COD - Current Output DAC

[Russ White]

Originally posted by 00940
I must have misunderstood this ?

No worries.

Sorry I understand your question a bit better now.

Ah, I think what Brian was referring to there was his particular test at the moment he had no caps, and more particularly the lack of output caps (AC coupling) was what he was trying to point out.

I don't think either of us (Brian or I) have any aversion to using filter caps in the TXD itself for I/V. Sorry if it sounded that way. I hope it is more clear now.

Perhaps the better to thing for us to say is that such things as filter caps are completely optional. And no DC blocking caps are required at all, because of the common mode rejection of THS4131.

Cheers!
Russ

[00940]

Everything's clear now

[Russ White]

Originally posted by 00940
Everything's clear now

Thanks, and actually your point on CFB is absolutely correct. It is something I should correct in the TXD documentation. I wrote that manual when the device was still pretty new to me, and the whole symmetric feedback was just soaking into my smallish brain.

I have actually designed a discrete amp very similar to the THS4131 and I know it is voltage feedback. So it stands to reason the THS is too.

Cheers!
Russ

[BrianDonegan]

h, I think what Brian was referring to there was his particular test at the moment he had no caps, and more particularly the lack of output caps (AC coupling) was what he was trying to point out.

This is correct.

[Russ White]

Just for reference.

This is not a complete schematic, the buffers are missing etc. But it gives you a good idea of what I am going to do.

This is also not the current version (it is actually one of my first simulation attempts) , it is just representative...

[MegaMe]

I look forward to hearing more about the different i/v stages you test...

[Russ White]

I have my COD (I have a couple of them) working.

I have tested the following scenarios:

1) Passive I/V, AC coupled (22uf Oscons) with 470,390,221, and 100R resistors for I/V into a Ballsie. The Ballsie gain is not very high so I would not go much lower than 100R with a stock Ballsie, but you could if you want. The input impedance of the Ballsie is high so 390 or 221 of output impedance (or close to that) is absolutely no problem at all.

2) Active I/V with TXD at 750R for the feedback resistor. Rf can be adjusted to get the output swing you desire.

Honestly both I/V options sound excellent

The noise floor is as low as the Opus, which is to say, quiet.

I will have to listen to each setup for a while longer to give an opinion on which I like best, right now I would put it at a dead tie.

Important note, filter caps are much more important on the COD than the opus. I used 1nf on the TXD and 1.2nf/22pf on the Ballsie.

Cheers!
Russ

[Variac]

It has balanced outputs?

[BrianDonegan]

Yup. When using the Ballsie as the output buffer, you get both balanced and single-ended.

[4real]

Looks very nice

I was planing on ordering the Opus, but this looks very promissing, too. Did you already make some kind of comparison between the two?

For current feedback IV opamps, you might try AD811's.