ADA4637 Opamp IV Stage Simulation - PCM1794 (some help and wisdom needed) - diyAudio
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Old 26th June 2012, 11:22 PM   #1
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Lightbulb ADA4637 Opamp IV Stage Simulation - PCM1794 (some help and wisdom needed)

Hi all,

I have been working on a USB DAC (that I will post up as I get my schematic finalized) and I have finally arrived at the DAC section.

I am using a PCM2707 to asynchronous up-sampling converter SRC4192 to the PCM1794 DAC.

I have been reading numerous threads on the IV stage and have seen many solutions involving discrete, passive and op-amp solutions but I don't want to blatantly just copy some one else's design and slap it onto my schematic (ZEN converther hehe). I have been trying to understand how the IV and amp/filter stage work, and I have taken a liking to the OPAMP solution.

So in my attempt no to copy other people and to understand how this works.... I have copied the design on page 20 of the PCM1794 datasheet . I have read up on finding an alternate opamp solution to the one proposed by the TI/BB designers. I have been looking into the ADA4627 and the ADA4637

Those opamps are pretty expensive at just under $10 on digikey/mouser and are supposed to be a rival to the OPA627 and OPA637 from TI/BB, therefore I need to know that this wont be a waste of money to performance that I can get.

ADA4637 has a higher Gain Bandwidth, Slew Rate, and Settling Time than the ADA4627 and I decided to run some simulations to see if it makes sense.... the results are below

Simulation done with LTSpice IV

Transient response at 1kHz and 10k load shows the highest harmonic at around 5kHz at 81.2106dB separation from the 1k (3kHz harmonic is has slightly larger dB separation around 82dB).

Frequency response shows a small roll-off at 10k to 20k at about -219.924mdB which should be good i guess, and a -3dB rolloff at around 79kHz

This the bare bones circuit from TI's datasheet and I hope to get some feedback from you guys and get a better THD than 81dB. Ideally I want to eliminate the IV/filter/gain stage being noise dominant over the DAC.

Also, all the components are ideal except for the ADA4637 models, so this might be a challenge.

All help will be appreciated.

Thanks
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Last edited by zxgravediggerxz; 26th June 2012 at 11:33 PM. Reason: crappy writing skills lol
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Old 27th June 2012, 03:36 AM   #2
jcx is offline jcx  United States
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As a decompensated op amp the ADA4637 requires cirucit modifications for use in DAC I/V which are "unity gain" from a stability standpoint

also the Vnoise isn't great - to max S/N you would need much higher than standard consumer audio 2.0 Vrms fs output

my guess is that the ADA4897 is "the best" candidate today for DAC I/V at the "high" ref current levels of the PCM1794, consumer output levels

but with 10 V max supply limit on the chip I would want to use it in a composite amp with a higher Vswing output op amp for the improved performance and because I have an applciation where higher, pro level output would be useful - but I've never seen a published, practical "multiloop" DAC I/V circuit - even though I've simmed some possibilities

Last edited by jcx; 27th June 2012 at 03:43 AM.
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Old 27th June 2012, 08:10 AM   #3
qusp is offline qusp  Australia
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you dont consider the hawksford multiloop practical?
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Old 27th June 2012, 06:38 PM   #4
jcx is offline jcx  United States
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I like the idea: The Other Hawksford IV (nested loop op amps)

but Hawsford didn't disclose what"s in the "input filter" box - my sims show there are a few stability issues not explained to the point others could confidently design/scale the multiloop I/V
even my "working" sims are still suspect due to the unrealistic op amp output Z models - I wouldn't say the circuits are "proved" without at least seeing working hardware looked over with 'scope much faster than the op amp GBW
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Old 27th June 2012, 08:47 PM   #5
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Quote:
Originally Posted by jcx View Post
As a decompensated op amp the ADA4637 requires cirucit modifications for use in DAC I/V which are "unity gain" from a stability standpoint

