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Old 28th November 2011, 02:37 AM   #41
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Originally Posted by glt View Post
Russ, what is the part number? (to take a look at the data sheet). Thanks...
I thought I had put that in the initial post.

DS91M040

At 250Mbit it is significantly (about double) faster than most isolators, but that is not really it's main function.
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Old 28th November 2011, 04:23 AM   #42
1audio is online now 1audio  United States
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I don't see the isolation here. There needs to be a common ground and the inputs (and outputs) are pretty fragile at least per the data sheet. There is no galvanic isolation in the part.

National has some very fast lvds parts that I looked at for HDMI at over 5 Gbps per lane. They have the technology if it survives being absorbed by TI.
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Old 28th November 2011, 11:44 AM   #43
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Quote:
Originally Posted by 1audio View Post
I don't see the isolation here. There needs to be a common ground and the inputs (and outputs) are pretty fragile at least per the data sheet. There is no galvanic isolation in the part.

National has some very fast lvds parts that I looked at for HDMI at over 5 Gbps per lane. They have the technology if it survives being absorbed by TI.
There is no need for a common ground. Isolation is based in differential signals.
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Old 28th November 2011, 12:09 PM   #44
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Originally Posted by 1audio View Post
I don't see the isolation here. There needs to be a common ground and the inputs (and outputs) are pretty fragile at least per the data sheet. There is no galvanic isolation in the part.
No shared ground is required in the least - though using one (say with a shielded cable) should not really hurt anything, it would just reduce the isolation. My applications so far do not connect the grounds in any way. This does require that the user be sure not to exceed the common mode DC rating of the chip. But that is generally no problem. If you have something like 3-5V or so common mode at the LVDS input/output (this should never occur and is easily cured if required) then you will need to join the grounds. Normally the only connection between boards is the LVDS signals. This does provide isolation, just not in the usual way.

As I stated early isolation is not the main goal. Signal integrity over more than an inch or two is. It would be perfectly acceptable to add an isolator in-between at either or both ends. I just have not run into a case where that would be necessary, and it would also add it's own noise/jitter.

I am not sure what you mean by "fragile". What would give you that idea? I don't see anything to indicate that on the contrary in my testing so far it has been robust and I have not been very easy on it.

I am quite pleased with the Teleporter so far. It is doing exactly what I intended it to do with superb results.

Cheers!
Russ
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Last edited by Russ White; 28th November 2011 at 12:43 PM.
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Old 28th November 2011, 03:27 PM   #45
1audio is online now 1audio  United States
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Most of these parts are pretty static sensitive. The faster the device the more sensitive. They will usually withstand a small discharge when used carefully but a real zap (the kind that has you going ouch) can fry the junctions. In this application without a ground the termination resistors need to take the full energy along with the input junctions. It seems that the termination resistors are defining the reference voltages in this application. Running the numbers in my head the ground impedance between source and destination will be around 12 Ohms. Most external uses of this technology use shielded twisted pairs and the ground reference is defined by the shield. My suggestion, in a preponderance of caution, would be to use CAT6 connectors and cables with their shield connections. A small increase in cost. You can still use CAT5 cables without the ground if you want to. The CAT6 cable will also reduce the radiated noise.

FWIW to help others looking in, Ethernet is transformer coupled and works as an ac coupled link which is why it does not need a ground. I2S (audio) has one link, the data line, that needs to be DC coupled and needs a real ground reference.

HDMI will have a higher bandwidth due to cable construction and connector performance but its really hard to justify 3 Gbps for I2S at any conceivable audio sample rate. The PS Audio standard takes advantage of the extra conductors to add an I2C link for communications between the boxes however does not support the clock originating at the destination (the DAC).

Any isolation scheme will add some jitter. LVDS will have very low added jitter but less isolation. Most optical links will have much higher added jitter.
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Old 28th November 2011, 04:02 PM   #46
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1) Most of these parts are pretty static sensitive.

2) Running the numbers in my head the ground impedance between source and destination will be around 12 Ohms.

3) Most external uses of this technology use shielded twisted pairs and the ground reference is defined by the shield. My suggestion, in a preponderance of caution, would be to use CAT6 connectors and cables with their shield connections. A small increase in cost. You can still use CAT5 cables without the ground if you want to. The CAT6 cable will also reduce the radiated noise.

4) FWIW to help others looking in, Ethernet is transformer coupled and works as an ac coupled link which is why it does not need a ground. I2S (audio) has one link, the data line, that needs to be DC coupled and needs a real ground reference.

5) Any isolation scheme will add some jitter. LVDS will have very low added jitter but less isolation. Most optical links will have much higher added jitter.
1) Yes one should always be careful here, but this is more or less the same situation when dealing with TTL input modules as well, even if they share a common GND reference. In practice I have not found it to be prone to failure of any sort (ESD or otherwise) and I am not taking any special precautions.

2) I am not sure I agree here, I will ask TI/National. Still it is a differential signal even if ground is common signal integrity will be intact because of the common mode rejection of the device. The LVDS part of the device is referenced to 0V(type 1) or 100mv (type 2) where 0V is relative to the differential signal. It is not relative to GND in any way. It is not referenced directly to GND at all. Pay attention to the table in the datasheet describing common mode receiver behavior. You will see that the impedance (other than the purely differential termination itself) is pretty high.

3) As I said earlier it is perfectly fine to use CAT6 and even HDMI cable. People should not be thinking there with be excessive transmissions in any case. It would be no worse than Ethernet.

4) All of the TTL signals do have a ground reference. And the signals can be anything from DC to 125Mhz. LVDS does not have to carry AC signals.

5) Exactly, Which is why I prefer to add as little as practical.

My practical experience is that it is working far better than I actually anticipated and I am excited to see the applications others will come up with.

The bottom line is that the option is there to join grounds if the builder so chooses, my recommendation is to do whatever makes you feel comfortable. This is DIY and so the choice is yours to make. I am perfectly comfortable leaving it floating myself. I couldn't be more pleased with the *actual* result.

Cheers!
Russ
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Last edited by Russ White; 29th November 2011 at 03:02 AM.
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Old 29th November 2011, 10:01 PM   #47
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Russ, is there a way to switch between i2s and dsd from the front panel or have the dac auto switch?
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Old 30th November 2011, 09:39 AM   #48
drone is offline drone  Germany
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I would like to connect a Philipps CD-Pro2 via IS2. Would instill need a metronome to use it with the buf II?

thx
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Old 30th November 2011, 09:41 AM   #49
NicMac is offline NicMac  Italy
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yes
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Old 4th December 2011, 09:17 AM   #50
karvid is offline karvid  Sweden
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Hi Russ,
when can we expect to see the "Teleporter" for sale?

Thanks!
Kenneth
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