ADAT in/out interface

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ccaudle

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It seems you simply didn't catch my point.

I think I did, but disagree with your assumptions stated below.

I claim that a 3 opamp instrumentation amplifier with no gain in the input (no cross-coupling) has almost no CMRR improvement over the one opamp type.

That is true with perfect source balance, since there is no difference in gain between common mode and differential signals.
However, as Whitlock and others have written in detail before, the output impedance of the driving stage and the input impedance of the receiver form a balanced bridge, so the sensitivity to source impedance imbalance is proportional to the input impedance (common mode input impedance, if the circuit is one that can have common mode and differential mode impedances set independently).
Real sources never have 0 Ohm output impedance, and never have 0 Ohm mismatch between the hot and cold driving impedances, so with real world sources, there will be a difference in how sensitive an instrumentation amp and single diff-amp configuration are to common mode signals if the instrumentation amp takes advantage of the ability to have greatly increased common mode impedance as I have suggested.

It is almost equally sensitive to resistor matching errors, which is usually dominant over source impedance unbalance and stray capacitance.

This may be the source of our disagreement. You can get 0.1% resistors at a reasonable price for DIY or boutique devices. If you go to 0.01% the prices get exorbitant, but I like to use the TI INA137 or ThatCorp 1246 devices, which are only a couple of dollars for op-amp with trimmed resistors in the package, and those have typically 0.005% resistor matching. There is never a situation where the combination of source impedance match and stray capacitance balance is better than 0.01%.

If you use those devices directly as input receivers, the very close resistor matching is lost due to source mismatch (of course the matching on the device is not lost, but the important value is the combination of source impedance and receiver impedance, and that will never be matched to 0.005%).
By preceding the integrated differential device with a pair of buffers with high common mode impedance you can better take advantage of the trimmed resistors. If you do not want the differential input impedance to be so high you can put a 10k or 20k resistor across the hot and cold lines to reduce the differential impedance without affecting the common mode impedance. That will also have some small benefit in reducing common-mode to differential conversion.
 

Dimitree

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Jul 26, 2011
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You will just have to look at the datasheets for the converters and the ADAT chips side by side and carefully compare the timing diagrams to see if there is any inversion needed.
Check the latency through the divider you have shown in the schematic and see what that does to the alignment of the various clock signals. If it delays those clocks too much you may have to reclock with a flip-flop and the higher frequency clock to get all the edges lined up again.

What quality level are you trying to achieve? The clock recovered from an audio signal (applies to S/PDIF, AES, ADAT, MADI, etc.) will be pretty high jitter. For low to medium quality interfaces you could use the recovered clock directly, but high quality would require a clean-up PLL, probably crystal based, but with careful design could probably use an LC based VCO.

Can you even still get those ADAT parts? I found a forum post from 2010 that said the Waveform Semi website was shutdown, and that there was no longer any R&D or even support, they were just selling parts until the sales volume dropped low enough to make that unprofitable, then they were going to shut down completely. The CoolAudio website mentioned in a previous post as a source of compatible replacement parts indicates that the original Waveform Semi (nee Alesis Semi) parts were discontinued in 2013. Were the CoolAudio the variants you were planning to use?

I think the main use I will do with this device (if I ever manage to build it) is using the internal oscillator as clock source, the clock recovered from ADAT is only there for convenience..probably will never be used. So I can skip the hassle of designing a PLL

Yes I'm going to use CoolAudio variants of the ADAT chips, those are still available.
 

abbey road d enfer

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there will be a difference in how sensitive an instrumentation amp and single diff-amp configuration are to common mode signals if the instrumentation amp takes advantage of the ability to have greatly increased common mode impedance as I have suggested.
Agreed. I stand corrected.
I am very much aware that CMRR of a balanced connection is a matter of balancing the bridge constituted by the combination of sender and receiver. Somehow I missed the forest for the tree (the fully diff amp).
 

cabintech

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Yes we (Cabintech Global) still supply the V1401 and V1402 chips - in fact we have a product under development now that use them. They are in a lot of current products and should be around for a long time yet. FYI we did consider the FPGA route and looked at open source ADAT cores, but the reliability and robustness were not proven, and the V140x chips have been around for years and will work with any other ADAT gear.
 

abbey road d enfer

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Thank you for chiming in, cabintech. Several members will be interested in some of your products, particularly the VTL5xx optocouplers and the LM394 substitute.
 
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