also the Vnoise isn't great - to max S/N you would need much higher than standard consumer audio 2.0 Vrms fs output

my guess is that the ADA4897 is "the best" candidate today for DAC I/V at the "high" ref current levels of the PCM1794, consumer output levels

but with 10 V max supply limit on the chip I would want to use it in a composite amp with a higher Vswing output op amp for the improved performance and because I have an applciation where higher, pro level output would be useful - but I've never seen a published, practical "multiloop" DAC I/V circuit - even though I've simmed some possibilities
Thanks for the catch! I overlooked the non unity gain of the ADA4637. I have found a paper by TI (Using a decompensated op amp for
improved performance) that describes how to compensate the op amp for gains lower than what is specced. Aparently you can get better THD by externaly compensating a decompensated op amp but I couldnt get my head around it yet. Perhaps I will draft up a ADA4627 IV stage with ADA4637 as the gain op amp with filter.

I was able to improve the circuit I presented by shuntig a 0.7uF capacitor at the output before the 100Ohm resistor. The noise level went down several dB and cleaned up the FFT a bit!!! I will post later

Quote:
Originally Posted by qusp View Post
you dont consider the hawksford multiloop practical?
Quote:
Originally Posted by jcx View Post
I like the idea: The Other Hawksford IV (nested loop op amps)

but Hawsford didn't disclose what"s in the "input filter" box - my sims show there are a few stability issues not explained to the point others could confidently design/scale the multiloop I/V
even my "working" sims are still suspect due to the unrealistic op amp output Z models - I wouldn't say the circuits are "proved" without at least seeing working hardware looked over with 'scope much faster than the op amp GBW
I read jcx's thread extensivelly and went over the Hawksford paper but my expertiese limits my understanding of the circuit and how to adjust it to my application. Also the amout of op amps is undesirable :/ (3x per Iout.... 2x3x2 = 12 opamps for the DAC?!?)

I will simulate a ADA4627 IV stage with ADA4637 differential gain/filter and see what comes out (i susspect the same thing but we will see)

Also the ADA4637 being decompensated and actually simulating somewhat reasonably good does not give me a lot of confidence in the simulation. It might show me that the ADA4637 PSpice model is stable at unity (might be using the ADA4627 model, slightly modified).



I am also looking at the LME49713 current feedback op amp for the IV
We will see how it simulates and if I can make the current feedback work.
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Old 27th June 2012, 08:54 PM   #6
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I found this in the ADA4627/37 models

Quote:
* CAUTION!! To aid in convergence, most Spice simulators add a
* conductance on every node to insure that no node is floating.
* This is GMIN, and the default value is usually 1E-12. To properly
* simulate the low input bias current and low current noise, the
* Spice simulator options have to be set to the following:
* .OPTIONS GMIN=0.01p
* .OPTIONS ABSTOL=0.01pA
* .OPTIONS ITL1=500
* .OPTIONS ITL2=200
* .OPTIONS ITL4=100
*
Will that significantly affect my simulation?

Is that a spice directive command that I have to copy and paste?
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Old 28th June 2012, 01:56 AM   #7
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I am not sure your simulation is giving you accurate results. If TI achieved ~120 dB DNR and -110 dB THD measured with this circuit with NE5534, I would not expect numbers so much worse with what is basically an OPA627 clone. Voltage noise is worse, however, at 6.1 nV / rtHz vs 3.5 for NE5534.

Is there some reason you are only considering 627/637?

Last edited by chris719; 28th June 2012 at 01:59 AM.
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Old 28th June 2012, 02:45 AM   #8
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Quote:
Originally Posted by chris719 View Post
I am not sure your simulation is giving you accurate results. If TI achieved ~120 dB DNR and -110 dB THD measured with this circuit with NE5534, I would not expect numbers so much worse with what is basically an OPA627 clone. Voltage noise is worse, however, at 6.1 nV / rtHz vs 3.5 for NE5534.

Is there some reason you are only considering 627/637?
Yeah I am not sure either but it looks a good start but my other simulations are not doing so well as this one.

