Best opamp for I/V conversion? (DAC)

[krishu]

Hello,

thanks guys,


i heard AD825 in this application - but could it be that the (newer) AD8065 is even better here? Highest slew rate etc.

Cheers
Christian.

[Charles Hansen]

Originally posted by ojg
Exactly, and I think this is why the AD825 with its higher slew-rate and shorter settling-time performs better than the OPA627 in this application.

This is kind of like making distinctions about what kind of dog poop you stepped in. It may be soft, or it may be hot, or it may be brown, but it's still dog poop that you stepped in. It's a lot better not to step in the dog poop in the first place (ie, not use an integrator to perform I-V conversion).

Originally posted by ojg
Using a simple resistor, you would still be limited by the slew-rate of the transformer or buffer following it. However I would be interested in seeing measurements of simple resistor I/V with the PCM1704. Do you have a link?

Both Muse and Resolution Audio have used the setup you describe for many years now. These have been tested in Stereophile magazine with full test results. Unfortunately neither of these seems to be in the on-line archives, so you will have to find a print copy.

[Jocko Homo]

Look for "simple I/V converter" threads. In the digital forum. Examples there.

Jocko

[Elso Kwak]

That's funny. must be something with my ears, hardhearing.....?
I am using the OPA604 with the TDA1543 NON-OS and it sounds very good to me.
Chris Owen is using the AD8610 in his ACk DACk.
Isn't an integrator just what we need to get rid of the steps (staircase)?
I did not like the AD8610 , tried two dozen opamps.

[Charles Hansen]

Elso,

Here is the link to an article by Walt Jung that touches on the problems of op-amps as I-V stages, although not in great detail:

http://www.elecdesign.com/Articles/A...7207/7207.html

The real information was in the articles by Barrie Gilbert that is referenced at the bottom of Jung's article, but these have unfortunately been deleted.

And no, you do not want to use an integrator as an I-V stage.

Have you ever tried a discrete I-V stage like the ones that Jocko has posted? If not, I think you will be pleasantly surprised at the sonic improvement they provide. Maybe even more important than a good clock....

[Jocko Homo]

Although the clock is important....especially with 8x, 24-bit stuff that I futz with.

And Charlie should know.......for years he demoed at CES with a cheap CD player that had my I/V inside.

And yes.....it did have a (yuk) TDA1541A and (double yuk) SAA7220 inside.

I hope the ex-employee who walked off with it is still enjoying it. Even though it is too good for him. Schmuck.

Jocko

[Elso Kwak]

Originally posted by Charles Hansen
Elso,

Have you ever tried a discrete I-V stage like the ones that Jocko has posted? If not, I think you will be pleasantly surprised at the sonic improvement they provide. Maybe even more important than a good clock....

Charles,
Thanks for the link. Yes I did try a common base amplifier and I was not impressed. Also tried a simple resitor as IV and was struck by the lack of dynamics and slam.....
Please note I am only using a non oversampling DAC TDA1543. So frequencies involved are not as high as with 8 times oversampling.

[Charles Hansen]

Just to give an idea of the problems that can happen when using an integrator (ie, op-amp) as an I-V converter:

a) Looking at the data sheet for the TDA1545 (sorry, I don't have the DS for the '1541), we can see that the analog output settling time is 0.2 uS. This translates to a 5 MHz signal.

b) The open-loop gain of the OPA604 at 5 MHz is approximately 5x. (Remember, it's really an integrator.)

c) Let's assume that you have the op-amp set to give 2 V output. With a gain of 5x (at 5 MHz), that means that the "virtual ground" inverting input is not a virtual ground at all. Instead, expect to see up to 400 mV of signal present there.

Now we can see what kinds of problems can occur. Sending 400 mV into a differential bipolar input stage with no emitter degeneration would cause all kinds of problems. Some op-amps have emitter degeneration resistors or other techniques to reduce these problems, but even here 400 mV is a mighty big number to handle well. The part you chose has FET inputs (presumably not a coincidence!), which will reduce the problem but not eliminate it.

As far as the common base amplifier you tried goes, it's hard to say without seeing the schematic.

[Charles Hansen]

Elso -

You edited in the comment about trying a passive resistor while I was composing my reply. I'm not surprised that you found some problems with that approach. It's not too hard to understand what causes them. Sort of a case of "out of the frying pan and into the fire".

That's why guys like Jocko have worked hard to create circuits that don't suffer from these problems, and at the same time avoiding the problems caused by using op-amps. You really should try one of his circuits...

[Elso Kwak]

Hi Charles, I put in a 0dB 1kHz tesignal into my IV and got about 0.01 V peak to peak signal at pin 2 of the opamp. Could not measure anything on pin 3 as it should. So I don't know where that 400 mV value comes from as I get 10mV p-p.