Building a DAC - question about PCM1794A

[pbalcer]

Hi there!

I am planning to build my own DAC based on the PCM1794A.
I have been wondering if somebody could advise me on the
analog portion of this chip.

I was trying to find some info on the function of the Iref pin
- I would like to know how to use it (rather than just blindly
following the data sheet and use 10k resistor there). How the
value of the resistor connected to the Iref pin affects the
signal output current and the sound quality? What difference
would it make if I used a current source/sink instaed of the
resistor?

And the second question is about the current outputs. I would
like to use passive circuit as I/V converter instead of opamps.
However I do not know how to pick the value of the resistors
connected to these outputs. I mean that the chip has been
designed with the virtual 0V at the current outputs in mind
(pure "current load"). If I connect the current output to a resistor
I will cause the potential to rise. Now my problem here is
that I do not know how high voltage the PC1794A can tolerate
at its outputs without compromising the sound quality (noise,
clipping, etc.). Is it 0.1V, 0.7V, 2V? I don't know. Does anyone
have any experience in this regard?

Unfortunately, I find the PCM1794's data sheet a bit lacking
when it comes to explaining the analog portion...

Thanks in advance!
Peter

 

[ezkcdude]

You can take a look at my project (see signature). It uses PCM1794 or PCM1798, and I've put up schematics and everything.

 

[Garbz]

Most DACs spit out a dedicated voltage at exactly half the value of Vcc to the IRef pin. In which case you would gain precisely zero benefit from using a current source. The current here is a reference, and does not have a changing load AFAIK.

My only experience is that passive I/V is not worth doing at all unless you use transformers for stepup and to form a natural filter. There are some DACs designed for passive IV, but in general most if not all of TIs DACs want to see exactly their reference voltage on the output to get the best performance. If you want something better than Opamp IV (yuk) look towards a discrete active stage. Compared to the rest of the DAC they are easily built.
http://www.garbz.com/blog/projects/dac.html <- some info on the Passlabs D1 variant I used is here.

 

[pbalcer]

Thanks a lot guys for the info and schematics! Much appreciate it.

The reason I am leaning towards passive output vs. opamps is:
1) I don't want to use global feedbacks that produce high-order
harmonics, 2) for this project I prefer only one, positive power supply

I will certainly want to add line driver though to make sure
the DAC will be able do easily drive different loads.

Thanks again!
Peter