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Old 19th May 2004, 10:24 AM   #1
krishu is offline krishu  Europe
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Default Best opamp for I/V conversion? (DAC)

Hi,

I am quite sure this has been discussed before but I did not find it using the search function.

I am going to bulid a DAC using CS8420, DF1704/06 and 2xPCM1704; followed by opamp for I/V conversion, passive HF filter and another opamp as output buffer.

The output buffer will be Analog Devices AD8610, but I am not sure which one would be best for I/V conversion. Maybe AD8065? Any suggestions are welcome.

Cheers
Christian.
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Old 19th May 2004, 10:56 AM   #2
borges is offline borges  Norway
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If you got cash: OPA627.

I've implemented this setup a couple of times. Most of the times I use BB OPA2134. For my latest board I have also collected a pile of OPA2604, OPA2227, and 5534.

But I must dissapoint you, I have not tested the sonic qualities of the different amps.

Greetings,

BÝrge
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Old 19th May 2004, 01:10 PM   #3
mandat is offline mandat  Poland
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The following ICs - OPA627&BUF634 were used in DIY DAC The First SE Mk2. The integrated buffer may be ommitted when a cable is not an output load.
Digital signal which comes from transport should be regenerated with fast TTL gates.
What do you think about AD8610/AD8620? These amps have excellent capacitive load driving capability, much better than OPA627, whitout ringing. Supply voltage +/- 13 V but Peranders use 15 V whithout problems.
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Old 19th May 2004, 03:24 PM   #4
ojg is offline ojg
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Hi krishu,

You might be interested in the measurements I did on my DAC that I posted here. It seems my design is similiar to yours, so maybe you should consider the AD825 as well?
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Old 19th May 2004, 03:32 PM   #5
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Quote:
Originally posted by ojg
Hi krishu,

You might be interested in the measurements I did on my DAC that I posted here. It seems my design is similiar to yours, so maybe you should consider the AD825 as well?
----------------------------------------------------------------------
I have the same experience as you. The 627 is inadequate and the 825 is better. Current load this to 3mA and the 825 sounds even better.
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Old 19th May 2004, 04:14 PM   #6
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The best op-amp for an I-V converter is no op-amp. As Barrie Gilbert (one of the top analog designers of all time) explains, an op-amp is in reality just an integrator. This causes all kinds of problems with input stage overload when presented with the step output of a DAC. Anything (transformer, resistor, discrete, etc.) will sound better than an op-amp for an I-V converter.
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Old 19th May 2004, 04:31 PM   #7
ojg is offline ojg
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Quote:
Originally posted by Charles Hansen
As Barrie Gilbert (one of the top analog designers of all time) explains, an op-amp is in reality just an integrator. This causes all kinds of problems with input stage overload when presented with the step output of a DAC.
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.

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?
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Old 19th May 2004, 04:35 PM   #8
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Default I'm glad that you told them first....

Somehow, I suspect that if I had been the first to say it, it would have been dismissed as an extension of my contant ranting about that.

Back around.......'92 I think it was......Charlie sent me a copy of a magazine article titled "The current state of CD reproduction: who is doing it right?" Too bad that the name of the mag that it appeared in was not on the copy, and Charlie can't remember.

They said........the best examples (in I/V stages) were from small American companies that were run by engineers, that used transimpedance amps, and not op-amps for that purpose.

(For those who may be curious.........I was probably the first.....going back to '89.)


Jocko
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Old 19th May 2004, 04:45 PM   #9
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what is a transimpedance amp?
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Old 19th May 2004, 05:04 PM   #10
Bricolo is offline Bricolo  France
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Quote:
Originally posted by till
what is a transimpedance amp?

impedance is something that converts volts into amperes, so I assume that a transimpedance converts amperes into volts
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