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Measurements of 19+20kHz IMD of various opamps at gain 40dB

pma

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This is a set of measurements of 19+20kHz IMD distortion of the following opamps: AD4627, ADA4898, AD8597, OPA140, OPA141, OPA1641, OPA209, OPA1611, OPA637, NE5534. Measured at non-inverting gain 40dB and 3.4Vp-p output.

CCIF_AD4627.png


CCIF_AD8597.png


CCIF_ADA4898.png


CCIF_NE5534.png


CCIF_OPA140.png


CCIF_OPA141.png


CCIF_OPA209.png


CCIF_OPA637.png


CCIF_OPA1611.png


CCIF_OPA1641.png
 

HpW

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May you consider to use the new TI OPA1656 :D as "SoundPlus™ ultra-low noise and distortion, Burr-Brown™ audio operational amplifier"
 

Hayabusa

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Blumlein 88

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All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.
 
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March Audio

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All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.
That's why I asked for a loopback, I wouldn't expect those products, especially at exactly the same level in each plot.
 
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pma

pma

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Can you post the loopback of the measurement system?
The measurements were done in January 2013. I may try to re-arrange the system, but I am not sure if I can replicate it perfectly. 2nd issue is that the DUT had 40dB gain so the DAC output is small and less distorted than if it works in loopback without gain. My guess is that the system distortion limit was close to OPA637 result.

All of your graphs show something at 8, 9 and 10 khz. Is this an aliasing/imaging product from using digital test gear? Looks like you used a 32 k FFT and 48 khz sampling.

Thanks for doing and sharing this btw. Good info.

Thank you. You are right, the 8,9,10kHz lines at -105 dBr were the soundcard artifacts present in every CCIF measurement with that card.

@restorer-john Yes +/-15V supply, 3.4Vp-p output as below
OZ_CCIF2_30January2013.PNG


--------------------------------------------
This was the test circuit. Double-sided PCB with ground plane.
opamp_40dB_test.png

--------------------------------------------

I have one more comment on 19+20kHz. Sometimes it is correctly stated that music does not contain such frequencies in high amplitudes. This is of course true, but the point of the 19+20kHz is different. Let's not forget that this test also shows static distortion of the transfer function. So if the DUT has frequency independent distortion (= static distortion only), it is also shown in the plot. H2 is shown as a difference tone, H3 as two lines at the side of base frequencies. Same for further even harmonics (they follow diff. tone) and odd harmonics ("skirts"). The bonus is that the 19+20kHz test also shows dynamic distortion, i.e. that related to higher frequency non-linearity or slew rate limit. These dynamic distortions remain almost invisible in the 1kHz sine spectrum test. So I suggest 19+20kHz test even at higher amplitudes (than those in natural music) for the test reasons to reveal dynamic behavior of the amp.

Below is the 19+20kHz test, simulated results, on a purely static DC nonlinearity, -80dB (re FS) H2 and H3. Original and distorted.

ccif_orig.png


ccif_dist.png
 
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guenthi_r

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May you consider to use the new TI OPA1656 :D as "SoundPlus™ ultra-low noise and distortion, Burr-Brown™ audio operational amplifier"

Betters my good old´s Asus STX (PCM1792a) I/V Stage (3dB improvement over LM4562/NE5532A):
OPA1656.png


THD.png

Not bad for a CMOS-Opamp.

greetings,
Günther
 

MakeMineVinyl

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I'm surprised that the "ancient" NE5534 apparently does so well.
 

scott wurcer

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I'm surprised that the "ancient" NE5534 apparently does so well.

Not really, it was the first op-amp (?) that compromised DC precision for audio applications. It has almost no use in precision instrumentation where drift, offset, and bias current issues are important. Just an historical observation that the money in the 60's and 70's was in military and aerospace applications.
 

MakeMineVinyl

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Not really, it was the first op-amp (?) that compromised DC precision for audio applications. It has almost no use in precision instrumentation where drift, offset, and bias current issues are important. Just an historical observation that the money in the 60's and 70's was in military and aerospace applications.
I think it was intended for audio applications from the start, so it makes sense that its DC performance was not what was required for instrumentation applications. If I remember correctly, it had compensation pins which were of use in particular instances. I know of one engineer who used these pins to optimize the audio performance for his particular application. The NE5532 (basically a dual NE5534 internally compensated) is still used, which I find interesting.
 

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AnalogSteph

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If I may, some considerations to put pma's results into context:
Output loading is a lowish 1 kOhm. Parts with good output driving are going to look better here.
Voltage gain = 40 dB. Common-mode voltage would have been very small at 34 mVpp.

Hence why the NE5534 was looking particularly good - output driving is more than decent, and the part's potential issues with common-mode distortion (though less pronounced than 5532) are not being highlighted. IMHO this part was the best single-chip compromise (voltage noise, current noise and output driving) for an MM phono stage for many years... I think it takes something quite recent like an OPA1656 to upstage it even in this application... truly low-noise FET parts have never been too common (the AD743 comes to mind, but that's not exactly super fast, and at 18-20 pF of input capacitance I don't dare asking what common-mode distortion is like).
 

DonH56

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At audio frequencies FET-input opamps tend to be JFETs to avoid MOSFETs' high 1/f noise (etc.) so they are less common... Bipolar inputs are worse for high-impedance sources like phono inputs, however, due to their bias current contributing current noise at the input.
 

scott wurcer

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Hence why the NE5534 was looking particularly good - output driving is more than decent, and the part's potential issues with common-mode distortion (though less pronounced than 5532) are not being highlighted. IMHO this part was the best single-chip compromise (voltage noise, current noise and output driving) for an MM phono stage for many years... I think it takes something quite recent like an OPA1656 to upstage it even in this application... truly low-noise FET parts have never been too common (the AD743 comes to mind, but that's not exactly super fast, and at 18-20 pF of input capacitance I don't dare asking what common-mode distortion is like).

Phono inherently has very low common mode voltage. The AD743 was designed for a giant project to plant hydrophones along our coasts to locate hostile submarines. The problem was all the subcontractors we talked to were bidding on the same contract, in the end GE used it in a generation of CT scanners (1024 channels each) much more satisfying. Every application is different.

BTW the uber-guru of all hydrophone designers called me personally at my desk to tell me these common mode capacitance effects were nonsense and didn't exist.
 
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