I tried using the ADA4627...... but there is something funky in the model and I get a "Time step is too small" type of error when trying to do a simple low pass filter... and it takes a million years to simulate in the IV circuit... aka it wont simulate and it looks like it can take days to finish.

I would like to simulate the TI cicuit BUT I cant even do that! I downloaded the Spice model for the NE5534 and put it in the circuit as per the datasheet and it is taking a million years to simulate. It has been an hour and it is at "Damped Pseudo-Transient Analysis: 18.41.. time constants done with no end in sight

I have also modeled and used the LME49713 in the IV portion and it simulates fast but the Harmonics and noise are ever present and I cant get rid of them :/. I used some app note by AD from 1988 to get the Current Feedback Amp to "work" but it's not looking good. Seems the IV stage is giving all the distortion once again.

Attachments show:
-ADA4637 updated circuit and freq response
-LME49713 IV and ADA4637 amp stage circuit and it's freq response


I want to use the 627/637 due to their better performance specs compared to some other audio op amps. Also the AD versions of the 627/637 are cheaper and you can get samples compared to the TI ones, and I don't see a lot of people using the AD ones so it's worth a try I guess.
The 627/637 gain bandwidth and slew rates are better than most of the other well known audio op amps and the opamps listed on the PCM1794 datasheet. I am open to suggestions tho , I do want it to sound good.


I would like to sim the TI reference circuit if i can but this LT spice is taking forever... and it will probably give an error at the end or some crappy output
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Old 28th June 2012, 03:44 AM   #9
jcx is offline jcx  United States
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I would say the biggest advantage of the TI OPA627/37 in Audio is the unusually high input Z, especially the dynamic, nonlinear common mode part - which only makes a big difference with large input common mode swing and high, unbalanced source&feedback Z
so the best feature of these op amps is not that important in a DAC I/V with 0 common mode V inverting input and for the PCM1794 currents giving < 1 kOhm feedback Z

I would rate GBW, input noise referred to the circuit conditions, low open loop output Z as more important in DAC I/V
"input linearity" should also be on the 1st order importance list, has some subtleties
slew rate is not 1st order important per se with feedback C, it has in the past been a proxy for good input linearity with higher Vdif, I think the new "highly linear" input stages that AD is doing such a poor job advertising are a "game changer" in the use of bjt input op amps for DAC I/V - I think these change the presumption of fet input being "more linear"


“noise gain compensation” of a decompensated op amp is good with resistive feedback, plenty of design guides, explanations out there

but for DAC I/V with feedback C the “noise gain” feedback becomes a C divider - I worry that a C divider feedback will have additional interactions with the op amp output Z – can still be dealt with if you have some practical info on op amp output Z near the gain intercept, possibly way beyond if local output Q oscillations are possible
Unfortunately maxing out GBW with today’s fast processes put us way beyond the spice macromodel output Z usefulness – sim can’t help us much here if the device designers don’t give use useful output models

We do have the example of Wurcer’s AD797 datasheet app with the input shunt C in a DAV I/V, he adds series R to the feedback C which partially defeats one of the advantages of having the feedback C, and the AD797 stability is known to be a compromise so this may not be a general answer
Attached Images
File Type: png wurcerIV.PNG (88.0 KB, 511 views)

Last edited by jcx; 28th June 2012 at 04:01 AM.
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Old 28th June 2012, 03:44 AM   #10
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Simulation will tell you very little about whether some I/V circuit will sound good. I've spent hours and hours trying to correlate listening results with the output of LTSpice, to no avail As you say your aim is good sound then the most effective route to that goal is breadboarding - listen as you tweak.

A week ago I'd have said abandon the 1794 if you really want good sound, but earlier this week I read a good review on Head-Fi of the Nuforce DAC9 which uses a sister part, the 1798. So decent results are indeed possible. Ultimately though the limitations of their digital noise-shaping loops will be the bottleneck I believe.
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