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Old 26th January 2013, 09:10 AM   #1
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Default Best place for digital isolator

I'm designing DF1704/AD1865 DAC wiith USB inputs (TAS1020B-based Asynchronous USB to I2S/SPDIF converter 24bit/96kHz). All critical signal will be reclocked by clean master clock before AD1865 (master clock will be aslo send too USB interface ). I would try isolating DAC chip with ADuM and i have dillema as Adum will itroduce additional jitter - is better place isolation before just reclocking circuit near AD1865 before digital filter or before digital filter?
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Old 26th January 2013, 08:24 PM   #2
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Were it me, I would put the isolation directly at the USB input. The largest source of jitter, in my view, is the noise pollution from the computer's power supply & ground lines. Computers are horrifically noisy environments, even when they have extremely quiet, linear power supplies(which they very rarely do, even on battery), as the high speed processing circuitry modulates the supply lines enormously, which in turn contaminates the ground. This has theoretically zero effect on data files, but I believe it can have a large effect when that data file is converted to an audio datastream, as timing becomes a critical factor at that point. So, if we assume that the TAS1020 has a negligible/manageable noise contribution, then isolating it's usb input from the computer's supply & ground lines should minimise the largest source of jitter. My own experiments have borne this out in practice. I prefer, however, to use ethernet instead of USB, as it has complete transformer isolation from the computer built into the format.
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Old 26th January 2013, 11:06 PM   #3
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The closet point to where the information leaves the digital domain.

If that D/A chips DA conversion is driven by the master clock, why would you reclock any other signals?

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Last edited by TNT; 26th January 2013 at 11:15 PM.
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Old 27th January 2013, 07:07 AM   #4
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D/ A isnt't directly driven by MCLK as AD1865 is R-2R dac so the most critical signal is word clock (LL/LR). In delta-sigma chips DA conversion is driven by MCLK.
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Old 28th January 2013, 12:38 AM   #5
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Are you sure an isolator's necessary? There's not exactly a shortage of non-isolated USB and Firewire implementations with THD+N and DNR 10+dB better than the AD1865's datasheet figures.
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