Hi Avik,

It looks like in the DDDAC schematic they have additional resistors to ground, which should take care of the common mode current.  Note that the series resistance of the primary winding will likely act like a voltage divider, so the common mode voltage on the two channels will not be identical.  This does not really cause a problem, but would mean that there is not exactly ±3.9mA through the winding, it would be lower depending on the resistance value and the shunt resistors to ground.  See the attached TINA-TI simulation.

PCM1794-coil-dc-transfer-function.TSC

In regards to your recent question "if i have dac with balanced outputs can i connect them directly to TPA3255 balanced inputs?"

That depends on 3 things:

1. Are we talking about balanced voltage output and inputs or balance current outputs and inputs?  Seeing as the PCM1794A is a current output device and the TPA3255 is a voltage input device, I will say no.  You must have some kind of I/V stage between the two.  The coil could do this, though I have not tested it.  Otherwise a op amp I/V circuit would also work.

2. What are the voltage input limits of the TPA?  The PDS shows that is is 7VPP, which should be pretty easy to have a I/V stage that can comply with that limit.

3. What is the common mode voltage of the balanced voltage output and the common mode voltage of the balanced voltage input?

This is why you see the DC-blocking capacitors in most designs.  If you consider the recommended I/V stage for the PCM1794A in its datasheet, you will see that it has a common mode voltage of 0V (ground centered).  The TPA3255 does not feature a negative supply, so the inputs must be biased away from ground to ensure that the input does not clip against the ESD protection diodes on the input pins.  Given that this has a max input of 7Vpeak-peak, the input common mode should be about 3.5V. 

So if you were to connect the balanced voltage output of the PCM1794A I/V (op amp) circuit to the input of the TPA3255 without the blocking caps, you would be shorting the 3.5V reference off the device to ground.  

For this reason you need some kind of DC isolation.  I still think the coil could be used for this, as the secondary winding does not need to be grounded.  In the TINA-TI sim I do have it grounded, but that is just because it does not like floating nodes.  

I think you could follow the DDDAC as a reference design.  Note that they use a VCC voltage that is beyond the absolute maximum value shown in the PCM1794A, which I do not recommend.

Thanks,

Paul