC was not a primary focus of what I am talking about now. Test I did seem to show this is not as big an issue as many previously supposed.
This post is primarily about the impact of the PDN on the generation of PS noise at sensitive chips in a DAC (main oscillator, DAC chip). In particular how a packetized data delivery (USB, Ethernet) significantly exacerbates this. Primarily because the packetized system produces current through the PDN with a much greater bandwidth than non-packetized systems (say I2S). Producing PDN to work well over this wide bandwidth is MUCH harder than for a non-packetized system.
On the question of WiFi: it is also a packetized system, and because of all the processing going on in WiFi, probably much worse than straight wired Ethernet.
On isolation, I have been including full isolation between digital sections and mixed signal sections for many many years. I do not use optical isolators, I do not like them at all, I prefer the GMR (Giant Magneto Resistive) isolators made by NVE. I think they work way better than opto isolators.
The important question here is how come an isolator doesn't completely fix things, it seems at first glance that having completely isolated power networks for the digital side and the mixed signal side (I'm calling it mixed signal because there are digital signals (I2S data, clocks) AND analog signals (output from the DAC chips) in the same power domain) should prevent PS noise from going between them. If the power domains were truly isolated they would, BUT the domains are NOT completely isolated, the data is going between them! This is the part that is usually forgotten in these types of discussions.
I hope I can convey what is happening here, let's follow a pulse through an isolator between domains and see what happens. Let's assume a real "dirty" digital side, a lot of ground plane noise and power supply noise, and noise riding on top of the digital signal. Lets look at the isolator, it has power and ground connections on the "dirty" side that run the driver that produces the whatever crosses the "barrier" (light, magnetic field, radio waves, whatever). The noise also modulates the "threshold" looking at the input signal. These and the noise and jitter in the signal all add up to a pretty large amount of variation in the field crossing the barrier.
On the other side of the barrier you have a much cleaner supply driving the receiver circuit, but the noisy field is going to cause a current in the receiver. Thus noise on the dirty side is going to cause current noise on the clean side as well. The isolator designers try and make them so the physical properties of the receivers have some form of thresholding so this transmitted noise is decreased, but a fair amount still gets through, and it is greater at the low frequency side. But that is not all, the data, the signal you WANT to cross the barrier, also causes current to flow through the PS pins of the clean side of the isolator, and that signal has a lot of jitter on it by now.
When the packet noise on the dirty side of the barrier is low, the current noise of the isolator will be lower, when the packet noise is high, the current noise of the isolator will be high. So even though the power supplies are completely separate, packet noise on the dirty side can still make it through an isolator and show up as current noise on the "clean side". If the PDN is very low impedance over a very wide bandwidth this current noise will produce very little voltage noise. If the PDN is not so great, there will be some significant voltage noise. It usually will be reduced from what it was on the dirty side, but still definitely there.
Yes putting a whole tracks worth of data in ram, shutting down the packet interface, and grabbing the data out of ram at the audio sample rate should help this, but this is frequently done by a processor and it's memory, that processor is usually producing it's own set of current noise which can cross the barrier. To be really effective it would take a system where the source (whatever it is) fills up the buffer then completely shuts down, nothing drawing power AT ALL from then on, the only thing drawing power is the counter walking through the ram and the ram itself. You definitely would want a simple ram structure, not something like a DDR3 DIMM which has all kinds of stuff going on all the time. The data from the RAM goes over the isolator and on to the DAC chip. This would probably be a very effective isolation scheme, but I don't think anybody has actually ever implemented this.
I have been doing some more experiments on this in the last week and have some results to share. I was working with the USB regen Alex mentioned, with the first version I was able to clearly see the packet noise on a scope. I made a new version with an improved PDN, this seemed to work, I could not see any packet noise any more, noise was still there but I could not discern any modulation due to the packet frequency. It sounded significantly better. Later I did some crude PDN analysis and discovered there was a raising in the impedance over a certain frequency range. I figured out I could fix this by adding a single capacitor in the right place. I soldered in that cap and started listening and was startled in the magnitude of the improvement in SQ. The noise looked identical with and without the capacitor, the sound significantly improved.
So I think I am on the right track, but it looks like I have already gone beyond what the simple measurements I was doing could detect. Next is to do these tests with the spectrum analyzer, it will probably be able to detect the packet noise buried in the over all noise floor.
I hope that answers some of the questions.
John S.
