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Hi guys,
I tried several hours to find an IV converter but I only found one for this DAC. Somebody can help me for this?
I would like to found a discrete iv without feedback and coupling cap. Thank you! Maxpou

Oh look, someone wants their work done for them.

Please do the following:

- Find as many discrete, no-coupling-cap, no-feedback I/V circuits as you can. Pick out a few which have the level of complexity and feature set that you want.
- For each of these, take note of the DAC chip that they're designed to be used with, and the DAC's output current and voltage compliance range. Compare that to the 1794/98. The 1794, especially, has a fairly high output current and you'll probably have to tweak the I/V circuit to work with it.
- For that experimentation, get yourself a copy of LTspice (it's free) and learn how to use it.

If all you do is build a schematic you find online, all you get good at is building schematics.

Here is one but it has an outputcap,maybee sommething to build from?
http://rockgrotto.proboards.com/index.c ... 653&page=1
I´m thinking of trying this later on myself (PCM1794),but if you find one without the cap please post it.

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Hi gmarsh,
the problem is i don't know how output current of DAC work. I know that a differential as Ryssen propose is OK ( thank you for your link) but what the difference with this one ( topology )? Maxpou

Stay simple.

From someone else here:
http://www.diyaudio.com/forums/digital-source/6121-easy-build-i-v-stage.html

It is worth understanding this circuit.

Someday, I'll build this, but I've got two headphone amps on my desk to build first.

Hi erik_s,
I know this circuit but I have differential current ouput, i can't use this circuit unless i built two of them for each channel. Maxpou

And your problem is?

The datasheet circuit for the PCM1794 is an I/V stage followed by a differential to SE stage. Any reason why you can't do the same?

erik,
do you have tried the difference between the IV resistor stage and I don't know the name but the same topology in second post? Almost all circuits that I found use this topology and not the IV resistor. It's the reason why i would like to use it.

question: the equation in 19 page of datasheet Vi= 7.8mA*Rf Rf is the IV resistor?

Thank you! maxpou

The stage you posted uses an IV resistor - it's clearly marked in the stage you posted as Riv.

I'm willing to be corrected, and likely will, but you need an IV resistor somewhere in the stage to create the voltage using Ohm's Law. The rest of the circuit is a way to "move" the current around to reduce the load on the output of the DAC.

The only stage that I've used personally is based upon the same topology as the ezDac. I know it is from an app note as I've seen reference to it here in the forum, but I can no longer find the relevant post.

Hi,



Well, I cannot help on the discrete without coulping cap, but why don't you make your life easy and just use the new Burr Brown Diamond transistors (OPA860/861). These make for a very easy open loop design that acts as "current conveyor" into a suitable I/V resistor. With the BB Parts you have 7.8mA PP per output.

So you can use a 750 Ohm I/U resistor after each OPA861 to ground and use that directly as output, no additional buffering etc. You probably need to add some offset current on the output to keep the outputs at 0V for digital silence, but that is an easy job for a small CCS that gives most of the correction and a servo to deal with thermal drift etc while having minimal contribution otherwise.

Ciao T

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Theory is when you know everything but nothing works.
Practice is when everything works but you don't know why.
Guruhood is when you know everything and everything works accordingly.

I just looked at the OPA861 datasheet again for about the twentieth time, and finally had the moment where I now understand how I can use it for IV. I knew it could be used, having seen reference to it in others work, but now I have an idea of how to do it.

Now just need to figure out the best way to do differential to SE.

Hi,



Just think of it as a simple transistor. Ignore the complexity. Input current into the Emitter node, ground base, load resistor on collector.



My suggestion would be to simply ignore it.

But if you must, you could also the OPA861 as Balanced/SE converter, as it has both inverting and non-inverting inputs. Just note that both have very different input impedances.

I think you can simulate this stuff using Tina-Ti.

Or you could use a 2:1 output transformer as the Balanced to SE converter, from the 4V RMS differential output at 1.5K to 2V RMS at around 400 Ohm.

Ciao T

_________________
Theory is when you know everything but nothing works.
Practice is when everything works but you don't know why.
Guruhood is when you know everything and everything works accordingly.

Hi Guys,
thank you for your suggestions but I would like to have a real discrete i/v. I will work on the circuit and I will be comeback with the schematic. Maxpou

Hi guys,
I would like to know if my arrangement of jocko's circuit ( easy-to-built ) works. As I said in my first post I want built a iv converter without feedback and coupling cap in signal path. Maxpou

No comment on my schema? I would like to know if i'm correct or no? Maxpou