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Old 15th June 2012, 01:29 AM   #1
nowhere is offline nowhere  Israel
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Default OpAmps for DACs

Yeah, I know, I know . And I use web search engines. And I know there are good and expensive OpAmps.

Still, why, for example, NE5322 is often despised ? Even though one can find positive opinions on it with some engineering background, like THD, spectrum of harmonics, driving ability.

And even more interesting, Texas Instruments in http://www.ti.com/lit/ds/symlink/pcm1798.pdf recommends the venerable NE5534 and not something fancy-schmancy.

So, what are the engineering style opinions ?

For example, I use NE5532 in my acoustic measurements gear, but ironically in it even 1% THD won't matter - other errors are much bigger. OTOH, NE5532 behaves quite decently.

So, what to pay attention to first ? Open loop gain at 20KHz, for example ? Slew rate ? Ability to drive capacitive load ? Often people do not protect OpAmp outputs from capacitive load by simple resistors.
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Old 15th June 2012, 03:21 AM   #2
BrianL is offline BrianL  United States
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If you look at the date of the PCM1798 - 2006 - you'll realize that at the time the NE5534 was probably the lowest distortion ('affordable') device for this application in TI's stable. Now there are other options - OPA1611/2, OPA1642, as well as the various National Semiconductor audio op-amps. Generally, such a schematic and/or demo/eval board is designed to give the customer a setup that will match the datasheet specs. One shouldn't infer that the chosen op-amp 'sounds best' or is the best possible part for the job. For example, mating an Analog Devices op-amp to the PCM1798 might give you better objective or subjective performance, but generally you won't see a TI demo board with other brand semiconductors; ditto for ADI's (or anyone else's) boards.
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Old 15th June 2012, 03:38 AM   #3
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You should pay attention to your circuit design.Good Opamps are not mean you can get good performance.
TI's recommends is just not bad.In this application,NE5534 is not good for I/V.You can try other Opamps like OPA132,OPA134 or anyother.
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Old 15th June 2012, 10:53 AM   #4
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Quote:
Originally Posted by BrianL View Post
If you look at the date of the PCM1798 - 2006 - you'll realize that at the time the NE5534 was probably the lowest distortion ('affordable') device for this application in TI's stable. Now there are other options - OPA1611/2, OPA1642, as well as the various National Semiconductor audio op-amps. Generally, such a schematic and/or demo/eval board is designed to give the customer a setup that will match the datasheet specs. One shouldn't infer that the chosen op-amp 'sounds best' or is the best possible part for the job. For example, mating an Analog Devices op-amp to the PCM1798 might give you better objective or subjective performance, but generally you won't see a TI demo board with other brand semiconductors; ditto for ADI's (or anyone else's) boards.
However, NE5534 is not even (originally) a TI-developed amplifier.
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Old 15th June 2012, 10:55 AM   #5
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Originally Posted by getter88 View Post
You should pay attention to your circuit design.Good Opamps are not mean you can get good performance.
TI's recommends is just not bad.In this application,NE5534 is not good for I/V.You can try other Opamps like OPA132,OPA134 or anyother.
TI recommends it because the DAC can meet its spec.
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Old 15th June 2012, 10:55 AM   #6
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NE5532/4 is perfectly serviceable for audio duty, however DAC I/V is a special case, being more video-like than audio. 5534 achieves the numbers for TI's datasheet (noise, distortion) but doesn't fare too well in terms of SQ due, I think to the high RF environment. For improved sound, I suggest a part which doesn't use an undegenerated LTP for input stage. GBW and slew rate are important factors beyond that.
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Old 15th June 2012, 11:10 AM   #7
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Originally Posted by abraxalito View Post
NE5532/4 is perfectly serviceable for audio duty, however DAC I/V is a special case, being more video-like than audio. 5534 achieves the numbers for TI's datasheet (noise, distortion) but doesn't fare too well in terms of SQ due, I think to the high RF environment. For improved sound, I suggest a part which doesn't use an undegenerated LTP for input stage. GBW and slew rate are important factors beyond that.
How about "natively" low impedance circuitry ?

For a different application I have a self-designed "OpAmp" whose input stage is common base amplifier. And it is an I -> V converter - it has negative feedback and resistor in the feedback is the I -> V conversion coefficient.

Because of relatively low open loop gain and local negative feedback the thing is stable without any capacitors.

I also think that because of relatively low open loop gain and local negative feedback the thing is much less prone to temporary internal overload -> IMD. I mean overload due to large step input signal at input.

Of course, I am not talking about OpAmp in DC sense of the word, but since this is audio, we do not care about DC anyway.

...

On THD in general. I doubt that "well behaved" THD is that important. For example, if music is played through speakers, the resulting THD is much higher than, say, THD of NE5532. I.e. I think what is important is IMD appearing at the moments of quick signal transitions at input.

By the way, if I'm correct regarding the IMD, OpAmps with local negative feedback implemented as emitter resistors in the first stage can be better.
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Old 15th June 2012, 11:29 AM   #8
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Could you please share your low-impedance i/v?
Common base is the way to go, prefferably with a nice behaviour without deep feedback (better transient response).
Take a look on ad844 internals, there is a nice current mirror on input with an output at TZ pin. This opamp is quite good for i/v conversion, but it gets distorted on larger >2ma currents. Input impedance is marginally high too.
Making it with transistors won't help much due to matching and temperature tracking problems.
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Old 15th June 2012, 11:35 AM   #9
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Quote:
Originally Posted by nowhere View Post
How about "natively" low impedance circuitry ?
I'm not sure of your meaning here.

Quote:
For a different application I have a self-designed "OpAmp" whose input stage is common base amplifier. And it is an I -> V converter - it has negative feedback and resistor in the feedback is the I -> V conversion coefficient.
Sounds to me rather like a CFB opamp - IME a good choice for I/V duty. The inverting input is low impedance by local transistor action rather than through depending on the opamp's output.

Quote:
By the way, if I'm correct regarding the IMD, OpAmps with local negative feedback implemented as emitter resistors in the first stage can be better.
That's my impression too, based on a very limited dataset
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Old 15th June 2012, 12:21 PM   #10
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Originally Posted by abraxalito View Post
I'm not sure of your meaning here.
...
I'm explaining.

Suppose you have an I -> V converter implemented using a classical book-perfect OpAmp.

A perfect OpAmp has infinite input impedance on both inputs. Zero input impedance on the inverting input in I -> V converter is the result of negative feedback and infinite open gain loop - I am still talking about about a perfect OpAmp.

So, in real world, because open loop gain is limited, especially at high frequencies, a fast transition on the inverting input of the I -> V converter first sees high impedance - because the overal negative feedback hasn't yet kicked in.

Because of this two things happen:

1) OpAmp input stage oveloads;
2) the inverting input voltage "jumps" thus somehow modulating the DAC circuitry, i.e. potentials on switches inside DAC jump.

Then negative feedback kicks in and things return to normal.

So, saying "native low impedance circuitry" I mean circuitry having low impedance because of physical reality of the devices involved and not because of overall negative feedback. Such circuitry is common base/gate/grid one.

By the way, in microphones they sometimes use connected in parallel common base transistors in order to further reduce input impedance. This is used with ribbon microphones - for obvious ribbon low impedance reasons.

IIRC, this approach was also used for moving coil amplifiers in vinyl LP era. Jellybean 2N4401/2N4403 transistors are actually pretty low noise - even according to their official specs.
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