PCM1704 or newer chips?

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#105
Hi Paolo,
Basically, what all that seems to be saying is that the PCM63 wants to see a low impedance on it's output. Looking at the data sheet for this part, we see that Burr-Brown engineers recommend using an OPA2604AP as a virtual earth I-V converter. There is no feedback resistor used, it's a direct connection to and from the DAC chip. That part then has enough drive to send the signal into the filter network and on out into the world.

This should make perfect sense, because the DAC is seeing a very low impedance to analog ground.

I don't buy using a resistor as an I-V converter. You still have to at least buffer it, and by using an op amp directly, the DAC seems to be happier with no voltage variations on it's output. Also, all the energy is used in that converter / buffer stage. Noise performance should be better too.

If it's good enough for the people who designed it DAC, and they use this circuit to prove the best numbers, it seems to me that this is the best way to do it.

The op amp datasheet link and the PCM63 data sheet link.

Now, here is the PCM1704 datasheet where they again run the current directly into an OPA627. This time the I-V converter is combined as a buffer and the start of the filter. They have eliminated one stage of amplification this time. Still, the idea is similar and I think they know how to squeeze the best performance out of their product. Now, if you want to try different op amps you can start by looking at the specs for the OPA627. You can see it is a pretty good performer.

-Chris
 
#106
This is an illustration from an AES paper (20 Bit "Colinear" DAC, a solution to Low Level Problems) by the PCM63 development team which shows the set up used for production trimming. DUT = PCM63. No resistor I/V here either.


There is an article at EDN.com Cascode circuit lessens DAC degradation which outlines some of the drawbacks of resistor I/V.
 

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#107
thanks guys for the help ( and the patience!😀 )
Very precious for me the 1704 datasheet also! 🙂
But I have jet made my priority: no op amps in my system!:angel: 😀
Right or wrong so it is, I would be try a circuit very similar to that illustred by Guido Tent. Do you know a schematic like that jet "ready"?
Excuse me again for my audiophile's obsessions!😀

Cheers,
Paolo
 
#110
nonsense

anatech said:
Looking at the data sheet for this part, we see that Burr-Brown engineers recommend using an OPA2604AP as a virtual earth I-V converter.
There is no feedback resistor used, it's a direct connection to and from the DAC chip. That part then has enough drive to send the signal into the filter network and on out into the world.

This should make perfect sense, because the DAC is seeing a very low impedance to analog ground.

If it's good enough for the people who designed it DAC, and they use this circuit to prove the best numbers, it seems to me that this is the best way to do it.

Now, here is the PCM1704 datasheet where they again run the current directly into an OPA627.
Click to expand...

I could ask you "how" the OPA2604AP provides the virtual earth without I-V resistor... :xeye:

The I/V resistor inside PCM63 is on pins 9 & 10. 😉

In the PCM1704 datasheet it is the 2k5 resistor.
 
#115
 
#119
there are some good resources , but I cant find the one where someone directly compared the 1862 to the 63 sadly (and oversampling filters)

"The D/A converter used is the AD1862. It is a conventional multibit 20-bit converter. Although the current trend in consumer audio is toward single bit converters, they don't seem to provide top of the line performance.While they have theoretically perfect linearity, they seem to be lacking in noise and transient performance. The AD1862 uses a digital offset technique to minimize the nonlinearity at center scale."

(instead of seperate positive, and seperate negative halfwaves as BB does, but differences might be blurred nowdays )
 
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